Fig. 562.—Examples of eruciform larvæ: a, Phryganea; b, Panorpa; c, Sesia; d, d, caterpillars; e, Selandria; f, Tipula; g, Simulium; h, Chionea; i, Musca; j, Tachina; k, Braula; l, flea; m, Tremex; n, coarctate larva of Meloë; o, bee (Andrena).
In the Meloidæ (Fig. 560, d) and Stylopidæ the first larval stage is Campodea-form; the changes will be described in the subsequent section on Hypermetamorphosis, and while these cases of change from a campodeoid to an inactive eruciform larva are very salient, if we compare the graduated series of larval forms throughout the order of Coleoptera, as represented by the illustrations in Fig. 561, we shall see that in nearly, if not each, case the form of the boring or mining, or bark or bud or seed-inhabiting grub is the result of a change of habit and commissariat from active predaceous larvæ, like those of the Carabidæ and other adephagous families, together with those of the Staphylinidæ, with their flat body, big mandibles, and well-developed maxillæ, to the cylindrical bodies of such larvæ as those of Dermestes and Anthrenus, which live a more sedentary life, to the root-feeding wire-worm or elaterid larvæ, and scarabæid grubs, onward to the phytophagous Chrysomelidæ, with the mining and boring buprestids and cerambycids,—in all these forms we see a gradual atrophy of the legs, which is fully carried out in the vermiform or maggot-like larva of the weevils. These changes throughout the members of the entire order are epitomized in the life-history of the Meloidæ, in which there are three typical forms of larva: the Campodea-form (triungulin stage), eruciform (second or carabidoid stage), and vermiform (coarctate) larva.
Fig. 563.—Prodoxus cinereus: a, apodous larva; b, head and prothoracic segment; c, anal hooks; d, pupa; e, cast pupal shell protruding from stalk of Yucca; f, female; g, side view of ♂ clasper.—After Riley, from Insect Life.
Fig. 564.—Larva of Limacodes scapha, nat. size.
In the Lepidoptera the eruciform, pedate type is adhered to throughout the order, with the rare exception of the nearly apodous mining larva of Prodoxus (Fig. 563, a), Phyllocnistis, and Nepticula, which have no thoracic legs, and the limacodid larvæ, whose abdominal legs are totally aborted, while the thoracic ones are much reduced (Fig. 564).
In the Hymenoptera the phytophagous forms are eruciform, while by the agency of the same factors as already mentioned, i.e. a sedentary or parasitic life and abundance of food within constant reach, the larvæ lose their legs and become vermiform.
In the Diptera, which are the most highly specialized of insects, the maggot or vermiform shape, and absence of any legs, prevails throughout the order, though the eucephalous larvæ show their origin from a primitive eruciform type of larva. The highly specialized larvæ of the Culicidæ and Simuliidae are undoubtedly related to the earliest and most generalized types, while the maggots of the parasitic flies (Tachinidæ) and other muscids are later degradational forms, and the result of adaptation induced, as in the previous cases, by a sedentary or parasitic mode of life, living as they do immersed in an abundance of rich nitrogenous food, with the result that the mouth-parts have become atrophied by disuse, while the limbs have become entirely aborted, though the thoracic imaginal discs develop normally in the embryonic or pre-larval stages.
It appears, therefore, highly probable that the metamorphoses of insects are the result of the action of change of conditions, just as the polymorphism of Termites is with little doubt the result of differences of food and other conditions. These matters will be farther discussed under the head of Causes of Metamorphosis.