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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 these 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.

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.

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.

CONTENTS.

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.

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.

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.

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.

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, says 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 have 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 the 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 from 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.”

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, having 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.