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] of 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. Brauer[58] and I[59] 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.
Fig. 52, Agrotis suffusa (after Packard). 53, Haltica (after Westwood).