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ORIGIN AND METAMORPHOSES

OF INSECTS.

ON THE

ORIGIN AND METAMORPHOSES

OF INSECTS

BY

SIR JOHN LUBBOCK, Bart., M.P., F.R.S., D.C.L., LL.D.

PRINCIPAL OF THE LONDON WORKING MEN’S COLLEGE; PRESIDENT OF THE LONDON
CHAMBER OF COMMERCE; AND VICE-CHAIRMAN OF THE LONDON COUNTY COUNCIL

WITH NUMEROUS ILLUSTRATIONS

London

MACMILLAN AND CO.

AND NEW YORK

1890

The Right of Translation and Reproduction is Reserved

Richard Clay and Sons, Limited,
london and bungay.

First Edition 1873. Reprinted 1874.
New Edition 1890.

vii

PREFACE.

For some years, much of my leisure time has
been devoted to the study of the anatomy, development,
and habits of the Annulosa, and especially of
Insects, on which subjects I have published various
memoirs, chiefly in the Transactions of the Royal,
Linnæan, and Entomological Societies: of these
papers I subjoin a list. Although the details, of
which these memoirs necessarily for the most part
consist, offer little interest, excepting to those persons
who are specially devoted to Entomology,
still there are portions which, having reference to
the nature of metamorphoses and to the origin of
insects, are of a more general character. I have
also briefly referred to these questions in a Monograph
of the Collembola and Thysanura, recently
published by the Ray Society, and in the Opening
Address to the Biological Section of the British
Association at Brighton in 1872. Under theseviii
circumstances, it has been suggested to me that a
small volume, containing, at somewhat greater length,
in a more accessible form, and with the advantage
of illustrations, the conclusions to which I have been
led on this interesting subject, might not be altogether
without interest to the general reader. The
result, which has already appeared in the pages of
Nature, is now submitted to the public, with some
additions. I am well aware that it has no pretence
to be in any sense a complete treatise; that the
subject itself is one as to which our knowledge
is still very incomplete, and on which the highest
authorities are much divided in opinion. Whatever
differences of opinion, however, there may be as to
the views here put forward, the facts on which they
are based will, I believe, be found correct. On
this point I speak with the more confidence, on
account of the valuable assistance I have received
from many friends: to Mr. and Mrs. Busk and Dr.
Hooker I am especially indebted.

The papers above referred to are as follows:—

1. On Labidocera.—Annals and Magazine of Natural History, vol.
xi., 1853.

2. On Two New Sub-genera of Calanidæ.—Annals and Magazine of
Natural History, vol. xii., 1853.

ix

3. On Two New Species of Calanidæ.—Annals and Magazine of
Natural History, vol. xii., No. lxvii., 1853.

4. On Two New Species of Calanidæ.—Annals and Magazine of
Natural History, vol. xii., No. lxix., 1853.

5. On some Arctic Calanidæ.—Annals and Magazine of Natural
History, 1854.

6. On the Freshwater Entomostraca of South America.—Transactions
of the Entomological Society, vol. iii., 1855.

7. On some New Entomostraca.—Transactions of the Entomological
Society, vol. iv., 1856.

8. On some Marine Entomostraca found at Weymouth.—Annals and
Magazine of Natural History, vol. xx., 1857.

9. On the Respiration of Insects.—Entomological Annual, 1857.

10. An Account of the Two Methods of Reproduction in Daphnia.—Transactions
of the Royal Society, 1857.

11. On the Ova and Pseudova of Insects.—Transactions of the Royal
Society, 1858.

12. On the Arrangement of the Cutaneous Muscles of Pygæra Bucephala.—Linnean
Society’s Transactions, vol. xxii., 1858.

13. On the Freshwater Entomostraca of South America.—Entomological
Society’s Transactions, 1858.

14. On Coccus Hesperidum.—Royal Society Proceedings, vol. ix.,
1858.

15. On the Distribution of Tracheæ in Insects.—Linnean Society’s
Transactions, vol. xxiii., 1860.

16. On the Generative Organs and on the Formation of the Egg in
Annulosa. Transactions of the Royal Society, 1861.

17. On Sphærularia Bombi.—Natural History Review, 1861.

18. On some Oceanic Entomostraca.—Linnean Society’s Transactions,
vol. xxiii., 1860.

19. On the Thysanura. Part 1.—Linnean Society’s Transactions, 1862.

20. On the Development of Lonchoptera.—Entomological Society’s
Transactions, 1862.

21. On the Thysanura. Part 2.—Linnean Society’s Transactions, 1862.

22. On the Development of Chloëon. Part 1.—Linnean Society’s
Transactions, 1863.

23. On Two Aquatic Hymenoptera.—Linnean Society’s Transactions,
1863.

24. On some little-known Species of Freshwater Entomostraca.—Linnean
Society’s Transactions, vol. xxiv., 1863.

25. On Sphærularia Bombi.—Natural History Review, 1864.

x

26. On the Development of Chloëon. Part 2.—Linnean Society’s
Transactions, 1865.

27. Metamorphoses of Insects.—Journal of the Royal Institution, 1866.

28. On Pauropus.—Linnean Society’s Transactions, 1866.

29. On the Thysanura. Part 3.—Linnean Society’s Transactions, 1867.

30. Address to the Entomological Society.—Entomological Society’s
Transactions, 1867.

31. On the Larva of Micropeplus Staphilinoides.—Entomological
Society’s Transactions, 1868.

32. On the Thysanura. Part 4.—Linnean Society’s Transactions, 1869.

33. Addresses to the Entomological Society.—Entomological Society’s
Transactions, 1867-1868.

34. On the Origin of Insects.—Journal of the Linnean Society, vol. xi.

35. Opening Address to the Biological Section of the British Association.—British
Association Report, 1872.

36. Observations on Ants, Bees, and Wasps. Part 1.—Journal of the
Linnean Society, 1873.

37. On British Wild Flowers considered in relation to Insects, 1874.

38. Observations on Ants, Bees, and Wasps. Part 2.—Journal of the
Linnean Society, 1874.

39. Observations on Ants, Bees, and Wasps. Part 3.—Journal of the
Linnean Society, 1875.

40. Observations on Ants, Bees, and Wasps. Part 4.—Journal of the
Linnean Society, 1877.

41. On some Points in the Anatomy of Ants.—Quekett Lecture,
1877.—Microscopical Journal.

42. On the Colors of Caterpillars.—Entomological Society’s Transactions,
1878.

43. Observations on Ants, Bees, and Wasps. Part 5.—Journal of the
Linnean Society, 1878.

44. Observations on Ants, Bees, and Wasps. Part 6.—Journal of the
Linnean Society, 1879.

45. On the Anatomy of Ants.—Linnean Society’s Transactions, 1880.

46. Observations on Ants, Bees, and Wasps. Part 7.—Journal of the
Linnean Society, 1880.

47. Observations on Ants, Bees, and Wasps. Part 8.—Journal of the
Linnean Society, 1881.

48. On Fruits and Seeds.—Journal of the Royal Institution, 1881.

49. Observations on Ants, Bees, and Wasps. Part 9.—Journal of the
Linnean Society, 1881.

50. On the Limits of Vision among some of the lower Animals.—Journal
of the Linnean Society, 1881.

51. Observations on Ants, Bees, and Wasps. Part 10.—Journal of the
Linnean Society, 1882.

xi

CONTENTS.

xiii

DESCRIPTION OF THE PLATES.

PLATE I. p. 7.

FIG.

1. Cricket. Westwood, Intro. to the Modern Classification of Insects,
vol. i. p. 440.

2. Earwig. Westwood, loc. cit. vol. i. p. 399.

3. Aphis. Packard, Guide to the Study of Insects, pp. 521, 522.

4. Scolytus. Westwood, loc. cit. vol. i. p. 350.

5. Anthrax. Westwood, loc. cit. vol. ii. p. 538.

6. Balaninus.

7. Cynips. Westwood, loc. cit. vol. ii. p. 121.

8. Ant (Formica). Westwood, loc. cit. vol. ii. p. 218.

9. Wasp. Ormerod, Nat. Hist. of Wasps, pl. i. fig. 1.

PLATE II. p. 8.

FIG.

1. Larva of Cricket. Westwood, loc. cit. vol. i. p. 440.

2. Larva of Aphis. Packard, loc. cit. pp. 521, 522.

3. Larva of Earwig. Westwood, loc. cit. vol. i. p. 399.

4. Larva of Scolytus. Westwood, loc. cit. vol. i. p. 350.

5. Larva of Anthrax. Westwood, loc. cit. vol. ii. p. 546.

6. Larva of Balaninus.

7. Larva of Cynips. Westwood, loc. cit. vol. ii. p. 121.

8. Larva of Ant (Formica). Westwood, loc. cit. vol. ii. p. 226.

9. Larva of Wasp. Newport, Art. Insecta, Todd’s Cycl. Anat. and
Phys., p. 871.

PLATE III. p. 14.

FIG.

1. Chloëon. Linn. Trans. 1866.

2. Meloë. Spry and Shuckard, Coleoptera Delineated, pl. 56.

3. Calepteryx.

4. Sitaris. Spry and Shuckard, loc. cit. pl. 56.

5. Campodea. Suites à Buffon. Aptéres.

xiv

6. Acilius. Westwood, loc. cit. vol. i. p. 100.

7. Termes. Westwood, loc. cit. vol. ii. p. 12.

8. Stylops. Duncan, Met. of Insects, p. 387; Packard, p. 482.

9. Thrips. Westwood, loc. cit. vol. ii. p. 1.

PLATE IV. p. 15.

FIG.

1. Larva of Chloëon. Linn. Trans. 1863.

2. Larva of Meloë. Chapuis and Candèze, Mem. Soc. Roy. Liége,
1853, pp. 1, 7.

3. Larva of Calepteryx. Dufour, Ann. Sci. Nat. 1852.

4. Larva of Sitaris. Duncan, Met. of Insects, p. 309.

5. Larva of Campodea. Gervais’ Suites à Buffon. Aptéres.

6. Larva of Acilius. Westwood, loc. cit. vol. i. p. 100.

7. Larva of Termes. Duncan, loc. cit. p. 348.

8. Larva of Stylops. Westwood, Trans. Ent. Soc. 1839, vol. ii.
pl. xv. fig. 13a.

9. Larva of Thrips. Westwood, loc. cit. vol. ii. p. i.

PLATE V. p. 99.

FIG.

1-5. Protamœba.

6-9. Protamyxa aurantiaca. Haeckel Beit. zur. Monog. der Moneren,
pl. 1.

10-18. Magosphœra planula. Haeckel, loc. cit. pl. v.

PLATE VI. p. 105.

FIG.

1-4. Yolk-segmentation in Laomedea. After Allman. Mon. of
Tubularian Hydroids. Ray Society.

5-9. Yolk-segmentation in Filaria. After Van Beneden. Mem.
sur les Vers Intestinaux.

10-13. Yolk-segmentation in Echinus. After Derbes. Ann. des. Sci.
Nat. 1847.

14-17. Yolk-segmentation in Lacinularia. After Huxley. J. of Mic.
Sci. 1853.

18-21. Yolk-segmentation in Purpura. After Koren and Danielssen.
Ann. des. Sci. Nat. 1853.

22-24. Yolk-segmentation in Amphioxus. After Haeckel. Naturliche
Schöpfungsgeschichte, pl. x.

25-29. Yolk-segmentation in Vertebrate. After Allen Thompson.
Art. Ovum. Cyclop. of Anatomy and Physiology.

xv

DESCRIPTION OF THE FIGURES.

FIG.

1. Larva of the Cockchafer (Melolontha)

2. Larva of Cetonia.

3. Larva of Trox.

4. Larva of Oryctes.

5. Larva of Aphodius.

6. Larva of Lucanus.

7. Larva of Brachytarsus.

8. Larva of Crioceris.

9. Larva of Sitaris humeralis.

10. Larva of Sitaris humeralis, in the second stage.

11. Larva of Sitaris humeralis, in the third stage.

12. Larva of Sitaris humeralis, in the fourth stage.

13. Pupa of Sitaris.

14. Larva of Sirex.

15. Egg of Rhynchites, showing the parasitic larva.

16. The parasitic larva, more magnified.

17. Egg of Platygaster.

18. Egg of Platygaster, showing the central cell.

19. Egg of Platygaster, after the division of the central cell.

20. Egg of Platygaster, more advanced.

21. Egg of Platygaster, more advanced.

22. Egg of Platygaster, showing the rudiment of the embryo.

23. Larva of Platygaster.—mo, mouth; a, antenna; kf, hooked feet;
r, toothed process; lfg, lateral process; f, branches of the tail.

24. Larva of another species of Platygaster. (The letters indicate the
same parts as in the preceding figure.)

25. Larva of a third species of Platygaster. (The letters indicate the
same parts as in the preceding figure.)

26. Larva of Platygaster in the second stage.—mo, mouth; slkf,
œsophagus; gsae, supra-œsophagal ganglion; lm, muscles;
bsm, nervous system; gagh, rudiments of the reproductive glands.

27. Larva of Platygaster in the third stage.—mo, mouth; ma, mandibles;
gsae, supra-œsophagal ganglion; slk, œsophagus; ag,
ducts of the salivary glands; bnm, ventral nervous system; sp,
salivary glands; msl, stomach; im, imaginal discs; tr, tracheæ;
fk, fatty tissue; ed, intestine; ga, rudiments of reproductive
organs; ew, wider portion of intestine; ao, posterior opening.

28. Embryo of Polynema.

29. Larva of Polynema.—asch, rudiments of the antennæ; flsch, of
the wings; bsch, of the legs; vfg, lateral projections; gsch,
rudiments of the ovipositor; fk, fatty tissue.

xvi
30. Egg of Phryganea (Mystacides).—A¹, mandibular segment; C¹–C⁵,
maxillary, labial, and three thoracic segments; D, abdomen.

31. Egg of Phryganea somewhat more advanced.—b, mandibles; c,
maxillæ; cfs, rudiments of the three pairs of legs.

32. Egg of Pholcus opilionides, showing the Protozonites.

33. Embryo of Julus.

34. Colony of Bougainvillea fruticosa, natural size, attached to the underside
of a piece of floating timber.

35. Portion of the same, more magnified.

36. The Medusa from the same species.

37. Larva of Prawn, Nauplius stage.

38. Larva of Prawn, more advanced, Zoëa stage.

39. Larva of Echino-cidaris œquituberculata seen from above ✕ 6/10.

40. Larva of Echinus ✕ 100.—A, front arm; F, arms of the mouth-process;
B, posterior side arm; E¹, accessory arm of the mouth-process;
a, mouth; a¹, œsophagus; b, stomach; b¹, intestine;
o, posterior orifice; d, ciliated bands; f, ciliated epaulets;
c, disc of future Echinus.

41. Comatula rosacea.

42. Larva of Comatula rosacea.

43. Larva of Comatula rosacea, more advanced.

44. Larva of Comatula rosacea, in the Pentacrinus state.

45. Larva of Starfish (Bipinnaria), ✕ 100.

46. Larva of Starfish (Bipinnaria), ✕ 100, seen from the side.—a,
mouth; b, œsophagus; c, stomach; c¹, intestine.

47. Larva of another Bipinnaria, showing the commencement of the
Starfish.—g, canal of the ciliated sac; i, rudiments of tentacles;
d, ciliated band.

48. Larva of Moth (Agrotis).

49. Larva of Beetle (Haltica).

50. Larva of Saw-fly (Cimbex).

51. Larva of Julus.

52. Agrotis suffusa.

53. Haltica.

54. Cimbex.

55. Julus.

56. Tardigrade.

57. Larva of Cecidomyia.

58. Lindia torulosa.

59. Prorhynchus stagnalis.

60. Egg of Tardigrade.

61. Egg of Tardigrade, after the yolk has subdivided.

62. Egg of Tardigrade, in the next stage.

63. Egg of Tardigrade, more advanced.

1

ON THE

ORIGIN AND METAMORPHOSES
OF INSECTS.

CHAPTER I.

THE CLASSIFICATION OF INSECTS.

About forty years ago the civil and ecclesiastical
authorities of St. Fernando in Chili arrested a certain
M. Renous on a charge of witchcraft, because he kept
some caterpillars which turned into butterflies.1 This
was no doubt an extreme case of ignorance; it is now
almost universally known that the great majority of
insects quit the egg in a state very different from
that which they ultimately assume; and the general
statement in works on entomology has been that the
life of an insect may be divided into four periods.

