CHAPTER V.

THE ORIGIN OF INSECTS.

The Origin of Insects.--Mistaken views of Darwinian theory.--Natural selection a _vera causa_.--Application of Darwin's views to Insects.--Similarity of young Crustacea as compared with mature forms; ditto in Insects.--Type of Insecta.--Two principal types of larvæ: Hexapod and Apod.--Conclusions to be drawn from them.--_Campodea_ the modern representative of the Insect-stock.--_Campodea_, perhaps derived from Tardigrade.--Vermiform or Apod type of larva.--Views of Fritz Müller, Brauer, and Packard.--Represents a still earlier ancestor.--Modern representatives.--_Notommata_, _Albertia_, _Lindia_.--Earlier forms difficult to trace.--Lowest forms of animal life.--Yolk-segmentation.--Embryology and Evolution.--Light thrown by the evolution of the individual on that of the species _page_ 82-108

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. _Protamoeba_.

6-9. _Protamyxa aurantiaca_. Haeckel Beit. zur. Monog. der Moneren, pl. 1.

10-18. _Magosphoera 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 Derbès. 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_, oesophagus; _gsae_, supra-oesophagal 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-oesophagal ganglion; _slk_, oesophagus; _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).--_A_^1, mandibular segment; _C_^1-_C_^5, 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 oequituberculata seen from above ×6/10.

40. Larva of _Echinus_ × 100.--_A_, front arm; _F_, arms of the mouth-process; _B_, posterior side arm; _E_^1, accessory arm of the mouth-process; _a_, mouth; _a_^1, oesophagus; _b_, stomach; _b_^1, 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_, oesophagus; _c_, stomach; _c_^1, 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.