Thus, according to Kirby and Spence,2 “The states
through which insects pass are four: the egg, the
larva, the pupa, and the imago.” Burmeister,3 also,
2says that, excluding certain very rare anomalies,
“we may observe four distinct periods of existence
in every insect,—namely, those of the egg, the larva,
the pupa, and the imago, or perfect insect.” In fact,
however, the various groups of insects differ widely
from one another in the metamorphoses they pass
through: in some, as in the grasshoppers and crickets,
the changes consist principally in a gradual increase
of size, and in the acquisition of wings; while others,
as for instance the common fly, acquire their full
bulk in a form very different from that which they
ultimately assume, and pass through a period of inaction
in which not only is the whole form of the
body altered, not only are legs and wings acquired,
but even the internal organs themselves are almost
entirely disintegrated and re-formed. It will be my
object, after having briefly described these changes,
to throw some light on the causes to which they are
due, and on the indications they afford of the stages
through which insects have been evolved.

The following list gives the orders or principal
groups into which the Class Insecta may be divided.
I will not, indeed, here enter upon my own views, but
will adopt the system given by Mr. Westwood in his
excellent “Introduction to the Modern Classification
of Insects,” from which also, as a standard authority,
most of the figures on Plates I. to IV., when not otherwise
acknowledged, have been taken. He divides
insects into thirteen groups, and with reference to
eight of them it may be said that there is little
difference of opinion among entomologists. These
orders are by far the most numerous, and I have3
placed them in capital letters. As regards the other
five there is still much difference of opinion. It must
also be observed that Prof. Westwood omits the
parasitic Anoplura, as well as the Thysanura and
Collembola.

ORDERS OF INSECTS ACCORDING TO WESTWOOD.

Of these thirteen orders, the eight which I have
placed in capital letters—namely the first, third, fifth,
seventh, ninth, eleventh, twelfth, and thirteenth, are
much the most important in the number and variety
of their species; the other five form comparatively
small groups. The Strepsiptera are minute insects,
parasitic on Hymenoptera: Rossi, by whom they
were discovered, regarded them as Hymenopterous;
Lamarck placed them among the Diptera; by others
they have been considered to be most closely allied
to the Coleoptera, but they are now generally treated
as an independent order.

The Euplexoptera or Earwigs are only too familiar
to most of us. Linnæus classed them among the4
Coleoptera, from which, however, they differ in
their transformations. Fabricius, Olivier, and Latreille
regarded them as Orthoptera; but Dr. Leach, on
account of the structure of their wings, considered
them as forming the type of a distinct order, in which
view he has been followed by Westwood, Kirby, and
many other entomologists.

The Thysanoptera, consisting of the Linnæan genus
Thrips, are minute insects well known to gardeners,
differing from the Coleoptera in the nature of their
metamorphoses, in which they resemble the Orthoptera
and Hemiptera. The structure of the wings and
mouth-parts, however, are considered to exclude them
from these two orders.

The Trichoptera, or Caddis worms, offer many
points of resemblance to the Neuroptera, while in
others they approach more nearly to the Lepidoptera.
According to Westwood, the genus Phryganea “forms
the connecting link between the Neuroptera and Lepidoptera.”

The last of these small aberrant orders is that of
the Aphaniptera, constituted for the family Pulicidæ.
In their transformations, as in many other respects,
they closely resemble the Diptera. Strauss Durckheim
indeed said that “la puce est un diptère sans
ailes.” Westwood, however, regards it as constituting
a separate order.

As indicated by the names of these orders, the
structure of the wings affords extremely natural and
convenient characters by which the various groups
may be distinguished from one another. The mouth-parts
also are very important; and, regarded from5
this point of view, the Insecta have been divided
into two series—the Mandibulata and Haustellata, or
mandibulate and suctorial groups, between which,
as I have elsewhere shown,4 the Collembola (Podura,
Smynthurus, &c.) occupy an intermediate position.
These two series are:—

Again—and this is the most important from my
present point of view—insects have sometimes been
divided into two other series, according to the nature
of their metamorphoses: “Heteromorpha,” to use the
terminology of Prof. Westwood,5 “or those in which
there is no resemblance between the parent and the
offspring; and Homomorpha, or those in which the
larva resembles the imago, except in the absence of
wings. In the former the larva is generally worm-like,
of a soft and fleshy consistence, and furnished
with a mouth, and often with six short legs attached
in pairs to the three segments succeeding the head.
In the Homomorpha, including the Orthoptera,
Hemiptera, Homoptera, and certain Neuroptera, the
body, legs, and antennæ are nearly similar in their
form to those of the perfect insect, but the wings
are wanting.”

6

But though the Homomorphic insects do not pass
through such striking changes of form as the
Heteromorphic, and are active throughout life, still
it was until within the last few years generally
(though erroneously) considered, that in them, as
in the Heteromorpha, the life fell into four distinct
periods; those of (1) the egg, (2) the larva, characterized
by the absence of wings, (3) the pupa with
imperfect wings, and (4) the imago, or perfect insect.

I have, however, elsewhere6 shown that there are
not, as a matter of fact, four well-marked stages, and
four only, but that in many cases the process is much
more gradual.

The species belonging to the order Hymenoptera
are among the most interesting of insects. To this
order belong the gallflies, the sawflies, the ichneumons,
and, above all, the ants and bees. We are
accustomed to class the Anthropoid apes next to
man in the scale of creation, but if we were to judge
animals by their works, the chimpanzee and the
gorilla must certainly give place to the bee and
the ant. The larvæ of the sawflies, which live on
leaves, and of the Siricidæ or long-tailed wasps,
which feed on wood, are very much like caterpillars,
7having three pairs of legs, and in the former case
abdominal pro-legs as well: but in the great majority
of Hymenoptera the larvæ are legless, fleshy grubs
(Plate II., Figs. 7-9); and the various modes by which
the females provide for, or secure to, them a sufficient
supply of appropriate nourishment constitutes one of
the most interesting pages of Natural History.

The species of Hymenoptera are very numerous;
in this country alone there are about 3,000 kinds,
most of which are very small. In the pupa state
they are inactive, and show distinctly all the limbs of
the perfect insect, encased in distinct sheaths, and
folded on the breast. In the perfect state they are
highly organized and very active. The working ants
and some few species are wingless, but the great
majority have four strong membranous wings, a character
distinguishing them at once from the true flies,
which have only one pair of wings.

The sawflies are so called because they possess at
the end of the body a curious organ, corresponding to
the sting of a wasp, but which is in the form of a
fine-toothed saw. With this instrument the female
sawfly cuts a slit in the stem or leaf of a plant, into
which she introduces her egg. The larva much resembles
a caterpillar, both in form and habits. To
this group belongs the nigger, or black caterpillar
of the turnip, which is often in sufficient numbers to
do much mischief. Some species make galls, but
the greater number of galls are formed by insects
of another family, the Cynipidæ.

8

PLATE I.7—MATURE INSECTS.

Fig. 1, Cricket; 2, Earwig; 3, Aphis; 4, Scolytus; 5, Anthrax; 6, Balaninus;
7, Cynips; 8, Ant; 9, Wasp.

9

PLATE II.—LARVÆ OF THE INSECTS REPRESENTED ON PLATE I.

Fig. 1, Larva of Cricket; 2, Larva of Aphis; 3, Larva of Earwig; 4, Larva of
Scolytus (Beetle); 5, Larva of Anthrax (Fly); 6, Larva of Balaninus (Nut
Weevil); 7, Larva of Cynips; 8, Larva of Ant; 9, Larva of Wasp.

10

In the Cynipidæ (Plate I., Fig. 7) the female is
provided with an organ corresponding to the saw of
the sawfly, but resembling a needle. With this she
stings or punctures the surface of leaves, buds, stalks,
or even roots of various plants. In the wound thus
produced she lays one or more eggs. The effects of
this proceeding, and particularly of the irritating fluid
which she injects into the wound, is to produce a
tumour or gall, within which the egg hatches, and on
which the larva, a thick fleshy grub (Plate II., Fig. 7),
feeds. In some species each gall contains a single
larva; in others, several live together.

The oak supports several kinds of gallflies:
one produces the well-known oak-apple, one a
small swelling on the leaf resembling a currant,
another a gall somewhat like an acorn, another
attacks the root; the species making the bullet-like
galls, which are now so common, has only
existed for a few years in this country; the
beautiful little spangles so common in autumn
on the under side of oak leaves are the work
of another species, the Cynus longipennis. One
curious point about this group is, that in some
of the commonest species the females alone are
known, no one yet having ever succeeded in
finding a male.

Another great family of the Hymenoptera is that
of the ichneumons; the females lay their eggs either
in or on other insects, within the bodies of which the
larvæ live. These larvæ are thick, fleshy, legless
grubs, and feed on the fatty tissues of their hosts,
but do not attack the vital organs. When full-grown,
the grubs eat their way through the skin of11
the insect, and turn into chrysalides. Almost every
kind of insect is subject to the attacks of these little
creatures, which are no doubt useful in preventing
the too great multiplication of insects, and especially
of caterpillars. Some species are so minute that they
actually lay their eggs within those of other insects
Figs. (15, 16). These parasites assume very curious
forms in their larval state.

But of all the Hymenoptera, the group containing
the ant, the bee, and the wasp is the most interesting.
This is especially the case with the social species,
though the solitary ones also are extremely remarkable.
The solitary bee or wasp, for instance, forms a
cell generally in the ground, places in it a sufficient
amount of food, lays an egg, and closes the cell. In
the case of bees, the food consists of honey; in that
of wasps, the larva requires animal food, and the
mother therefore places a certain number of insects
in the cell, each species having its own special
prey, some selecting small caterpillars, some beetles,
some spiders. Cerceris bupresticida, as its name
denotes, attacks beetles belonging to the genus Buprestis.
Now if the Cerceris were to kill the beetle
before placing it in the cell, it would decay, and the
young larva, when hatched, would find only a mass
of corruption. On the other hand, if the beetle
were buried uninjured, in its struggles to escape it
would be almost certain to destroy the egg. The
wasp has, however, the instinct of stinging its prey
in the centre of the nervous system, thus depriving
it of motion, and let us hope of suffering, but not
of life; consequently, when the young larva leaves12
the egg, it finds ready a sufficient store of wholesome
food.

Other wasps are social, and, like the bees and ants,
dwell together in communities. They live for one
season, dying in autumn, except some of the females,
which hibernate, awake in the spring, and form new
colonies. These, however, do not, under ordinary
circumstances, live through a second winter. One
specimen which I kept tame through last spring and
summer, lived until the end of February, but then
died. The larvæ of wasps (Plate II., Fig. 9) are fat,
fleshy, legless grubs. When full-grown they spin for
themselves a silken covering, within which they turn
into chrysalides. The oval bodies which are so numerous
in ants’ nests, and which are generally called
ants’ eggs, are really not eggs but cocoons. Ants are
very fond of the honey-dew which is formed by the
Aphides, and have been seen to tap the Aphides with
their antennæ, as if to induce them to emit some
of the sweet secretion. There is a species of Aphis
which lives on the roots of grass, and some ants
collect these into their nests, keeping them, in fact,
just as we do cows. Moreover they collect the eggs in
the autumn and tend them through the winter (when
they are of no use) with the same care as their own,
so as to have a supply of young Aphides in the spring.
This is one of the most remarkable facts I know in the
whole history of animal life. One species of red ant
does no work for itself, but makes slaves of a black
kind, which then do everything for their masters. The
slave makers will not even put food into their own
mouths, but would starve in the midst of plenty, if they13
had not a slave to feed them. I found, however, that
I could keep them in life and health for months if I
gave them a slave for an hour or two in a week to clean
and feed them.

14

PLATE III.–MATURE INSECTS.

Fig. 1, Chloëon; 2, Meloë (after Shuckard); 3, Calepteryx; 4, Sitaris (after Shuckard);
5, Campodea (after Gervais); 6, Acilius; 7, Termes; 8, Stylops (female);
9, Thrips.

15

PLATE IV. YOUNG FORMS OF THE INSECTS REPRESENTED ON
PLATE III.–Fig. 1, Larva of Chloëon; 2, Larva of Meloë (after Chapuis and Candèze);
3, Larva of Calepteryx (after Léon Dufour); 4, Larva of Sitaris; 5, Larva
of Campodea; 6, Larva of Acilius; 7, Larva of Termes (after Blanchard); 8,
Larva of Stylops; 9, Larva of Thrips.

Ants also keep a variety of beetles and other insects
in their nests. That they have some reason for this
seems clear, because they readily attack any unwelcome
intruder; but what that reason is, we do not yet
know. If these insects are to be regarded as the
domestic animals of the ants, then we must admit that
the ants possess more domestic animals than we do.

Some indeed of these beetles produce a secretion
which is licked by the ants like the honeydew; there
are others, however, which have not yet been shown
to be of any use to the ants, and yet are rarely, if‘
ever, found, excepting in ants’ nests.

M. Lespès, who regards these insects as true
domestic animals, has recorded8 some interesting
observations on the relations between one of them
(Claviger Duvalii) and the ants (Lasius niger) with
which it lives. This species of Claviger is never met
with except in ants’ nests, though on the other hand
there are many communities of Lasius which possess
none of these beetles; and M. Lespès found that
when he placed Clavigers in a nest of ants which
had none of their own, the beetles were immediately
killed and eaten, the ants themselves being on the
other hand kindly received by other communities of
the same species. He concludes from these observations
that some communities of ants are more advanced
in civilization than others; the suggestion is
16
no doubt ingenious, and the fact curiously resembles
the experience of navigators who have endeavoured
to introduce domestic animals among barbarous
tribes; but M. Lepès has not yet, so far as I am
aware, published the details of his observations, without
which it is impossible to form a decided opinion.
I have sometimes wondered whether the ants have
any feeling of reverence for these beetles; but the
whole subject is as yet very obscure, and would well
repay careful study.

The order Strepsiptera are a small, but very remarkable
group of insects, parasitic on bees and wasps.
The larva (Pl. IV., Fig. 8) is minute, six-legged, and
very active; it passes through its transformations
within the body of the bee or wasp. The male and
female are very dissimilar. The males are minute,
very active, short-lived, and excitable, with one pair
of large membranous wings. The females (Pl. III., Fig.
8), on the contrary, are almost motionless, and shaped
very much like a bottle; they never quit the body of
the bee, but only thrust out the top of the bottle
between the abdominal rings of the bee.

In the order Coleoptera, the larvæ differ very much
in form. The majority are elongated, active, hexapod,
and more or less depressed; but those of the Weevils
(Pl. II., Fig. 6), of Scolytus (Pl. II., Fig. 4), &c., which
are vegetable feeders, and live surrounded by their
food,—as, for instance, in grain, nuts, &c.,—are apod,
white, fleshy grubs, not unlike those of bees and ants.
The larvæ of the Longicorns, which live inside trees,
are long, soft, and fleshy, with six short legs. The
Geodephaga, corresponding with the Linnæan genera17
Cicindela and Carabus, have six-legged, slender, carnivorous
larvæ; those of Cicindela, which waylay
their prey, being less active than the hunting larvæ of
the Carabidæ. The Hydradephaga, or water-beetles
(Dyticidæ and Gyrinidæ), have long and narrow larvæ
(Pl. IV., Fig. 6), with strong sickle-shaped jaws, short
antennæ, four palpi, and six small eyes on each side
of the head; they are very voracious. The larvæ of
the Staphylinidæ are by no means unlike the perfect
insect, and are found in similar situations; their jaws
are powerful, and their legs moderately strong. The
larvæ of the Lamellicorn beetles Figs. (1-6)—cockchafers,
stag-beetles, &c.—feed on vegetable substances
or on dead animal matter. They are long,
soft, fleshy grubs, with the abdomen somewhat curved,
and generally lie on their side. The larvæ of the
Elateridæ, known as wireworms, are long and slender,
with short legs. That of the glowworm (Lampyridæ)
is not unlike the apterous female. The male glowworm,
on the contrary, is very different. It has long,
thin, brown wing-cases, and often flies into rooms at
night, attracted by the light, which it probably mistakes
for that of its mate.

The metamorphoses of the Cantharidæ are very
remarkable, and will be described subsequently.
The larvæ are active and hexapod. The Phytophaga
(Crioceris, Galeruca, Haltica, Chrysomela,
&c.) are vegetable feeders, both as larvæ and in
the perfect state. The larvæ are furnished with
legs, and are not unlike the caterpillars of certain
Lepidoptera.

The larva of Coccinella (the Ladybird) is somewhat
depressed, of an elongated ovate form, with a18
small head, and moderately strong legs. It feeds
on Aphides.

Thus, then, we see that there are among the Coleoptera
many different forms of larvæ. Macleay considered
that there were five principal types.

1. Carnivorous hexapod larvæ, with an elongated,
more or less flattened body, six eyes on each side of
the head, and sharp falciform mandibles (Carabus,
Dyticus, &c.).

2. Herbivorous hexapod larvæ, with fleshy, cylindrical
bodies, somewhat curved, so that they lie on
their side.

3. Apod grub-like larvæ, with scarcely the rudiments
of antennæ (Curculio).

4. Hexapod antenniferous larvæ, with a subovate
body, the second segment being somewhat larger
than the others (Chrysomela, Coccinella).

5. Hexapod antenniferous larvæ, of oblong form,
somewhat resembling the former, but with caudal
appendages (Meloë, Sitaris).

The pupa of the Coleoptera is quiescent, and “the
parts of the future beetle are plainly perceivable,
being encased in distinct sheaths; the head is applied
against the breast; the antennæ lie along the sides
of the thorax; the elytra and wings are short and
folded at the sides of the body, meeting on the under
side of the abdomen; the two anterior pairs of legs
are entirely exposed, but the hind pair are covered by
wing-cases, the extremity of the thigh only appearing
beyond the sides of the body.”9

In the next three orders—namely, the Orthoptera
(grasshoppers, locusts, crickets, walking-stick insects,
19cockroaches, &c.), Euplexoptera (earwigs), and Thysanoptera,
a small group of insects well known to
gardeners under the name of Thrips (Pl. I. and II.,
Figs. 1 and 2)—the larvæ when they quit the egg
already much resemble the mature form, differing, in
fact, principally in the absence of wings, which are
more or less gradually acquired, as the insect increases
in size. They are active throughout life.
Those specimens which have rudimentary wings are,
however, usually called pupæ.

The Neuroptera present, perhaps, more differences
in the character of their metamorphoses than any
other order of insects. Their larvæ are generally
active, hexapod little creatures, and do not vary
from one another in appearance so much, for instance,
as those of the Coleoptera, but their pupæ
differ essentially; some groups, namely, the Psocidæ,
Termitidæ, Libellulidæ, Ephemeridæ, and Perlidæ,
remaining active throughout life, like the Orthoptera;
while a second division, including the Myrmeleonidæ,
Hemerobiidæ, Sialidæ, Panorpidæ, Raphidiidæ, and
Mantispidæ, have quiescent pupæ, which, however, in
some cases, acquire more or less power of locomotion
shortly before they assume the mature state; thus
that of Raphidia, though motionless at first, at length
acquires strength enough to walk, even while still
enclosed in the pupa skin, which is very thin.10

One of the most remarkable families belonging
to this order is that of the Termites, or white ants.
They abound in the tropics, where they are a perfect
pest, and a serious impediment to human development.
Their colonies are extremely numerous, and
20they attack woodwork and furniture of all kinds,
generally working from within, so that their presence
is often unsuspected, until it is suddenly found that
they have completely eaten away the interior of
some post or table, leaving nothing but a thin outer
shell. Their nests, which are made of earth, are
sometimes ten or twelve feet high, and strong enough
to bear a man. One species, Termes lucifugus, is
found in the South of France, where it has been
carefully studied by Latreille. He found in these
communities five kinds of individuals—(1) males;
(2) females, which grow to a very large size, their
bodies being distended with eggs, of which they
sometimes lay as many as 80,000 in a day; (3) a
form described by some observers as Pupæ, but by
others as neuters. These differ very much from the
others, having a long, soft body without wings, but
with an immense head, and very large, strong jaws.
These individuals act as soldiers, doing apparently
no work, but keeping watch over the nest and attacking
intruders with great boldness. (4) Apterous,
eyeless individuals, somewhat resembling the winged
ones, but with a larger and more rounded head;
these constitute the greater part of the community,
and, like the workers of ants and bees, perform all the
labour, building the nest and collecting food. (5)
Latreille mentions another kind of individual which
he regards as the pupa, and which resembles the
workers, but has four white tubercles on the back,
where the wings afterwards make their appearance.
There is still, however, much difference of opinion
among entomologists, with reference to the true
nature of these different classes of individuals. M.21
Lespès, who has recently studied the same species,
describes a second kind of male and a second kind of
female, and the subject, indeed, is one which offers a
most promising field for future study.

Another interesting family of Neuroptera is that of
the Ephemeræ, or Mayflies (Pl. III., Fig. 1), so well
known to fishermen. The larvæ (Pl. IV., Fig. 1) are
semi-transparent, active, six-legged little creatures,
which live in water; having at first no gills, they respire
through the general surface of the body. They
grow rapidly and change their skin every few days.
After one or two moults they acquire seven pairs
of branchiæ, or gills, which are generally in the form
of leaves, one pair to the segment. When the larvæ
are about half grown, the posterior angles of the two
posterior thoracic segments begin to elongate. These
elongations become more and more marked with
every change of skin. One morning, in the month
of June, some years ago, I observed a full-grown
larva, which had a glistening appearance, owing to
the presence of a film of air under the skin. I put
it under the microscope, and, having added a drop
of water with a pipette, looked through the glass.
To my astonishment, the insect was gone, and an
empty skin only remained. I then caught a second
specimen, in a similar condition, and put it under
the microscope, hoping to see it come out. Nor
was I disappointed. Very few moments had elapsed,
when I had the satisfaction of seeing the thorax open
along the middle of the back; the two sides turned
over; the insect literally walked out of itself, unfolded
its wings, and in an instant flew up to the window.
Several times since, I have had the pleasure of22
witnessing this marvellous change, and it is really wonderful
how rapidly it takes place: from the moment
when the skin first cracks, not ten seconds are over
before the insect has flown away.

Another family of Neuroptera, the Dragon-flies,
or Horse-stingers, as they are sometimes called, from
a mistaken idea that they sting severely enough to
hurt a horse, though in fact they are quite harmless,
also spend their early days in the water. The larvæ
are brown, sluggish, ugly creatures, with six legs.
They feed on small water-animals, for which they
wait very patiently, either at the bottom of the
water, or on some aquatic plant. The lower jaws
are attached to a long folding rod; and when any
unwary little creature approaches too near the larva,
this apparatus is shot out with such velocity that the
prey which comes within its reach seldom escapes.
In their perfect condition, also, Dragon-flies feed on
other insects, and may often be seen hawking round
ponds. The so-called Ant-lions in many respects
resemble the Dragon-flies, but the habits of the
larvæ are very dissimilar. They do not live in the
water, but prefer dry places, where they bury themselves
in the loose sand, and seize with their long
jaws any small insect which may pass. The true Ant-lion
makes itself a round, shallow pit in loose ground
or sand, and buries itself at the bottom. Any inattentive
little insect which steps over the edge of this pit
immediately falls to the bottom, and is instantaneously
seized by the Ant-lion. Should the insect escape, and
attempt to climb up the side of the pit, the Ant-lion
is said to throw sand at it, knocking it down again.

One other family of Neuroptera which I must23
mention, is the Hemerobiidæ. The perfect insect is
a beautiful, lace-winged, very delicate, green creature,
something like a tender Dragon-fly, and with bright,
green, touching eyes. The female deposits her eggs
on leaves, not directly on the plant itself, but attached
to it by a long white slender footstalk. The larva
has six legs and powerful jaws, and makes itself very
useful in destroying the Hop-fly.

The insects forming the order Trichoptera are well
known in their larval condition, under the name of
caddis worms. These larvæ are not altogether unlike
caterpillars in form, but they live in water—which is
the case with very few lepidopterous larvæ—and form
for themselves cylindrical cases or tubes, built up of
sand, little stones, bits of stick, leaves, or even shells.
They generally feed on vegetable substances, but will
also attack minute freshwater animals. When full
grown, the larva fastens its case to a stone, the stem
of a plant, or some other fixed substance, and closes
the two ends with an open grating of silken threads,
so as to admit the free access of water, while excluding
enemies. It then turns into a pupa which bears some
resemblance to the perfect insect, “except that the
antennæ, palpi, wings, and legs are shorter, enclosed
in separate sheaths, and arranged upon the breast.”
The pupa remains quiet in the tube until nearly
ready to emerge, when it comes to the surface, and
in some cases creeps out of the water. It is not
therefore so completely motionless as the pupæ of
Lepidoptera.

The Diptera, or Flies, comprise insects with two
wings only, the hinder pair being represented by
minute club-shaped organs called “haltères.” Flies24
quit the egg generally in the form of fat, fleshy,
legless grubs. They feed principally on decaying
animal or vegetable matter, and are no doubt useful
as scavengers. Other species, as the gadflies, deposit
their eggs on the bodies of animals, within which the
grubs feed, when hatched. The mouth is generally
furnished with two hooks which serve instead of jaws.
The pupæ of Diptera are of two kinds. In the true
flies, the outer skin of the full-grown larva is not shed,
but contracts and hardens, thus assuming the appearance
of an oval brownish shell or case, within which
the insect changes into a chrysalis. The pupæ of the
gnats, on the contrary, have the limbs distinct and
enclosed in sheaths. They are generally inactive, but
some of the aquatic species continue to swim about.

One group of Flies, which is parasitic on horses,
sheep, bats, and other animals, has been called the
Pupipara, because it was supposed that they were
not born until they had arrived at the condition of
pupæ. They come into the world in the form of
smooth, ovate bodies, much resembling ordinary dipterous
pupæ, but as Leuckart has shown,11 they are
true, though abnormal, larvæ.

The next order, that of the Aphaniptera, is very
small in number, containing only the different species
of Flea. The larva is long, cylindrical, and legless;
the chrysalis is motionless, and the perfect insect is
too well known, at least, as regards its habits, to need
any description.

The Heteroptera, unlike the preceding orders of
insects, quit the egg in a form differing from that of
25the perfect insect principally in the absence of wings,
which are gradually acquired. In their metamorphoses
they resemble the Orthoptera, and are active
through life. The majority are dull in colour, though
some few are very beautiful. The species constituting
this group, though very numerous, are generally small,
and not so familiarly known to us as those of the
other large orders, with indeed one exception, the
well-known Bug. This is not, apparently, an indigenous
insect, but seems to have been introduced.
The word is indeed used by old writers, but either
as meaning a bugbear, or in a general sense, and not
with reference to this particular insect. In this country
it never acquires wings, but is stated to do so sometimes
in warmer climates. The Heteroptera cannot
exactly be said either to sting or bite. The jaws, of
which, as usual among insects, there are two pairs,
are like needles, which are driven into the flesh, and
the blood is then sucked up the lower lip, which has
the form of a tube. This peculiar structure of the
mouth prevails throughout the whole order; consequently
their nutriment consists almost entirely of
the juices of animals or plants. The Homoptera
agree with the Heteroptera in the structure of the
mouth, and in the metamorphoses. They differ principally
in the front wings, which in Homoptera are
membranous throughout, while in the Heteroptera,
the front part is thickened and leathery. As in the
Heteroptera, however, so also in the Homoptera,
some species do not acquire wings. The Cicada,
celebrated for its chirp, and the lanthorn fly, belong
to this group. So also does the so-called Cuckoo-spit,
so common in our gardens, which has the curious26
faculty of secreting round itself a quantity of frothy
fluid which serves to protect it from its enemies.
But the best known insects of this group are the
Aphides or Plant-lice; while the most useful belong
to the Coccidæ, or scale insects, from one species of
which we obtain the substance called lac, so extensively
used in the manufacture of sealing-wax and
varnish. Several species also have been used in
dyeing, especially the Cochineal insect of Mexico, a
species which lives on the cactus. The male Coccus
is a minute, active insect, with four large wings;
while the female, on the contrary, never acquires
wings, but is very sluggish, broad, more or less
flattened, and in fact, when full grown, looks like a
small brown, red, or white scale.

The larva of the order Lepidoptera are familiar to
us all, under the name of caterpillars. The insects
of this order in their larval condition are almost all
phytophagous, and are very uniform both in structure
and in habits. The body is long and cylindrical, consisting
of thirteen segments; the head is armed with
powerful jaws; the three following segments, the
future prothorax, mesothorax, and metathorax, each
bears a pair of simple articulated legs. Of the posterior
segments, five also bear false or pro-legs, which
are short, unjointed, and provided with a number of
hooklets. A caterpillar leads a dull and uneventful
life; it eats ravenously, and grows rapidly, casting its
skin several times during the process, which generally
lasts only a few weeks; though in some cases, as for
instance that of the goat-moth, it extends over a
period of two or three years, after which the larva
changes into a quiescent pupa or chrysalis.

27

CHAPTER II.

THE INFLUENCE OF EXTERNAL CONDITIONS
ON THE FORM AND STRUCTURE OF LARVÆ.

The facts recapitulated briefly in the preceding
chapter show, that the forms of insect larvæ depend
greatly on the group to which they belong. Thus
the same tree may harbour larvæ of Diptera,
Hymenoptera, Coleoptera, and Lepidoptera; each
presenting the form typical of the family to which
it belongs.

If, again, we take a group, such, for instance, as
the Lamellicorn beetles, we shall find larvæ extremely
similar in form, yet very different in habits. Those,
for instance, of the common cockchafer (Fig. 1) feed
on the roots of grass; those of Cetonia aurata (Fig. 2)
inhabit ants’ nests; the larvæ of the genus Trox
(Fig. 3) are found on dry animal substances; of
Oryctes (Fig. 4) in tan-pits; of Aphodius (Fig. 5) in
dung; of Lucanus (the stag-beetle, Fig. 6) in wood.

Fig. 1, Larva of the Cockchafer (Melolontha). (Westwood, Int. to the
Modern Classification of Insects, vol. i. p. 194.). 2, Larva of Cetonia.
3, Larva of Trox. 4, Larva of Oryctes. 5, Larva of Aphodius (Chapuis
and Candèze, Mém. Soc. Roy. Liège, 1853). 6, Larva of Lucanus.
(Packard, Guide to the Study of Insects, Fig. 403).

On the other hand, in the present chapter it will be
my object to show that the form of the larva depends
very much on the conditions of its life. Thus, those
larvæ which are internal parasites, whether in animals28
or plants, are vermiform, as are those which live in
cells, and depend on their parents for food. On the
other hand, larvæ which burrow in wood have strong
jaws and generally somewhat weak thoracic legs;
whilst those which feed on leaves have the thoracic
legs more developed, but less so than the carnivorous
species. Now, the Hymenoptera, as a general
rule, belong to the first category: the larvæ of the
Ichneumons, &c., which live in animals,—those of
the Cynipidæ, inhabiting galls,—and those of ants,
bees, wasps, &c., which are fed by their parents, are
fleshy, apodal grubs; though the remarkable fact
that the embryos of bees in one stage of their development
possess rudiments of thoracic legs which
subsequently disappear, seems to show, not indeed
that the larvæ of bees were ever hexapod, but that
bees are descended from ancestors which had hex29apod
larvæ, and that the present apod condition of
these larvæ is not original, but results from their
mode of life.

On the other hand, the larvæ of Sirex (Fig. 14)
being wood-burrowers, possess well-developed thoracic
legs. Again, the larvæ of the Tenthredinidæ,
which feed upon leaves, closely resemble the caterpillars
of Lepidoptera, even to the presence of
abdominal pro-legs.

Fig. 7, Larva of Brachytarsus (Ratzeburg, Forst. Insecten). 8, Larva of Crioceris
(Westwood, loc. cit.).

The larvæ of most Coleoptera (Beetles) are active,
hexapod, and more or less flattened: but those which
live inside vegetable tissues, such as the weevils, are
apod fleshy grubs, like those of Hymenoptera. Pl. II.,
Fig. 6, represents the larva of the nut-weevil, Balaninus
(Pl. I., Fig. 6), and it will be seen that it closely
resembles Pl. II., Fig. 5, which represents that of a fly
(Anthrax), Pl. I., Fig. 5, and Pl. II., Figs. 7, 8, and 9,
which represent respectively those of a Cynips or
gall-fly (Pl. I., Fig. 7), an ant (Pl. I., Fig. 8), and wasp
(Pl. I., Fig. 9). Nor is Balaninus the only genus of
Coleoptera which affords us examples of this fact.
Thus in the genus Scolytus (Pl. I., Fig. 4), the larvæ
(Pl. II., Fig. 4), which, as already mentioned, feed on
the bark of the elm, closely resemble those just described,
as also do those of Brachytarsus (Fig. 7). On
the other hand, the larvæ of certain beetles feed on30
leaves, like the caterpillars of Lepidoptera; thus that
of Crioceris Asparagi (Fig. 8)—which, as its name
denotes, feeds on the asparagus—closely resembles
the larvæ of certain Lepidoptera, as for instance of
Thecla spini. From this point of view the transformations
of the genus Sitaris (Pl. III., Fig. 4), which
have been very carefully investigated by M. Fabre,
are peculiarly interesting.12

Fig. 9, Larva of Sitaris humeralis (Fabre, Ann. des Sci. Nat., sér.
4, tome vii.). 10, Larva of Sitaris humeralis, in the second stage.
11, Larva of Sitaris humeralis, in the third stage. 12, Larva of Sitaris humeralis, in the fourth stage. 13, Pupa of Sitaris.

The genus Sitaris (a small beetle allied to Cantharis,
the blister-fly, and to Meloë, the oil-beetle) is
parasitic on a kind of Bee (Anthophora), which excavates
subterranean galleries, each leading to a cell.
The eggs of the Sitaris, which are deposited at the
entrance of these galleries, are hatched at the end of
September or beginning of October; and M. Fabre not
31unnaturally expected that the young larvæ, which are
active little creatures with six serviceable legs (Fig. 9),
would at once eat their way into the cells of the Anthophora.
No such thing: till the month of April
following they remain without leaving their birthplace,
and consequently without food; nor do they in this
long time change either in form or size. M. Fabre
ascertained this, not only by examining the burrows
of the Anthophoras, but also by direct observation of
some young larvæ kept in captivity. In April, however,
his captives at last awoke from their long lethargy,
and hurried anxiously about their prisons. Naturally
inferring that they were in search of food, M. Fabre
supposed that this would consist either of the larvæ
or pupæ of the Anthophora, or of the honey with
which it stores its cell. All three were tried without
success. The first two were neglected, and the larvæ,
when placed on the latter, either hurried away, or
perished in the attempt, being evidently unable to
deal with the sticky substance. M. Fabre was in
despair: “Jamais expérience,” he says, “n’a éprouvé
pareille déconfiture. Larves, nymphes, cellules, miel,
je vous ai tous offert; que voulez-vous donc, bestioles
maudites?” The first ray of light came to him from
our countryman, Newport, who ascertained that a
small parasite found by Léon Dufour on one of the
wild bees, and named by him Triungulinus, was, in
fact, the larva of Meloë;. The larvæ of Sitaris much
resembled Dufour’s Triungulinus; and acting on this
hint, M. Fabre examined many specimens of Anthophora,
and found on them at last the larvæ of his
Sitaris. The males of Anthophora emerge from the32
pupæ sooner than the females, and M. Fabre ascertained
that, as they come out of their galleries, the
little Sitaris larvæ fasten upon them. Not, however,
for long: instinct teaches them that they are not yet
in the straight path of development; and, watching
their opportunity, they pass from the male to the
female bee. Guided by these indications, M. Fabre
examined several cells of the Anthophora: in some,
the egg of the Anthophora floated by itself on the
surface of the honey; in others, on the egg, as on a
raft, sat the still more minute larva of the Sitaris.
The mystery was solved. At the moment when the
egg is laid the Sitaris larva springs upon it. Even
while the poor mother is carefully fastening up her
cell, her mortal enemy is beginning to devour her
offspring: for the egg of the Anthophora serves not
only as a raft, but as a repast. The honey which is
enough for either, would be too little for both; and
the Sitaris, therefore, at its first meal, relieves itself
from its only rival. After eight days the egg is consumed,
and on the empty shell the Sitaris undergoes
its first transformation, and makes its appearance
in a very different form, as shown in Fig. 10.

The honey which was fatal before is now necessary;
the activity which before was necessary is now useless;
consequently, with the change of skin, the
active, slim larva changes into a white, fleshy grub,
so organized as to float on the surface of the honey,
with the mouth beneath, and the spiracles above the
surface: “grâce à l’embonpoint du ventre,” says M.
Fabre, “la larve est à l’abri de l’asphyxie.” In this
state it remains until the honey is consumed; then the33
animal contracts, and detaches itself from its skin,
within which the further transformations take place.
In the next stage, which M. Fabre calls the pseudo-chrysalis
(Fig. 11), the larva has a solid corneous
envelope and an oval shape; and in its colour, consistency,
and immobility reminds one of a Dipterous
pupa. The time passed in this condition varies much.
When it has elapsed, the animal moults again, again
changes its form, and assumes that shown in Fig. 12;
after this it becomes a pupa (Fig. 13) without any
remarkable peculiarities. Finally, after these wonderful
changes and adventures, in the month of August the
perfect Sitaris (Pl. III., Fig. 4) makes its appearance.

On the other hand, there are cases in which larvæ
diverge remarkably from the ordinary type of the
group to which they belong, without, as it seems in
our present imperfect state of information, any sufficient
reason.

Thus the ordinary type of Hymenopterous larva, as
we have already seen, is a fleshy apod grub; although
those of the leaf-eating and wood-boring groups,
Tenthredinidæ and Siricidæ (Fig. 14), are caterpillars,
more or less closely resembling those of Lepidoptera.
There is, however, a group of minute Hymenoptera,
the larvæ of which reside within the eggs or larvæ
of other insects. It is difficult to understand why
these larvæ should differ from those of Ichneumons,
which are also parasitic Hymenoptera, and should be,
as will be seen by the accompanying figures, of such
remarkable and grotesque forms. The first known of
these curious larvæ was observed by De Filippi,13 who,
34having collected some of the transparent eggs of
a small Beetle (Rhynchites betuleti), to his great
surprise found more than half of them attacked by
a parasite, which proved to be the larva of a minute
Hymenopterous insect belonging to the Pteromalidæ.
Fig. 15 shows the egg of the Beetle, with the parasitic
larva, which is represented on a larger scale in
Fig. 16.

Fig. 14, Larva of Sirex (Westwood, loc. cit.). 15, Egg of Rhynchites, showing the
parasitic Larva in the interior. 16, the parasitic Larva more magnified.

More recently this group has been studied by M.
Ganin,14 who thus describes the development of Platygaster.
The egg, as in allied Hymenopterous families,
for instance in Cynips, is elongated and club-shaped
(Fig. 17). After a while a large nucleated cell appears
in the centre (Fig. 18). This nucleated cell
divides (Fig. 19) and subdivides. The outermost cells
continue the same process, thus forming an outer
investing layer. The central, on the contrary, enlarges
considerably, and develops within itself a
number of daughter cells (Figs. 20 and 21), which
gradually form a mulberry-like mass, thus giving
rise to the embryo (Fig. 22).

35

Fig. 17, Egg of Platygaster (after Ganin). 18, Egg of Platygaster
showing the central cell. 19, Egg of Platygaster after the division of
the central wall. 20, Egg of Platygaster more advanced. 21, Egg of
Platygaster more advanced. 22, Egg of Platygaster showing the rudiment
of the embryo.

Ganin met with the larvæ of Platygaster in those
of a small gnat, Cecidomyia. Sometimes as many as
fifteen parasites occurred in one gnat, but as a rule
only one of these attained maturity. The three
species of Platygaster differ considerably in form, as
shown in Figs. 23-25. They creep about within
the larva of Cecidomyia by means of the strong
hooked feet, kf, somewhat aided by movements
of the tail. They possess a mouth, stomach, and
muscles, but the nervous, vascular, and respiratory
systems do not make their appearance until later.
After some time the larva (Fig. 23) changes its
skin, assuming the form represented in Fig. 26. In
this moult the last abdominal segment of the first
larva is entirely thrown off: not merely the outer
skin, as in the case of the other segments, but also36
the hypodermis and the muscles. This larva, as will
be seen by the figure, resembles a barrel or egg in
form, and is .870 mm. in length, the external appendages
having disappeared, and the segments being37
indicated only by the arrangement of the muscles.
slkf is the œsophagus leading into a wide stomach
which occupies nearly the whole body, gsae is the
rudiment of the supra-œsophageal ganglia, bsm the
ventral nervous cords. The ventral nervous mass has
the form of a broad band, with straight sides; it
consists of embryonal cells, and remains in this undeveloped
condition during the whole larval state.

Fig. 23, Larva of Platygaster (after Ganin)—mo, mouth; a, antenna;
kf, hooked feet; z, toothed process; lfg, lateral process; f, branches
of the tail. 24, Larva of another species of Platygaster. The letters indicate
the same parts as in the preceding figure. 25, Larva of a third
species of Platygaster. The letters indicate the same parts as in the preceding
figures. 26, Larva of Platygaster in the second stage—mo, mouth;
slkf, œsophagus; gsae, supra-œsophageal ganglion; lm, muscles;
bsm, nervous system; ga, gh, rudiments of the reproductive glands.
27, Larva of Platygaster in the third stage—mo, mouth; md,
mandibles; gsae, supra-œsophageal ganglion; slk, œsophagus; ag,
ducts of the salivary glands; bnm, ventral nervous system; sp, salivary
glands; msl, stomach; im, imaginal discs; tr, tracheæ; fk, fatty
tissue; ed, intestine; ga, rudiments of reproductive organs; ew, wider
portion of intestine; ao, posterior opening.

At the next moult the larva enters its third state,
which, as far as the external form (Fig. 27) is concerned,
differs from the second only in being somewhat
more elongated. The internal organs, however,
are much more complex and complete. The tracheæ
have made their appearance, and the mouth is provided
with a pair of mandibles. From this point
the metamorphoses of Platygaster do not appear to
differ materially from those of other parasitic Hymenoptera.

An allied genus, Polynema, has also very curious
larvæ. The perfect insect is aquatic in its habits,
swimming by means of its wings; flying, if we may
say so, under water.15 It lays its eggs inside those
of Dragon-flies; and the embryo, as shown in
Fig. 28, has the form of a bottle-shaped mass of undifferentiated
embryonal cells, covered by a thin cuticle,
but without any trace of further organization. Protected
by the egg-shell of the Dragon-fly, and bathed
in the nourishing fluid of the Dragon-fly’s egg, the
young Polynema imbibes nourishment through its
whole surface, and increases rapidly in size. The
digestive canal gradually makes its appearance; the
38cellular mass forms a new skin beneath the original
cuticle, distinctly divided into segments, and provided
with certain appendages. After a while the old
cuticle is thrown off, and the larva gradually assumes
the form shown in Fig. 29. The subsequent metamorphoses
of Polynema offer no special peculiarities.

Fig. 28, Embryo of Polynema (after Ganin). 29, Larva of Polynema—asch,
rudiments of the antenna; flsch, rudiments of the wings; bsch,
rudiments of the legs; vfg. lateral projections; gsch, rudiments of
the ovipositor; fk, fatty tissue.

From these facts—and, if necessary, many more of
the same nature might have been brought forward—it
seems to me evident that while the form of any
given larva depends to a certain extent on the group
of insects to which it belongs, it is also greatly
influenced by the external conditions to which it is
subjected; that it is a function of the life which the
larva leads and of the group to which it belongs.

The larvæ of insects are generally regarded as being
nothing more than immature states—as stages in the39
development of the egg into the imago; and this
might more especially appear to be the case with
those insects in which the larvæ offer a general resemblance
in form and structure (excepting of course
so far as relates to the wings) to the perfect insect.
Nevertheless we see that this would be a very incomplete
view of the case. The larva and pupa undergo
changes which have no relation to the form which
the insect will ultimately assume. With a general
tendency to this goal, as regards size and the development
of the wings, there are coincident other
changes having reference only to existing wants
and condition. Nor is there in this, I think,
anything which need surprise us. External circumstances
act on the insect in its preparatory states,
as well as in its perfect condition. Those who
believe that animals are susceptible of great, though
gradual, change through the influence of external
conditions, whether acting, as Mr. Darwin has suggested,
through natural selection, or in any other
manner, will see no reason why these changes should
be confined to the mature animal. And it is evident
that creatures which, like the majority of insects,
live during the successive periods of their existence
in very different circumstances, may undergo considerable
changes in their larval organization, in
consequence of forces acting on them while in that
condition; not, indeed, without affecting, but certainly
without affecting to any corresponding extent, their
ultimate form.

I conclude, therefore, that the form of the larva
in insects, whenever it departs from the hexapod40
Campodea type, has been modified by the conditions
under which it lives. The external forces acting
upon it are different from those which affect the
mature form; and thus changes are produced in
the young which have reference to its immediate
wants, rather than to its final form.

And, lastly, as a consequence, that metamorphoses
may be divided into two kinds, developmental and
adaptional or adaptive.

41

CHAPTER III.

ON THE NATURE OF METAMORPHOSES.

In the preceding chapters we have considered the
life history of insects after they have quitted the
egg; but it is obvious that to treat the subject in
a satisfactory manner we must take the development
as a whole, from the commencement of the
changes in the egg, up to the maturity of the animal,
and not suffer ourselves to be confused by the fact
that insects leave the egg in very different stages
of embryonal development. For though all young
insects when they quit the egg are termed “larvæ,”
whatever their form may be (the case of the so-called
Pupipara not constituting a true exception), still it
must be remembered that some of these larvæ are
much more advanced than others. It is evident that
the larva of a fly, as regards its stage of development,
corresponds in reality neither with that of a
moth nor with that of a grasshopper. The maggots
of flies, in which the appendages of the head are rudimentary,
belong to a lower grade than the grubs of
bees, &c., which have antennæ, mandibles, maxillæ,42
labrum, labium, and, in fact, all the mouth parts of a
perfect insect.

The caterpillars of Lepidoptera are generally classed
with the vermiform larva of Diptera and Hymenoptera,
and contrasted with those of Orthoptera,
Hemiptera, &c.; but, in truth, the possession of thoracic
legs places them, together with the similar larvæ
of the Tenthredinidæ, on a decidedly higher level.
Thus, then, the period of growth (that in which the
animal eats and increases in size) occupies sometimes
one stage in the development of an insect, sometimes
another; sometimes, as for instance in the case of
Chloëon, it continues through more than one; or, in
other words, growth is accompanied by development.
But, in fact, the question is even more complicated
than this. It is not only that the larvæ of insects at
their birth offer the most various grades of development,
from the grub of a fly to the young of a grasshopper
or a cricket; but that, if we were to classify
larvæ according to their development, we should have
to deal, not with a simple case of gradations only, but
with a series of gradations, which would be different
according to the organ which we took as our test.

Apart, however, from the adaptive changes to which
special reference was made in the previous chapter,
the differences which larvæ present are those of gradation,
not of direction. The development of a grasshopper
does not pursue a different course from that
of a butterfly, but the embryo attains a higher state
before quitting the egg in the former than in the
latter: while in most Hymenoptera, as for instance
in Bees, Wasps, Ants, &c., the young are hatched43
without thoracic appendages; in the Orthoptera, on
the contrary, the legs are fully developed before the
young animal quits the egg.

Prof. Owen,16 indeed, goes so far as to say that the
Orthoptera and other Homomorphous insects are, “at
one stage of their development, apodal and acephalous
larvæ, like the maggot of the fly; but instead of quitting
the egg in this stage, they are quickly transformed
into another, in which the head and rudimental
thoracic feet are developed to the degree which characterizes
the hexapod larvæ of the Carabi and
Petalocera.”

I quite believe that this may have been true of
such larvæ at an early geological period, but the
fact now appears to be, so far at least as can be
judged from the observations yet recorded, that the
legs of those larvæ which leave the egg with these
appendages generally make their appearance before
the body-walls have closed, or the internal organs
44have approached to completion. Indeed, when the
legs first appear, they are merely short projections,
which it is not always easy to distinguish from the
segments themselves. It must, however, be admitted,
that the observations are neither so numerous, nor
in most cases so full, as could be wished.

Fig. 30, Egg of Phryganea (Mystacides)—A¹, mandibular segment; C¹
to C⁵, maxillary, labial, and three thoracic segments; D, abdomen
(after Zaddach). 31, Egg of Phryganea somewhat more advanced—b,
mandibles; c, maxillæ; cfs, rudiments of the three pairs of legs.

Fig. 30, represents an egg of a May-fly (Phryganea),
as represented by Zaddach in his excellent
memoir,17 just before the appearance of the appendages.
It will be seen that a great part of the
yolk is still undifferentiated, that the side walls are
incomplete, the back quite open, and the segments
merely indicated by undulations. This stage is
rapidly passed through, and Zaddach only once met
with an egg in this condition; in every other specimen
which had indications of segments, the rudiments
of the legs had also made their appearance,
as in Fig. 31, which, however, as will be seen, does
not in other respects show much advance on Fig. 30.

Again in Aphis, the embryology of which has been
so well worked out by Huxley,18 the case is very
similar, although the legs are somewhat later in
making their appearance. When the young was 1/140th
of an inch in length, he found the cephalic portion
of the embryo beginning, he says, “to extend upwards
again over the anterior face of the germ, so
as to constitute its anterior and a small part of its
superior wall. This portion is divided by a median
fissure into two lobes, which play an important part
45in the development of the head, and will be termed
the ‘procephalic lobes.’ I have already made use
of this term for the corresponding parts in the embryos
of Crustacea. The rudimentary thorax presents
traces of a division into three segments; and
the dorso-lateral margins of the cephalic blastoderm,
behind the procephalic lobes, have a sinuous margin.
It is in embryos between this and 1/100th of an inch in
length, that the rudiments of the appendages make
their appearance; and by the growth of the cephalic,
thoracic, and abdominal blastoderm, curious changes
are effected in the relative position of those regions.”

In Chrysopa oculata, one of the Hemerobiidæ,
Packard has described19 and figured a stage in which
the body segments have made their appearance, but
in which he says “there are no indications of limbs.
The primitive band is fully formed, the protozorites
being distinctly marked, the transverse impressed
lines indicating the primitive segments being distinct,
and the median furrow easily discerned.” Here
also, again, the dorsal walls are incomplete, and the
internal organs as yet unformed.

In certain Dragon-flies (Calepteryx), and Hemiptera
(Hydrometra), the legs, according to Brandt,20 appear
at a still earlier stage.

According to the observations of Kölliker,21 it
would appear that in the Coleopterous genus Donacia
the segments and appendages appear simultaneously.

46

Kölliker himself, however, frankly admits that “meæ
de hoc insecto observationes satis sunt manca,” and
it is possible that he may never have met with an
embryo in the state immediately preceding the appearance
of the legs; especially as it appears from
the observations of Kowalevski that in Hydrophilus
the appendages do not make their appearance until
after the segments.22

On the whole, as far as we can judge from the
observations as yet recorded, it seems that in Homomorphous
insects the ventral wall is developed and
divided into segments, before the appearance of the
legs; but that the latter are formed almost simultaneously
with the cephalic appendages, and before
either the dorsal walls of the body or the internal
organs.

Fig. 32.—Egg of Pholcus opilionides (after Claparède).

As it is interesting, from this point of view, to
compare the development of other Articulata with
that of insects, I give a figure (Fig. 32), representing
an early stage in the development of a spider
(Pholcus) after Claparède,23 who says, “C’est à ce
47moment qu’a lieu la formation des protozonites ou
segments primordiaux du corps de l’embryon. Le
rudiment ventral s’épaissit suivant six zônes disposées
transversalement entre le capuchon anal et le capuchon
céphalique.”

Fig. 33.—Embryo of Julus (after Newport).

Among Centipedes the development of Julus has
been described by Newport.24 The first period, from
the deposition of the egg to the gradual bursting of
the shell, and exposure of the embryo within it,
which, however, remains for some time longer in
connection with the shell, lasts for twenty-five days.
The segments of the body, originally six in number,
make their appearance on the twentieth day after
the deposition of the egg, at which time there
were no traces of legs. The larva, when it leaves
the egg, is a soft, white, legless grub (Fig. 33), consisting
of a head and seven segments, the head being
somewhat firmer in texture than the rest of the body.
It exhibits rudimentary antennæ, but the legs are
still only represented by very slight papilliform processes48
on the undersides of the segments to which
they belong.

As already mentioned, it is possible that at one time
the vermiform state of the Homomorphous insects—which,
as we have seen, is now so short, and passed
through at so early a stage of development—was
more important, more prolonged, and accompanied by
a more complete condition of the internal organs.
The compression, and even disappearance of those embryonal
stages which are no longer adapted to the
mode of life—which do not benefit the animal—is
a phenomenon not without a parallel in other parts
of the animal or even of the vegetable kingdom.
Just as in language long compound words have
a tendency to concision, and single letters sometimes
linger on, indicating the history of a word, like
the “l” in “alms,” or the “b” in “debt,” long
after they have ceased to influence the sound; so in
embryology useless stages, interesting as illustrations
of past history, but without direct advantage
under present conditions, are rapidly passed through,
and even, as it would appear, in some cases altogether
omitted.

Fig. 34.—Colony of Bougainvillea fruticosa, natural size, to the underside
of a piece of floating timber (after Allman).

For instance, among the Hydroida, in the great
majority of cases, the egg produces a body more
or less resembling the common Hydra of our ponds,
and known technically as the “trophosome,” which
develops into the well-known Medusæ or jelly-fishes.
The group, however, for which Prof. Allman has proposed
the term Monopsea,25 and of which the genus
49
Ægina may be taken as the type, is, as he says,
distinguished by the absence of a hydriform stage,
“the ovum becoming developed through direct
metamorphosis into a medusiform body, just as in
the other orders it is developed into a hydriform
body.” Fig. 34 represents, after Allman, a colony of
Bougainvillea fruticosa of the natural size. It is a
British species, which is found growing on buoys,
floating timber, &c., and, says Allman,26 “when in
health and vigour, offers a spectacle unsurpassed in
interest by any other species—every branchlet
crowned by its graceful hydranth and budding with
50
Medusæ in all stages of development (Fig. 35), some
still in the condition of minute buds, in which no
trace of the definite Medusa-form can yet be detected;
others, in which the outlines of the Medusa can be
distinctly traced within the transparent ectothèque (external
layer); others, again, just casting off this thin
outer pellicle, and others completely freed from it,51
struggling with convulsive efforts to break loose
from the colony, and finally launched forth in the
full enjoyment of their freedom into the surrounding
water. I know of no form in which so many
of the characteristic features of a typical hydroid
are more finely expressed than in this beautiful
species.”

Fig. 35.—Portion of colony of Bougainvillea fruticosa, more magnified.

Fig. 36.—The Medusa form of the same species.

Fig. 36 represents the Medusa form of this species,
and the development thus described may be regarded
as typical of the Hydroida; yet, as already
mentioned, the Æginidæ do not present us with
any stage corresponding to the fixed condition of
Bougainvillea, but, on the contrary, are developed
into Medusæ direct from the egg.

On the other hand, there are groups in which52
the Medusiform stage becomes less and less important.

Fig. 37, Larva of Prawn, Nauplius stage (after F. Müller). 38, Larva of Prawn,
more advanced, Zoëa stage.

The great majority of the higher Crustacea go
through well-marked metamorphoses. Figs. 37 and
38 represent two stages in the development of the
prawn. In the first (Fig. 37), representing the young
animal as it quits the egg, the body is more or less
oval and unsegmented; there is a median frontal
eye, and three pairs of natatory feet, the first pair
simple, while the two posterior are two-branched.
Very similar larvæ occur in various other groups of
Crustacea. They were at first regarded as mature53
forms, and O. F. Müller gave them the name of Nauplius.
So also, the second or Zoëa form (Fig. 38) was
at first supposed to be a mature animal, until its true
nature was discovered by Vaughan Thompson.

The Zoëa form of larva differs from the perfect
prawn or crab in the absence of the middle portion of
the body and its appendages. The mandibles have
no palpi, the maxillipeds or foot-jaws are used as
feet, whereas in the mature form they serve as jaws.
Branchiæ are either wanting or rudimentary, respiration
being principally effected through the walls of
the carapace. The abdomen and tail are destitute of
articulate appendages. The development of Zoëa
into the perfect animal has been well described by
Mr. Spence Bate27 in the case of the common crab
(Carcinus mænas).

All crabs, as far as we know, with the exception of
a species of land crab (Gegarcinus), described by
Westwood, pass through a stage more or less resembling
that shown in Fig. 38. On the other hand,
the great group of Edriopthalma, comprising Amphipoda
(shore-hoppers, &c.) and Isopoda (wood-lice, &c.)
pass through no such metamorphosis; the development
is direct, as in the Orthoptera. It is true that
one species, Tanais Dulongii, though a typical Isopod
in form and general character, is said to retain in some
points, and especially in the mode of respiration,
some peculiarities of the Zoëa type; but this is quite
an exceptional case. In Mysis, says F. Müller,28
“there is still a trace of the Nauplius stage; being
54transferred back to a period when it had not to
provide for itself, the Nauplius has become degraded
into a mere skin; in Ligia this larva-skin has lost
the traces of limbs, and in Philoscia it is scarcely
demonstrable.”

The Echinodermata in most cases “go through a
very well-marked metamorphosis, which often has
more than one larval stage…. The mass of more
or less differentiated sarcode, of which the larva, or
pseud-embryo, as opposed to the Echinoderm within
it, is made up, always carries upon its exterior certain
bilaterally-arranged ciliated bands, by the action of
which the whole organism is moved from place to
place; and it may be strengthened by the super-addition
to it of a framework of calcareous rods.”29
Müller considered that the mouth and pharynx of
the larva were either absorbed or cast off with the
calcareous rods, but were never converted into the
corresponding organs of the perfect Echinoderm.
According to A. Agassiz, however, this is not the
case, but on the contrary “the whole larva and all
its appendages are gradually drawn into the body,
and appropriated.”30

Fig. 39.—Larva of Echino-cidaris, seen from above ✕ 6/10 (after Müller).

Fig. 39 represents the larva of a sea-egg (Echino-cidaris)
after Müller.31 The body is transparent,
shaped somewhat like a double easel, but with two
long horns in front, which, as well as the posterior
55processes, are supported by calcareous rods. This
larva swims by means of minute vibratile hairs, or
ciliæ. It has a mouth, stomach, and in fact a well-defined
alimentary canal; but no nerves or other internal
organs have yet been discovered in it. After
swimming about in this condition for a while, it
begins to show signs of change. An involution of the
integument takes place on one side of the back, and
continues to deepen till it reaches a mass or store of
what is called blastema, or the raw material of the
animal body. This blastema then begins to change,
and gradually assumes the form of the perfect
Echinoderm.32

56

Fig. 40, Larva of Echinus, ✕ 100. A, front arm; F, arms of the mouth
process; B, posterior side arm; E₁, accessory arm of the mouth process;
a, mouth; a´, œsophagus; b, stomach; b´, intestine; o, posterior
orifice; d, ciliated bands; f, ciliated epaulets; c, disc of future Echinus
(after Müller).

Fig. 40 represents a larva, probably of another sea-egg
(Echinus lividus), from the Mediterranean, and
shows the commencement of the sea-egg within the
body of the larva. The capital letters denote the
different arms: a is the mouth, a´ the œsophagus, b
the stomach, b´ the intestine, f the ciliated lobes or
epaulets, c the young sea-egg.

Fig. 41.—Comatula rosacea (after Forbes).

The development of the beautiful Comatula rosacea
(Fig. 41) has been described in the “Philosophical
Transactions,” by Prof. Wyville Thomson and Dr.
Carpenter.33 The larva quits the egg, as shown in
Fig. 42, in the form of an oval body about 1/30 inch
57in length, something like a barrel, surrounded by
four bands orops of long vibratile hairs or ciliæ.
There is also a tuft of still longer hairs at the
narrower posterior end of the body. Gradually a
number of minute calcareous spines and plates make
their appearance (Fig. 43) in the body of this larva,
and at length arrange themselves in a definite order,
so as to form a bent calcareous club or rod with an
enlarged head.

58

Fig. 42, Larva of Comatula rosacea (after Thomson). 43, Larva of Comatula
rosacea, more advanced. 44, Larva of Comatula rosacea, in the
Pentacrinus state.

59

As this process continues, the little creature gradually
loses its power of swimming, and, sinking to the
bottom, looses the bands of ciliæ, and attaches itself
by its base to some stone or other solid substance,
the knob of the club being free. The calcareous
framework increases in size, and the expanded head
forms itself into a cup, round which from five to
fifteen delicate tentacles, as shown in Fig. 44, make
their appearance.

In this stage the young animal resembles one of
the stalked Crinoids, a family of Echinoderms very
abundant in earlier geological periods, but which
has almost disappeared, being, as we see, now represented
by the young states of existing more advanced,
free, species. This attached, plant-like condition
of Comatula was indeed at first supposed
to be a mature form, and was named Pentacrinus;
but we now know that it is only a stage in the development
of Comatula. The so-called Pentacrinus
increases considerably in size, and after various gradual
changes, which time does not now permit me
to describe, quits the stalk, and becomes a free
Comatula.

The metamorphoses of the Starfishes are also very
remarkable. Sars discovered, in the year 1835, a
curious little creature about an inch in length, which
he named Bipinnaria asterigera (Figs. 45-47), and
which he then supposed to be allied to the ciliograde
Medusæ. Subsequent observations, however, made
in 1844, suggested to him that it was the larva of a
Starfish, and in 1847 MM. Koren and Danielssen
satisfied themselves that this was the case.

Figs. 45 and 46 represent the front and side view60
of a Bipinnaria found by Müller34 near Marseilles.
a is the mouth, b the œsophagus, c the stomach, c´
the intestine. Fig. 47 represents a somewhat older
specimen, in which the Starfish (k) is already beginning
to make its appearance.

Fig. 45, Larva of Starfish (Bipinnaria), ✕ 100 (after Müller). 46, Larva of
Starfish (Bipinnaria), ✕ 100, seen from the side—a, mouth; b, œsophagus;
c, stomach; c´, intestine. 47, Larva of another Bipinnaria,
showing the commencement of the Starfish—g, canal of the ciliated sac;
i, rudiments of tentacles; d, ciliated band.

But while certain Starfishes thus go through metamorphoses
similar in character, and not less remarkable
than those of sea-eggs, there are others—as, for
instance, the genus Asteracanthion—in which development
may be said to be direct—the organs and
appendages special to the Pseudembryo being in
abeyance; while in another genus, Pteraster, they
are reduced to a mere investing membrane.35

61

Among the Ophiurans also we find two well-marked
types of development. Some passing through metamorphoses,
while others, as for instance Ophiopholis
bellis, “is developed very much after the method of
Asteracanthion Mülleri, without passing through the
Plutean stage.”36

Even in the same species of Echinoderm the degree
of development attained by the larva differs to a
certain extent according to the temperature, the
supply of food, &c. Thus in Comatula, specimens
which are liberally supplied with sea-water, and kept
warm, hurry as it were through their early stages, and
the free larva becomes distorted by the growing
Pentacrinus (see Fig. 43), almost before it has attained
its perfect form. On the other hand, under less
favourable conditions, if the temperature is low and
food less abundant, the early stages are prolonged,
the larva is longer lived, and reaches a much higher
degree of independent development. Similar differences
occur in the development of other animals,
as for instance, in the Hydroids,37 and among the insects
themselves, in Flies;38 and it is obvious that these
facts throw much light on the nature and origin of
the metamorphoses of insects, which subject we
shall now proceed to consider.

62

CHAPTER IV.

ON THE ORIGIN OF METAMORPHOSES.

The question still remains, Why do insects pass
through metamorphoses? Messrs. Kirby and Spence
tell us they “can only answer that such is the will of
the Creator;”39 this, however, is a general confession
of faith, not an explanation of metamorphoses.
So indeed they themselves appear to have felt; for
they immediately proceed to make a suggestion.
“Yet one reason,” they say, “for this conformation
may be hazarded. A very important part assigned
to insects in the economy of nature, as we shall hereafter
show, is that of speedily removing superabundant
and decaying animal and vegetable matter. For such
agents an insatiable voracity is an indispensable
qualification, and not less so unusual powers of multiplication.
But these faculties are in a great degree
incompatible; an insect occupied in the work of
reproduction could not continue its voracious feeding.
Its life, therefore, after leaving the egg, is divided into
three stages.”

But there are some insects—as, for instance, the
63Aphides—which certainly are not among the least
voracious, and which grow and breed at the same
time. There are also many scavengers among
other groups of animals—such, for instance, as the
dog, the pig, and the vulture—which undergo no
metamorphosis.

It is certainly true that, as a general rule, growth
and reproduction do not occur together; and it follows,
almost as a necessary consequence, that in such cases
the first must precede the second. But this has no
immediate connection with the occurrence of metamorphoses.
The question is not, why an insect does
not generally begin to breed until it has ceased to
grow, but why, in attaining to its perfect form, it
passes through such remarkable changes; why these
changes are so sudden and apparently violent; and
why they are so often closed by a state of immobility—that
of the chrysalis or pupa; for undoubtedly
the quiescent and death-like condition of the pupa is
one of the most remarkable phenomena of insect-metamorphoses.

In the first place, it must be observed that many
animals which differ considerably in their mature state,
resemble one another more nearly when young. Thus
birds of the same genus, or of closely allied genera,
which, when mature, differ much in colour, are often
very similarly coloured when young. The young of
the lion and the puma are often striped, and the fœtal
Black whale has teeth, like its ally the Sperm whale.

In fact, the great majority of animals do go through
well-marked metamorphoses, though in many cases
they are passed through within the egg, and thus do64
not come within the popular ken. “La larve,” says,
Quatrefages, “n’est qu’un embryon à vie indépendante.”40
Those naturalists who accept in any form
the theory of evolution, consider that “the embryonal
state of each species reproduces more or less completely
the form and structure of its less modified progenitors.”41
“Each organism,” says Herbert Spencer,42
“exhibits within a short space of time a series of
changes which, when supposed to occupy a period
indefinitely great, and to go on in various ways
instead of one way, give us a tolerably clear conception
of organic evolution in general.”

The naturalists of the older school do not, as
Darwin and Fritz Müller have already pointed out,
dispute these facts, though they explain them in a different
manner—generally by the existence of a supposed
tendency to diverge from an original type.
Thus Johannes Müller says, “The idea of development
is not that of mere increase of size, but that of
progress from what is not yet distinguished, but which
potentially contains the distinction in itself, to the
actually distinct. It is clear that the less an organ is
developed, so much the more does it approach the
type, and that during its development it acquires
more and more peculiarities. The types discovered
by comparative anatomy and developmental history
must therefore agree.” And again, “What is true in
this idea is, that every embryo at first bears only the
65type of its section, from which the type of the class,
order, &c., is only afterwards developed.” Agassiz
also observes that “the embryos of different animals
resemble each other the more the younger they are.”

There are, no doubt, cases in which the earlier
states are rapidly passed through, or but obscurely
indicated; yet we may almost state it as a general
proposition, that either before or after birth animals
undergo metamorphoses. The state of development
of the young animal at birth varies immensely.
The kangaroo (Macropus major), which attains
a height of seven feet ten inches, does not when
born exceed one inch and two lines in length;
the chick leaves the egg in a much more advanced
condition than the thrush; and so, among insects,
the young cricket is much more highly developed,
when it leaves the egg, than the larva of the fly
or of the bee; and, as I have already mentioned,
differences occur even within the limit of one species,
though not of course to anything like the same
extent.

In oviparous animals the condition of the young
at birth depends much on the size of the egg: where
the egg is large, the abundant supply of nourishment
enables the embryo to attain a high stage of
development; where the egg is small, and the yolk
consequently scanty, the embryo requires an additional
supply of food before it can do so. In the
former case the embryo is more likely to survive; but
when the eggs are large, they cannot be numerous,
and a multiplicity of germs may be therefore in some
circumstances a great advantage. Even in the same66
species the development of the egg presents certain
differences.43

The metamorphoses of insects depend then primarily
on the fact that the young quit the egg at
a more or less early stage of development; and
that consequently the external forces, acting upon
them in this state, are very different from those by
which they are affected when they arrive at maturity.

Hence it follows that, while in many instances
mature forms, differing greatly from one another,
arise from very similar larvæ, in other cases, as we
have seen, among some the parasitic Hymenoptera,
insects agreeing closely with one another, are produced
from larvæ which are very unlike. The same
phenomenon occurs in other groups. Thus, while in
many cases very dissimilar jelly-fishes arise from
almost identical Hydroids, we have also the reverse
of the proposition in the fact that in some species,
Hydroids of an entirely distinct character produce
very similar Medusæ.44

We may now pass to the second part of our subject:
the apparent suddenness and abruptness of
the changes which insects undergo during metamorphosis.
But before doing so I must repeat that these
changes are not always, even apparently, sudden and
great. The development of an Orthopterous insect,
say a grasshopper, from its leaving the egg to
maturity, is so gradual that the ordinary nomen67clature
of entomological works (larva state and pupa
state) does not apply to it; and even in the case
of Lepidoptera, the change from the caterpillar to
the chrysalis and from this to the butterfly is in
reality less rapid than might at first sight be supposed;
the internal organs are metamorphosed very
gradually, and even the sudden and striking change
in external form is very deceptive, consisting merely
of a throwing off of the outer skin—the drawing
aside, as it were of a curtain and the revelation of
a form which, far from being new, has been in preparation
for days; sometimes even for months.

Swammerdam, indeed, supposed (and his view was
adopted by Kirby and Spence) that the larva contained
within itself “the germ of the future butterfly,
enclosed in what will be the case of the pupa, which
is itself included in three or more skins, one over the
other, that will successively cover the larva.” This
was a mistake; but it is true that, if a larva be examined
shortly before it is full grown, the future
pupa may be traced within it. In the same manner,
if we examine a pupa which is about to disclose
the butterfly, we find the future insect, soft indeed
and imperfect, but still easily recognizable, lying
more or less loosely within the pupa-skin.

One important difference between an insect and a
vertebrate animal is, that whereas in the latter—as, for
instance, in ourselves—the muscles are attached to an
internal bony skeleton, in insects no such skeleton
exists. They have no bones, and their muscles are
attached to the skin; whence the necessity for the
hard and horny dermal investment of insects, so68
different from the softness and suppleness of our
own skin. The chitine, or horny substance, of which
the outside of an insect consists, is formed by a layer
of cells lying beneath it, and, once secreted, cannot be
altered. From this the result is, that without a change
of skin, a change of form is impossible. In some
cases, as for instance in Chloëon, each change of skin
is accompanied by a change of form, and thus the
perfect insect is gradually evolved. In others, as
in caterpillars, several changes of skin take place
without any material alteration of form, and the
change, instead of being spread over many, is confined
to the last two moults.

One explanation of this difference between the
larvæ which change their form with every change of
skin, and those which do not, is, I believe, to be found
in the structure of the mouth. That of the caterpillar
is provided with a pair of strong jaws, fitted
to eat leaves; and the digestive organs are adapted
for this kind of food. On the contrary, the mouth of
the butterfly is suctorial; it has a long proboscis,
beautifully adapted to suck the nectar from flowers,
but which would be quite useless, and indeed only
an embarrassment to the larva. The digestive organs
also of the butterfly are adapted for the assimilation,
not of leaves, but of honey. Now it is evident that
if the mouth-parts of the larva were slowly metamorphosed
into those of the perfect insect, through
a number of small changes, the insect would in the
meantime be unable to feed, and liable to perish of
starvation in the midst of plenty. In the Orthoptera,
and among those insects in which the changes are69
gradual, the mouth of the so-called larva resembles
that of the perfect insect, and the principal difference
consists in the presence of wings.

Similar considerations throw much light on the
nature of the chrysalis or pupa state—that remarkable
period of death-like quiescence which is one of
the most striking characteristics of insect metamorphosis.
The quiescence of the pupa is mainly owing
to the rapidity of the changes going on in it. In
that of a butterfly, not only (as has been already
mentioned) are the mouth and the digestive organs
undergoing change, but the muscles are in a similar
state of transition. The powerful ones which move
the wings are in process of formation; and even the
nervous system, by which the movements are set
on foot and regulated, is in a state of rapid change.45

It must not be forgotten that all insects are inactive
for a longer or shorter space of time after
each moult. The slighter the change, as a general
rule, the shorter is the period of inaction. Thus,
after the ordinary moult of a caterpillar, the insect
only requires a short rest until the new skin is
hardened. When, however, the change is great, the
period of inaction is correspondingly prolonged.
Most pupæ indeed have some slight powers of
motion; those which assume the chrysalis state in
wood or beneath the ground usually come to the surface
when about to assume the perfect state, and the
aquatic pupæ of certain Diptera swim about with
much activity. Among the Neuroptera, certain families
have pupæ as quiescent as those of the Lepidoptera:
70others—as, for instance, Raphidia—are quiescent at
first, but at length acquire sufficient strength to walk,
though still enclosed within the pupa-skin: a power
dependent partly on the fact that this skin is very
thin. Others again—as, for instance, dragon-flies—are
not quiescent on assuming the so-called pupa state
for any longer time than at their other changes of
skin. The inactivity of the pupa is therefore not
a new condition peculiar to this stage, but a prolongation
of the inaction which has accompanied
every previous change of skin.

Nevertheless the metamorphoses of insects have
always seemed to me one of the greatest difficulties
of the Darwinian theory. In most cases, the development
of the individual reproduces to a certain extent
that of the race; but the motionless, imbecile pupa
cannot represent a mature form. No one, so far as
I know, has yet attempted to explain, in accordance
with Mr. Darwin’s views, a life-history in which the
mouth is first mandibulate and then suctorial, as, for
example, in a butterfly. A clue to the difficulty may,
I think, be found in the distinction between developmental
and adaptive changes; to which I have
called attention in a previous chapter. The larva
of an insect is by no means a mere stage in the development
of the perfect animal. On the contrary,
it is subject to the influence of natural selection,
and undergoes changes which have reference entirely
to its own requirements and condition. It is evident,
then, that while the embryonic development
of an animal in the egg may be an epitome of its
specific history, this is by no means the case with71
species in which the immature forms have a separate
and independent existence. If an animal which,
when young, pursues one mode of life, and lives on
one kind of food, subsequently, either from its own
growth in size and strength, or from any change
of season, alters its habits or food, however slightly,
it immediately becomes subject to the action of new
forces: natural selection affects it in two different,
and, it may be, very distinct manners, gradually
tending to changes which may become so great as
to involve an intermediate period of change and
quiescence.

There are, however, peculiar difficulties in those
cases in which, as among the Lepidoptera, the same
species is mandibulate as a larva, and suctorial as an
imago. From this point of view Campodea and the
Collembola (Podura, &c.) are peculiarly interesting.
There are in insects three principal types of mouth:—

in which the mandibles and maxillæ are retracted,
but have some freedom of motion, and can be used
for biting and chewing soft substances. This type is,
in some respects, intermediate between the other two.
Assuming that certain representatives of such a type
were placed under conditions which made a suctorial
mouth advantageous, those individuals in which the
mandibles and maxillæ were best calculated to pierce
or prick would be favoured by natural selection, and
their power of lateral motion would tend to fall into72
abeyance; while, on the other hand, if masticatory
jaws were an advantage, the opposite process would
take place.

There is yet a third possibility—namely, that
during the first portion of life, the power of mastication
should be an advantage, and during the second
that of suction, or vice versâ. A certain kind of food
might abound at one season and fail at another;
might be suitable for the animal at one age and
not at another. Now in such cases we should have
two forces acting successively on each individual, and
tending to modify the organization of the mouth in
different directions. It cannot be denied that the
innumerable variations in the mouth-parts of insects
have special reference to their mode of life, and are of
some advantage to the species in which they occur.
Hence, no believer in natural selection can doubt
the possibility of the three cases above suggested,
the last of which seems to throw some light on the
possible origin of species which are mandibulate in
one period of life and not in another. Granting then
the transition from the one condition to the other, this
would no doubt take place contemporaneously with a
change of skin. At such times we know that, even
when there is no change in form, the softness of the
organs temporarily precludes the insect from feeding
for a time, as, for instance, in the case of caterpillars.
If, however, any considerable change were
involved, this period of fasting must be prolonged,
and would lead to the existence of a third condition,
that of the pupa, intermediate between the other two.
Since the acquisition of wings is a more conspicuous73
change than any relating to the mouth, we are apt
to associate with it the existence of a pupa-state:
but the case of the Orthoptera (grasshoppers, &c.) is
sufficient proof that the development of wings is perfectly
compatible with permanent activity; the necessity
for prolonged rest is in reality much more intimately
connected with the change in the constitution
of the mouth, although in many cases, no doubt, this
is accompanied by changes in the legs, and in the
internal organization. An originally mandibulate
mouth, however, like that of a beetle, could not, I
think, have been directly modified into a suctorial
organ like that of a butterfly or a gnat, because the
intermediate stages would necessarily be injurious.
Neither, on the other hand, for the same reasons,
could the mouth of the Hemiptera be modified into
a mandibulate type like that of the Coleoptera. But
in Campodea and the Collembola we have a type of
animal closely resembling certain larvæ which occur
both in the mandibulate and suctorial series of insects,
possessing a mouth neither distinctly mandibulate
nor distinctly suctorial, but constituted on a
peculiar type, capable of modification in either direction
by gradual change, without loss of utility.

In discussing this subject, it is necessary also to
take into consideration the nature and origin of wings.
Whence are they derived? why are there normally
two pairs? and why are they attached to the meso-and
meta-thorax? These questions are as difficult
as they are interesting. It has been suggested, and
I think with justice, that the wings of insects originally
served for aquatic and respiratory purposes.

74
In the larva of Chloëon (Pl. IV., Fig. 1), for instance,
which in other respects so singularly resembles
Campodea (Pl. III., Fig. 5), several of the segments
are provided with foliaceous expansions which serve
as respiratory organs. These so-called branchiæ are in
constant agitation, and the muscles which move them
in several points resemble those of true wings. It
is true that in Chloëon the vibration of the branchiæ
is scarcely, if at all, utilized for the purpose of locomotion;
the branchiæ are, in fact, placed too far back
to act efficiently. The situation of these branchiæ
differs in different groups; indeed, it seems probable
that originally there were a pair on each segment.
In such a case, those branchiæ situated near the
centre of the body, neither too much in front nor too
far back, would serve the most efficiently as propellers:
the same causes which determined the position of
the legs would also affect the wings. Thus a division
of labour would be effected; the branchiæ on the
thorax would be devoted to locomotion; those on
the abdomen to respiration. This would tend to
increase the development of the thoracic segments,
already somewhat enlarged, in order to receive the
muscles of the legs.

That wings may be of use to insects under water
is proved by the very interesting case of Polynema
natans,46 which uses its wings for swimming. This,
however, is a rare case, and it is possible that the
principal use of the wings was, primordially, to
enable the mature forms to pass from pond to pond,
thus securing fresh habitats and avoiding in-and-in
75breeding. If this were so, the development of wings
would gradually have been relegated to a late period
of life; and by the tendency to the inheritance of
characters at corresponding ages, which Mr. Darwin
has pointed out,47 the development of wings would
have thus become associated with the maturity of
the insect. Thus the late acquisition of wings in the
Insecta generally seems to be itself an indication of
their descent from a stock which was at one period,
if not originally, aquatic, and which probably resembled
the present larvæ of Chloëon in form, but
had thoracic as well as abdominal branchiæ.

Finally, from the subject of metamorphosis we
pass naturally to that most remarkable phenomenon
which is known as the “Alternation of Generations:”
for the first systematic view of which we are indebted
to my eminent friend Prof Steenstrup.48

I have always felt it very difficult to understand
why any species should have been created in this
double character; nor, so far as I am aware, has any
explanation of the fact yet been attempted. Nevertheless
insects offer, in their metamorphoses, a phenomenon
not altogether dissimilar, and give a clue to
the manner in which alternation of generations may
have originated.

The caterpillar owes its difference from the butterfly
to the undeveloped state in which it leaves the
egg; but its actual form is mainly due to the influence
of the conditions under which it lives. If the cater76pillar,
instead of changing into one butterfly, produced
several, we should have an instance of alternation of
generations. Until lately, however, we knew of no
such case among insects; each larva produced one
imago, and that not by generation, but by development.
It has long been known, indeed, that there
are species in which certain individuals remain always
apterous, while others acquire wings. Many entomologists,
however, regard these abnormal individuals as
perfect, though wingless insects; and therefore I shall
found no argument upon these cases, although they
appear to me deserving of more attention than they
have yet received.

Recently, however, Prof. Wagner49 has discovered
that, among certain small gnats, the larvæ do not
directly produce in all cases perfect insects, but give
birth to other larvæ, which undergo metamorphoses
of the usual character, and eventually become gnats.
His observations have been confirmed, as regards
this main fact, by other naturalists; and Grimm has
met with a species of Chironomus in which the pupæ
lay eggs.50

Here, then, we have a distinct case of alternation of
generations, as characterized by Steenstrup. Probably
other cases will be discovered in which insects undeniably
in the larval state will be found fertile. Nay,
it seems to me possible, if not probable, that some
larvæ which do not now breed may, in the course of
ages, acquire the power of doing so. If this idea is
correct, it shows how the remarkable phenomenon,
77known as alternation of generations, may have originated.

Summing up, then, the preceding argument, we find
among insects various modes of development; from
simple growth on the one hand, to well-marked
instances of the so-called alternation of generation on
the other. In the wingless species of Orthoptera
there is little external difference, excepting in size,
between the young larva and the perfect insect. The
growth is gradual, and there is nothing which would,
in ordinary language, be called a metamorphosis. In
the majority of Orthoptera, though the presence of
wings produces a marked difference between the
larva and the imago, the habits are nearly the same
throughout life, and consequently the action of external
circumstances affects the larva in the same
manner as it does the perfect insect.

This is not the case with the Neuroptera. The
larvæ do not live under the same conditions as the
perfect insects: external forces accordingly affect them
in a different manner; and we have seen that they
pass through some changes which bear no reference
to the form of the perfect insect: these changes, however,
are for the most part very gradual. The caterpillars
of Lepidoptera have even more extensive
modifications to undergo; the mouth of the larva,
for instance, being remarkably unlike that of the
perfect insect. A change in this organ, however,
could hardly take place while the insect was growing
fast, and consequently feeding voraciously; nor,
even if the change could be thus effected, would the
mouth, in its intermediate stages, be in any way fitted78
for biting and chewing leaves. The same reasoning
applies also to the digestive organs. Hence the
caterpillar undergoes little, if any, change, except
in size, and the metamorphosis is concentrated, so
to say, into the last two moults. The changes then
become so rapid and extensive, that the intermediate
period is necessarily one of quiescence. In
some exceptional cases, as in Sitaris (ante, p. 30) we
even find that, the conditions of life not being
uniform throughout the larval period, the larva itself
undergoes metamorphoses.

Owing to the fact that the organs connected with
the reproduction of the species come to maturity at a
late period, larvæ are generally incapable of breeding.
There are, however, some flies which have viviparous
larvæ, and thus offer a typical case of alternation of
generations.

Thus, then, we find among insects every gradation,
from simple growth to alternation of generations; and
see how, from the single fact of the very early period
of development at which certain animals quit the
egg, we can throw some light on their metamorphoses,
and for the still more remarkable phenomenon that,
among many of the lower animals, the species is
represented by two very different forms. We may
even conclude, from the same considerations, that this
phenomenon may in the course of ages become still
more common than it is at present. As long, however,
as the external organs arrive at their mature form
before the internal generative organs are fully developed,
we have metamorphosis; but if the reverse is
the case, then alternation of generations often results.79

The same considerations throw much light on the
remarkable circumstance, that in alternation of generations
the reproduction is, as a general rule, agamic
in one form. This results from the fact that reproduction
by distinct sexes requires the perfection both
of the external and internal organs; and if the phenomenon
arise, as has just been suggested, from the
fact that the internal organs arrive at maturity before
the external ones, reproduction will result in those
species only which have the power of agamic multiplication.

Moreover, it is evident that we have in the animal
kingdom two kinds of dimorphism.

This term has usually been applied to those cases
in which animals or plants present themselves at
maturity under two forms. Ants and Bees afford us
familiar instances among animals; and among plants
the interesting case of the genus Primula has recently
been described by Mr. Darwin. Even more recently
he has made known to us the still more remarkable
phenomenon afforded by the genus Lythrum,
in which there are three distinct forms, and which
therefore offers an instance of polymorphism.51

The other kind of dimorphism or polymorphism
differs from the first in being the result of the differentiating
action of external circumstances, not on the
mature, but on the young individual. Such different
forms, therefore, stand towards one another in the
relation of succession. In the first kind the chain
of being divides at the extremity; in the other it
80is composed of dissimilar links. Many instances of
this second form of dimorphism have been described
under the name of alternation of generations.

The term, however, has met with much opposition,
and is clearly inapplicable to the differences exhibited
by insects in various periods of their life. Strictly
speaking, the phenomena are frequently not alternate,
and in the opinion of some eminent naturalists they
are not, strictly speaking, cases of generation at all.52

In order, then, to have some name for these remarkable
phenomena, and to distinguish them from those
cases in which the mature animal or plant is represented
by two or more different forms, I think it would
be convenient to retain exclusively for these latter
the terms dimorphism and polymorphism; and those
cases in which animals or plants pass through a succession
of different forms might be distinguished by
the name of dieidism or polyeidism.

The conclusions, then, which I think we may draw
from the preceding considerations, are:—

1. That the occurrence of metamorphoses arises
from the immaturity of the condition in which some
animals quit the egg.

2. That the form of the insect larva depends in
great measure on the conditions in which it lives.
The external forces acting upon it are different
from those which affect the mature form; and thus
changes are produced in the young, having refer81ence
to its immediate wants, rather than to its final
form.

3. That metamorphoses may therefore be divided
into two kinds, developmental and adaptional or
adaptive.

4. That the apparent abruptness of the changes
which insects undergo, arises in great measure from
the hardness of their skin, which admits of no gradual
alteration of form, and which is itself necessary in
order to afford sufficient support to the muscles.

5. The immobility of the pupa or chrysalis depends
on the rapidity of the changes going on in it.

6. Although the majority of insects go through
three well-marked stages after leaving the egg, still a
large number arrive at maturity through a greater or
smaller number of slight changes.

7. When the external organs arrive at this final
form before the organs of reproduction are matured,
these changes are known as metamorphoses; when,
on the contrary, the organs of reproduction are
functionally perfect before the external organs, or
when the creature has the power of budding, then the
phenomenon is known as alternation of generations.

82

CHAPTER V.

ON THE ORIGIN OF INSECTS.

“Personne,” says Carl Vogt, “en Europe au moins,
n’ose plus soutenir la Création indépendante et de
toutes pièces des espèces,” and though this statement
is perhaps not strictly correct, still it is no
doubt true, that the Doctrine of Evolution, in some
form or other, is accepted by most, if not by all, the
greatest naturalists of Europe. Yet it is surprising
how much, in spite of all that has been written,
Mr. Darwin’s views are still misunderstood. Thus
Browning, in one of his recent poems, says:—

This theory, though it would be regarded by many as
a fair statement of his views, is one which Mr. Darwin
would entirely repudiate. Whether fish and insect,
reptile, bird and beast, are derived from one original
stock or not, they are certainly not links in one
83
sequence. I do not, however, propose to discuss the
question of Natural Selection, but may observe that
it is one thing to acknowledge that in Natural Selection,
or the survival of the fittest, Mr. Darwin has
called attention to a vera causa, has pointed out
the true explanation of certain phenomena; but
it is quite another thing to maintain that all animals
are descended from some primordial source.

For my own part, I am satisfied that Natural
Selection is a true cause, and, whatever may be the
final result of our present inquiries—whether animated
nature be derived from one ancestral source,
or from many—the publication of the Origin of
Species will none the less have constituted an epoch
in the History of Biology. But, how far the present
condition of living beings is due to that cause; how
far, on the other hand, the action of Natural Selection
has been modified and checked by other natural
laws—by the unalterability of types, by atavism, &c.;
how many types of life originally came into being;
and whether they arose simultaneously or successively,—these
and many other similar questions
remain unsolved, even admitting the theory of Natural
Selection. All this has indeed been clearly pointed
out by Mr. Darwin himself, and would not need repetition
but for the careless criticism by which in
too many cases the true question has been obscured.
Without, however, discussing the argument for and
against Mr. Darwin’s conclusions, so often do we
meet with travesties of it like that which I have
just quoted, that it is well worth while to consider
the stages through which some group, say for in84stance
that of insects, have probably come to be what
they are, assuming them to have developed under
natural laws from simpler organisms. The question
is one of great difficulty. It is hardly necessary to
say that insects cannot have passed through all the
lower forms of animal life, and naturalists do not at
present agree as to the actual line of their development.

In the case of insects, the gradual course of evolution
through which the present condition of the
group has probably been reached, has been discussed
by Mr. Darwin, by Fritz Müller, Haeckel, Brauer,
myself and others.

In other instances Palæontology throws much light
on this question. Leidy has shown that the milk-teeth
of the genus Equus resemble the permanent
teeth of the ancient Anchitherium, while the milk-teeth
of Anchitherium again approximate to the dental system
of the still earlier Merychippus. Rütimeyer, while
calling attention to this interesting observation, adds
that the milk-teeth of Equus caballus in the same way,
and still more those of E. fossilis, resemble the permanent
teeth of Hipparion.

“If we were not acquainted with the horse,” says
Flower,54 “we could scarcely conceive of an animal
whose only support was the tip of a single toe on
each extremity, to say nothing of the singular conformation
of its teeth and other organs. So striking
have these characters appeared to many zoologists,
that the animals possessing them have been reckoned
as an order apart, called Solidungula; but palæon85tology
has revealed that in the structure of its skull,
its teeth, its limbs, the horse is nothing more than a
modified Palæotherium; and though still with gaps
in certain places, many of the intermediate stages
of these modifications are already known to us, being
the Palæotherium, Anchitherium, Merychippus, and
Hipparion.”

“All Echinoids,” says A. Agassiz,55 “pass, in their
early stages, through a condition which recalls to us
the first Echinoids which made their appearance in
geological ages.” On embryological grounds, he
observes, we should “place true Echini lowest, then
the Clypeastroids, next the Echinolamps, and finally
the Spatangoids.” Now among the Echinoids of the
Trias there are no Clypeastroids, Echinolamps, or
Spatangoids. The Clypeastroids make their appearance
in the Lias, the Echinolamps in the Jurassic, while
the Spatangoids commence in the Cretaceous period.

Again56 “in the Radiates, the Acalephs in their
first stages of growth, that is, in their Hydroid condition,
remind us of the adult forms among Polyps,
showing the structural rank of the Acalephs to be
the highest, since they pass beyond a stage which
is permanent with the Polyps; while the Adult forms
of the Acalephs have in their turn a certain resemblance
to the embryonic phases of the class next
above them, the Echinoderms; within the limits
of the classes, the same correspondence exists as
between the different orders; the embryonic forms of
the highest Polyps recall the adult forms of the lower
86ones, and the same is true of the Acalephs as far as
these phenomena have been followed and compared
among them.” Indeed, the accomplished authors
from whom I have taken the above quotation, do
not hesitate to say57 that “whenever such comparisons
have been successfully carried out, the result is
always the same; the present representatives of the
fossil types recall in their embryonic condition the
ancient forms, and often explain their true position
in the animal kingdom.”

Fossil insects are unfortunately rare, there being
but few strata in which the remains of this group
are well preserved. Moreover, well-characterized
Orthoptera and Neuroptera occur as early as the
Devonian strata; Coleoptera and Hemiptera in the
Coal-measures; Hymenoptera and Diptera in the
Jurassic; Lepidoptera, on the contrary, not until the
Tertiary. But although it appears from these facts
that, as far as our present information goes, the
Orthoptera and Neuroptera are the most ancient
orders, it is not, I think, conceivable that the latter
should have been derived from any known species of
the former; on the other hand, the earliest known
Neuroptera and Orthoptera, though in some respects
less specialized than existing forms, are as truly, and
as well characterized, Insects, as any now existing;
nor are we acquainted with any earlier forms, which
in any way tend to bridge over the gap between them
and lower groups, though, as we shall see, there are
types yet existing which throw much light on the
subject.

87

In the consideration then of this question, we must
rely principally on Embryology and Development.
I have already referred to the cases in which species,
very unlike in their mature condition, are very similar
one to another when young. Haeckel, in his “Naturliche
Schöpfungsgeschichte,” gives a diagram which
illustrates this very well as regards Crustacea. Pls. 1-4
show the same to be the case with Insects.

The Stag-beetle, the Dragon-fly, the Moth, the
Bee, the Ant, the Gnat, the Grasshopper,—these and
other less familiar types seem at first to have little
in common. They differ in size, in form, in colour,
in habits, and modes of life. Yet the researches of
entomologists, following the clue supplied by the
illustrious Savigny, have proved, not only that while
differing greatly in details, they are constructed on
one common plan; but also that other groups, as
for instance, Crustacea (Lobsters, Crabs, &c.) and
Arachnida (Spiders and Mites), can be shown to be
fundamentally similar. In Pl. IV I have figured the
larvæ of an Ephemera (Fig. 1), of a Meloë (Fig. 2),
of a Dragon-fly (Fig. 3), of a Sitaris (Fig. 4), of a
Campodea (Fig. 5), of a Dyticus (Fig. 6), of a Termite
(Fig. 7), of a Stylops (Fig. 8), and of a Thrips (Fig. 9).
All these larvæ possess many characters in common.
The mature forms are represented in the corresponding
figures of Plate 3, and it will at once be seen
how considerably they differ from one another. The
same fact is also illustrated in Figs. 48-55, where
Figs. 48-51 represent the larval states of the mature
forms represented in Figs. 52-55. Fig. 48 is the
larva of a moth, Agrotis suffusa (Fig. 52); Fig. 49 of88
a beetle, Haltica (Fig. 53); Fig. 50 of a Saw-fly,
Cimbex (Fig. 54); and Fig. 51 of a Centipede, Julus
(Fig. 55).

Fig. 48, Larva of Moth (Agrotis suffusa), after Packard. 49, Larva of Beetle
(Haltica), after Westwood. 50, Larva of Sawfly (Cimbex), Brischke and
Zaddach. Beob. ub d. arten. der Blatt und Holzwespen, Fig. 8. 51, Larva of
Julus. Newport, Philos. Transactions, 1841.

Thus, then, although it can be demonstrated that
perfect insects, however much they differ in appearance,
are yet reducible to one type, the fact becomes much
more evident if we compare the larvæ. M. Brauer58
and I59 have pointed out that two types of larvæ,
which I have proposed to call Campodea-form and
Lindia-form, and which Packard has named Leptiform
and Eruciform, run through the principal groups of
insects. This is obviously a fact of great importance:
as all individual Meloës are derived from a form
resembling Pl. II, Fig. 2, it is surely no rash hypothesis
to suggest that the genus itself may have been so.

89

Fig. 52, Agrotis suffusa (after Packard). 53, Haltica (after Westwood).

Fig. 54, Cimbex, Brischae and Zaddach. l.c. T. 2, Fig. 9.

Fig. 55. Julus (after Gervais).

Firstly, however, let me say a word as to the
general Insect type. It may be described shortly as
consisting of animals possessing a head, with mouth
parts, eyes and antennæ; a many segmented body,
with three pairs of legs on the segments immediately
following the head; with, when mature, either one or
two pairs of wings, generally with caudal appendages
I will not now enter into a description of
their internal anatomy. It will be seen that, except90
as regards the wings, Pl. IV, Fig. 4, representing the
larva of a small beetle named Sitaris, answers very
well to this description. Many other Beetles are
developed from larvæ closely resembling those of
Meloë (Pl. IV, Fig. 2), and Sitaris (Pl. IV, Fig. 4); in
fact—except those species the larvæ of which, as, for
instance of the Weevils (Pl. II, Fig. 6), are internal
feeders, and do not require legs—we may say that
the Coleoptera generally are derived from larvæ of
this type.

I will now pass to a second order, the Neuroptera.
Pl. IV, Fig. 1, represents the larva of Chloëon, a
species the metamorphoses of which I described
some years ago in the Linnean Transactions,60 and
it is obvious that in essential points it closely resembles
the form to which I have just alluded.

The Orthoptera, again, the order to which Grasshoppers,
Crickets, Locusts, &c. belong, commence
life in a similar condition; and the same may also
be said of the Trichoptera.

The larvæ of Bees when they quit the egg are
entirely legless, but in an earlier stage they possess
well-marked rudiments of thoracic legs, showing, as
it seems to me, that their apodal condition is an
adaptation to their circumstances. Other Hymenopterous
larvæ, those for example of Sirex (Fig. 9),
and of the Saw-flies (Fig. 50) have well-developed
thoracic legs.

From the difference in external form, and especially
from the large comparative size of the abdomen,
these larvæ, as well as those of Lepidoptera (Fig. 48),
91have generally been classed with the maggots of Flies,
Weevils, &c., rather than with the more active form
of larva just adverted to. This seems to me, as I
have already pointed out,61 to be a mistake. The
caterpillar type differs, no doubt, in its general appearance,
owing to its greater clumsiness, but still
essentially agrees with that already described.

No Dipterous larva, so far as I know, belongs truly
to this type; in fact, the early stages of the pupa
in the Diptera seem in some respects to correspond
to the larvæ of other Insect orders. The Development
of the Diptera is, however, as Weissman62 has
shown, very abnormal in other respects.

Thus, then, we find in many of the principal
groups of insects that, greatly as they differ from
one another in their mature condition, when they
leave the egg they more nearly resemble the typical
insect type; consisting of a head; a three-segmented
thorax, with three pairs of legs; and a many-jointed
abdomen, often with anal appendages. Now,
is there any mature animal which answers to this
description? We need not have been surprised if
this type, through which it would appear that
insects must have passed so many ages since (for
winged Neuroptera have been found in the carboniferous
strata) had long ago become extinct. Yet it
is not so. The interesting genus Campodea (Pl. III,
Fig. 5) still lives; it inhabits damp earth, and
closely resembles the larva of Chloëon (Pl. II, Fig. 1),
constituting, indeed, a type which, as shown in Pl. 4,
92occurs in many orders of insects. It is true that the
mouth-parts of Campodea do not resemble either
the strongly mandibulate form which prevails among
the larvæ of Coleoptera, Orthoptera, Neuroptera,
Hymenoptera, Lepidoptera; or the suctorial type of
the Homoptera and Heteroptera. It is, however, not
the less interesting or significant on that account,
since, as I have elsewhere63 pointed out, its mouth-parts
are intermediate between the mandibulate
and haustellate types; a fact which seems to me
most suggestive.

It appears, then, that there are good grounds for
considering that the various types of insects are
descended from ancestors more or less resembling
the genus Campodea, with a body divided into head,
thorax, and abdomen: the head provided with
mouth-parts, eyes, and one pair of antennæ; the
thorax with three pairs of legs; and the abdomen, in
all probability, with caudal appendages.

If these views are correct, the genus Campodea
must be regarded as a form of remarkable interest
since it is the living representative of a primæval
type, from which not only the Collembola and Thysanura,
but the other great orders of insects have
derived their origin.

From what lower group the Campodea type was
itself derived is a question of great difficulty. Fritz
Müller indeed says,64 “if all the classes of Arthropoda
(Crustacea, Insecta, Myriopoda, and Arachnida) are
indeed all branches of a common stem (and of this
there can scarcely be a doubt), it is evident that
93the water-inhabiting and water-breathing Crustacea
must be regarded as the original stem from which
the other terrestrial classes, with their tracheal respiration,
have branched off.” Haeckel, moreover, is of
the opinion that the Tracheata are developed from
the Crustacea, and probably from the Zoëpoda.
For my own part, though I feel very great diffidence
in expressing an opinion at variance with that of
such high authorities, I am rather disposed to suggest
that the Campodea type may possibly have been
derived from a less highly developed one, resembling
the modern Tardigrade,65 a (Fig. 56) smaller and much
less highly organized being than Campodea. It possesses
two eyes, three anterior pairs of legs, and one
at the posterior end of the body, giving it a curious
resemblance to some Lepidopterous larvæ.

Fig. 56, Tardigrade (after Dujardin).

These legs, however, as will be seen, are reduced
to mere projections. But for them, the Tardigrada
94would closely resemble the vermiform larva so common
among insects. Among Trichoptera the larva early
acquires three pairs of legs, but as Zaddach has
shown,66 there is a stage, though it is quickly passed
through, in which the divisions of the body are indicated,
but no trace of legs is yet present. Indeed,
there appear to be reasons for considering that while
among Crustacea the appendages appear before the
segments, in Insects the segments precede the appendages,
although this stage of development is very
transitory, and apparently, in some cases, altogether
suppressed. I say “apparently,” because, as I have
already mentioned, I am not yet satisfied that it will
not eventually be found to be so in all cases.
Zaddach, in his careful observations of the embryology
of Phryganea, only once found a specimen
in this stage, which also, according to the researches
of Huxley,67 seems to be little more than indicated
in Aphis. It is therefore possible that in other cases,
when no such stage has been observed, it not really
may be absent, but, from its transitoriness, may have
hitherto escaped attention.

Fritz Müller has expressed the opinion68 that this
vermiform type is of comparatively recent origin. He
says: “The ancient insects approached more nearly to
the existing Orthoptera, and perhaps to the wingless
Blattidæ, than to any other order, and the complete
metamorphosis of the Beetles, Lepidoptera, &c., is of
later origin.” “There were,” he adds, “perfect insects
95before larvæ and pupæ.” This opinion has been
adopted by Mr. Packard69 in his “Embryological
Studies on Hexapodous Insects.”

M. Brauer70 also considers that the vermiform larva
is a more recent type than the Hexapod form, and is
to be regarded not as a developmental form, but as
an adaptational modification of the earlier active
hexapod type. In proof of this he quotes the case of
Sitaris.

Fig. 57, Larva of Cecidomyia (After Packard). 58, Lindia
torulosa (after Dujardin).

Considering, however, the peculiar habits of this
genus, to which I have already referred, and also that
the vermiform type is altogether lower in organization
and less differentiated than the Campodea form, I
cannot but regard this case as exceptional; one in
which the development has been, as it were, to use an
expression of Fritz Müller’s, “falsified” by the struggle
for existence, and which therefore does not truly indicate
the successive stages of evolution. On the
whole, the facts seem to me to point to the conclusion
that, though the grublike larvæ of Coleoptera
and some other insects, owe their present form mainly
to the influence of external circumstances, and partially
also to atavism, still the Campodea type is
itself derived from earlier vermiform ancestors.
Nicolas Wagner has shown in the case of a small
gnat, allied to Cecidomyia, that even now, in some
instances, the vermiform larvæ possess the power of
reproduction. Such a larva (as, for instance, Fig. 57)
very closely resembles some of the Rotatoria, such
for instance as Albertia or Notommata, which however
96
possess vibratile cilia. There is, indeed,
one genus—Lindia (Fig. 58)—in which these ciliæ
are altogether absent, and which, though resembling
Macrobiotus in many respects, differs from that genus
in being entirely destitute of legs. I have never
met with it myself, but it is described by Dujardin,
who found it in a ditch near Paris, as being oblong,
vermiform, divided into rings, and terminating posteriorly
in two short conical appendages. The jaws
are not unlike those of the larvæ of Flies, and indeed
many naturalists meeting with such a creature would,
I am sure, regard it as a small Dipterous larva; yet97
Dujardin figures a specimen containing an egg, and
seems to have no doubt that it is a mature form.71

For the next descending stage we must, I think,
look among the Infusoria, through such genera as
Chætonotus or Ichthydium. Other forms of the
Rotatoria, such for instance as Rattulus, and still
more the very remarkable species discovered in 1871
by Mr. Hudson,72 and described under the name of
Pedalion mira, seem to lead to the Crustacea through
the Nauplius form. Dr. Cobbold tells me that he
regards the Gordii as the lowest of the Scolecida;
Mr. E. Ray Lankester considers some of the Turbellaria,
such genera as Mesostomum, Vortex, &c., to be
the lowest of existing worms; excluding the parasitic
groups. Haeckel73 also regards the Turbellaria as
forming the nearest approach to the Infusoria. The
true worms seem, however, to constitute a separate
branch of the animal kingdom.

Fig. 59, Prorhynchus stagnaus.75

We may take, as an illustration of the lower worms,
the genus Prorhynchus (Fig. 59), which consists of
a hollow cylindrical body, containing a straight
simple tube, the digestive organ.

But however simple such a creature as this may be,
there are others which are far less complex, far less
differentiated; which therefore, on Mr. Darwin’s principles,
may be considered still more closely to repre98sent
the primæval ancestor from which these more
highly-developed types have been derived, and which,
in spite of their great antiquity—in spite of, or perhaps
in consequence of, their simplicity, still maintain
themselves almost unaltered.

Thus the form which Haeckel has described74 under
the name Protamœba primitiva, Pl. V, Fig. 1-5, consists
of a homogeneous and structureless substance,
which continually alters its form; putting out and
drawing in again more or less elongated processes,
and creeping about like a true Amœba, from which,
however, Protamœba differs, in the absence of a
nucleus. It seems difficult to imagine anything
simpler; indeed, as described, it appears to be an
illustration of properties without structure. It takes
into itself any suitable particle with which it comes in
contact, absorbs that which is nutritious, and rejects
the rest. From time to time a constriction appears at
the centre (Pl. V, Fig. 2), its form approximates more
and more to that of an hour-glass (Pl. V, Fig. 3), and
at length the two halves separate, and each commences
an independent existence (Pl. V, Fig. 5).

99

PLATE V.

Figs. 1-5, Protamœba 6-9, Protamyxa aurantiaca, Haeckel, Beit. zur Monog.
der Moneren, pl. 1; 10-18, Magosphœra planula, Haeckel, loc. cit. pl. 5.]100

In the true Amœbas, on the contrary, we find a
differentiation between the exterior and the interior:
the body being more or less distinctly divisible into an
outer layer and an inner parenchyme. In the Amœbas,
as in Protamœba, multiplication takes place by self-division,
and nothing corresponding to sexual reproduction
has yet been discovered.

Somewhat more advanced, but still of great simplicity,
is the Protomyxa aurantiaca (Pl. V, Fig. 8), discovered
by Haeckel76 on dead shells of Spirula, where
it appears as a minute orange speck, which shows well
against the clear white of the Spirula. Examined
with a microscope, the speck is seen to be a spherical
mass of orange-coloured, homogeneous, albuminous
matter, surrounded by a delicate, structureless membrane.
It is obvious from this description that
these bodies closely resemble eggs, for which indeed
Haeckel at first mistook them. Gradually, however,
the yellow sphere broke itself up into smaller
spherules (Pl. V, Fig. 9), after which the containing
membrane burst, and the separate spherules, losing
their globular form, crept out as small Amœbæ (Pl. V,
Fig. 6), or amœboid bodies. These little bodies moved
about, assimilated the minute particles of organic
matter, with which they came in contact, and gradually
increased in size (Pl. V, Fig. 7) with more or less
rapidity according to the amount of nourishment they
were able to obtain. They threw out arms in various
directions, and if divided each section maintained its
individual existence. After a while their movements
ceased, they contracted into a ball, and again secreted
round themselves a clear structureless envelope.

101

This completes their life history as observed by
Haeckel, who found it easy to retain them in his
glasses in perfect health, and who watched them
closely.

As another illustration I may take the Magosphæra
planula, discovered by Haeckel on the coast of
Norway.

In one stage of its existence (Pl. V, Fig. 10) it is a
minute mass of gelatinous matter, which continually
alters its form, moves about, feeds, and in fact behaves
altogether like the Amœba just described. It does
not, however, remain always in this condition. After
a while it contracts into a spherical form (Pl. V, Fig.
ii), and secretes round itself a structureless envelope,
which, with the nucleus, gives it a very close resemblance
to a minute egg.

Gradually the nucleus divides, and the protoplasm
also separates into two spherules (Pl. V, Fig. 12); these
two subdivide into four (Pl. V, Fig. 13), and so on
(Pl. 5, Fig 14), until at length thirty-two are present,
compressed into a more or less polygonal form (Pl. V,
Fig. 15). Here this process ends. The separate
spherules now begin to lose their smooth outline, to
throw out processes, and to show amœboid movements
like those of the creatures just described. The
processes or pseudopods grow gradually longer, thinner,
and more pointed. Their movements become more
active, until at length they take the form of ciliæ.
The spherical Magosphæra, the upper surface of
which has thus become covered with ciliæ, now begins
to rotate within the cyst or envelope, which at length
gives way and sets free the contained sphere, which102
then swims about freely in the water (Pl. V, Fig. 16),
thus closely resembling Synura, or one of the Volvocineæ.
After swimming about in this condition for a
certain time, the sphere breaks up into the separate
cells of which it is composed (Pl. V, Fig. 17). As long
as the individual cells remained together, they had
undergone no changes of form, but after separating
they show considerable contractility, and gradually
alter their form, until they become undistinguishable
from true Amœbæ (Pl. V, Fig. 18). Finally, according
to Haeckel, these amœboid bodies, after living for a
certain time in this condition, return to a state of rest,
again contract into a spherical form, and secrete round
themselves a structureless envelope. The life history
of some other low organisms, as for instance Gregarina,
is of a similar character.

It may be said, and said truly, that the difference
between such beings as these and the Campodea, or
Tardigrade, is immense. But if it be considered
incredible that even during the long lapse of geological
time such great changes should have taken
place as are implied in the belief that there is genetic
connection between them and these lower groups, let
us consider what happens under our eyes in the
development of each one of these little creatures in
the proverbially short space of their individual life.

I will take for instance the first stages, and for the
sake of brevity only the first stages, of the life-history
of a Tardigrade.77 As shown in Fig. 60, the egg is at
first a round body or cell, with a clear central nucleus—the germinal
103vesicle; it increases in size, and after
a while the yolk and the germinal vesicle divide into
two (Fig. 61), then into four (Fig. 62), and so on, just
as we have seen to be the case in Magosphæra. From
the minute cells (Fig. 63) arising through this process
of yolk-segmentation, the body of the Tardigrade is
then built up.78

Fig. 60, Egg of Tardigrade, Kaufmann, Zeit f. Wiss. Zool. 1851, Pl. 1. 61, Egg
of Tardigrade after the yolk has subdivided. 62, Egg of Tardigrade in the
next stage. 63, Egg of Tardigrade more advanced.

Though I will not now attempt to point out the full
bearing of these facts on the study of embryology
generally, yet I cannot resist calling attention to the
similarity of the development of Magosphœra with
the first stages of development of other animals,
because it appears to me to possess a significance, the
importance of which it would be difficult to overestimate.

Among the Zoophytes Prof. Allman thus describes79
the process in Laomedea, as representing the Hydroids
(Pl. VI, Fig. 1, represents the young egg):—”The first
step observable in the segmentation-process is the
104cleavage of the yolk into two segments (Pl. VI, Fig. 2),
immediately followed by the cleavage of these into
other two, so that the vitellus is now composed of
four cleavage spheres (Pl. VI, Fig. 3).” These spheres
again divide (Pl. VI, Fig. 4) and subdivide, thus at
length forming minute cells, of which the body of the
embryo is built up.

In Pl. VI, Figs. 5-9 represent the corresponding
stages in the development of a small parasitic worm—the
Filaria mustelarum—as given by Van Beneden.80
The first process is that within the egg, which represents,
so to say, the encysted condition of Magosphœra,
the yolk divides itself into two balls (Pl. VI,
Fig. 6), then into four, eight, and so on, the cells
thus constituted finally forming the young worm.
I have myself observed the same stages in the eggs
of the very remarkable and abnormal Sphærularia
bombi.81

Among the Echinoderms M. Derbès thus describes
the first stages (Pl. VI, Figs. 10-13) in the development
of the egg of an Echinus (Echinus esculentus):—”Le
jaune commence à se segmenter, d’abord en
deux, puis en quatre et ainsi de suite, chacune des
nouvelles cellules se partageant à son tour en deux.”82
Sars has observed the same thing in the star-fish.83

PLATE. 6.

In the Rotatoria, as shown by Huxley in Lacinularia,84
and by Williamson in Melicerta,85 the yolk is at
106105first a single globular mass, the first changes which take
place in it being as follows:—”The central nucleus
becomes drawn out and subdivides into two, this
division being followed by a corresponding segmentation
of the yolk. The same process is repeated again
and again, until at length the entire yolk is converted
into a mass of minute cells.” Among the Crustacea
the total segmentation of the yolk occurs among the
Copepoda, Rhizocephala, and Cirripedia. Sars has
described the same process in one of the nudibranchiate
mollusca86 (Tritonia), Müller in Entochocha,87
Haeckel in Ascidia,88 Lacaze Duthiers in Dentalium.89
Figures 18 to 21, Pl. VI, are taken from Koren and
Danielssen’s90 memoir on the development of Purpura
lapillus.

Figs. 22-24 show the same stages in a fish
(Amphioxus) as given by Haeckel, and it is unnecessary
to point out the great similarity.

Lastly, figures 25 to 29, Pl. 6, are given by Dr.
Allen Thomson,91 as illustrating the first stages in the
development of the vertebrata.

I might have given many other examples, but the
above are probably sufficient, and will show that the
processes which constitute the life-history of the
lowest organized beings very closely resemble the
first stages in the development of more advanced
107groups; that as Allen Thomson has truly observed,92
“the occurrence of segmentation and the regularity of
its phenomena are so constant that we may regard it
as one of the best established series of facts in organic
nature.”

It is true that normal yolk-segmentation is not
universal in the animal kingdom; that there are
great groups in which the yolk does not divide in
this manner,—perhaps owing to some difference in
its relation to the germinal vesicle, or perhaps because
one of the suppressed stages in embryological
development, many examples might be given, not
only in zoology, but, as I may state on the authority
of Dr. Hooker, in botany also. But, however, this
may be, it is surely not uninteresting, nor without
significance, to find that changes which constitute
the life-history of the lowest creatures for the initial
stages even of the highest.

Returning, in conclusion, to the immediate subject
of this work, I have pointed out that many beetles and
other insects are derived from larvæ closely resembling
Campodea.

Since, then, individual insects are certainly in many
cases developed from larvæ closely resembling the
genus Campodea, why should it be regarded as incredible
that insects as a group have gone through
similar stages? That the ancestors of beetles under
the influence of varying external conditions, and in
the lapse of geological ages, should have undergone
changes which the individual beetle passes
through under our own eyes and in the space of a few
108
days, is surely no wild or extravagant hypothesis.
Again, other insects come from vermiform larvæ
much resembling the genus Lindia, and it has
been also repeatedly shown that in many particulars
the embryo of the more specialized forms
resembles the full-grown representatives of lower
types. I conclude, therefore, that the Insecta generally
are descended from ancestors resembling the
existing genus Campodea, and that these again have
arisen from others belonging to a type represented
more or less closely by the existing genus Lindia.

Of course it may be argued that these facts have
not really the significance which they seem to me to
possess. It may be said that when Divine power
created insects, they were created with these remarkable
developmental processes. By such arguments the
conclusions of geologists were long disputed. When
God made the rocks, it was tersely said, He made the
fossils in them. No one, I suppose, would now be
found to maintain such a theory; and I believe the
time will come when it will be generally admitted
that the structure of the embryo, and its developmental
changes, indicate as truly the course of
organic development in ancient times as the contents
of rocks and their sequence teach us the past history
of the earth itself.

FOOTNOTES:

THE END.

RICHARD CLAY AND SONS, LIMITED, LONDON AND BUNGAY.

109BY THE SAME AUTHOR.

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