Fig. 560.—Examples of campodeoid nymphs and larvæ: a, Campodea; b, Podura (Degeeria); c, Lepisma; d, triungulin larva of Meloë; e, Perla; f, Forficula; g, Chloëon; h, May-fly (Palingenia); i, Æschna; j, Atropos; k, Myrmeleon; l, Sialis; m, Corydalus; n, Cicada.
b. The eruciform type of larva
Brauer also sagaciously pointed out that “a larger part of the most highly developed insects assume another larva form, which appears not only as a later acquisition, through adaptation to certain definite conditions, but also arises as such before our eyes. The larvæ of Lepidoptera, of saw-flies, and Panorpidæ show the form most distinctly, and I call this the caterpillar form (Raupenform). That this is not the primitive form, but one later acquired, we see illustrated in certain beetles. The larvæ of Meloë and of Sitaris, in their fully grown conditions, possess the caterpillar form, but the new-born larvæ of these genera show the Campodea-form. The last form is lost as soon as the larva begins its parasitic mode of life.... The larger part of the beetles, the Neuroptera (in part), the bees and flies (the last with the most degraded maggot form), possess larvæ of this second form.” In 1871 we adopted these views, giving the name eruciform to this type of larvæ, and afterwards Lubbock adopted Brauer’s views. Brauer considered that the eruciform larva was the result of living a stationary semi-parasitic life on plants, in carrion, or burrowing in the trunks and branches or leaves and buds of trees, where they do not have to move about in search of their food. The change from the Campodea-form to the eruciform larva is a process of degeneration and often of atrophy of the limbs, and, in the footless forms of dipterous and hymenopterous insects, of the gnathites, accompanied by a tendency of the body to become more or less cylindrical.
The first steps in the origination of the eruciform larva were apparently taken in the order Neuroptera, as restricted by Brauer and by myself, where, though the larvæ are campodeoid, there is a true resting pupal stage. The most generalized larval form is perhaps that of the Sialidæ (Fig. 560, l), in which the body tends to be slightly cylindrical, though the legs are long, and the gnathites well developed for seizing and biting their living prey. The terrestrial larvæ of the Hemerobiidæ, though modifications of the sialid larval form, are considerably specialized in adaptation to their active carnivorous habits. But the life-history of Mantispa, where there are two larval stages, gives us plainly enough the key to the mode in which the complete metamorphosis was brought about. The larva, born a true Campodea-like form, with large, long, 4–jointed legs, has a structure which would enable it to move about freely after its prey, beginning at once to live a sedentary life in the egg-sac of a spider; before the first moult it loses the use of its legs, while the antennæ are partly aborted. The result is that, owing to this change of habits and surroundings from those of its active ancestors, it changes its form, and the fully grown larva becomes cylindrical, with small slender legs, and, owing to the partial disuse of its jaws, acquires a small, round head.
Examples of coleopterous larvæ, showing the passage from the campodeoid to the eruciform type of larvæ.
Fig. 561.—Coleopterous larvæ showing passage from campodeoid to eruciform larvæ: a, b,
Harpalus; c, Dyticus; d, Staphylinus; e, Silpha; f, Melanactes; g, Ludius; h, Elater; i, Donacia;
j, Chrysobothris; k, Orthosoma; l, Coccinella; m, Byrrhus; n, Trox; o, p, Lachnosterna; q,
Labidomera; r, Ptinus; s, Anobium; t, Balaninus (entirely apodous).
Its antennæ, mouth-parts, and legs not only retarded in growth, but retrograding and becoming vestigial, the body meanwhile becoming fat and cylindrical, an apparent acceleration of growth goes on within, with probably an enlargement of the intestine and fat-body, and thus the pupal form is perfected while the larva is full-fed and quiescent. It is not improbable that in the primitive neuropteron, as the result of a mode of life like that of Mantispa, the quiescent life of the later stages graduated into a quiescent, inactive pupal life, allowing the changes going on in the internal organs to result in a complete metamorphosis, which was transmitted to the later Neuroptera, thus making the complete metamorphosis a fixed, normal condition. It thus appears that a change of habits and of food, and more especially the fact that the nymph became so surrounded with an abundance of food close at hand that it did not have to run actively about and seize it in a haphazard manner, were the factors bringing about a change from the Campodea-form nymph to the eruciform larva, thus inducing a hypermetamorphosis.
The larvæ of the Mecoptera (Panorpidæ, Fig. 562, b) are still more caterpillar-like, and besides their cylindrical body, rounded head, small short gnathites, small thoracic legs, they have what appear to be 2–jointed legs to each of the nine abdominal segments, and the close resemblance to caterpillars is farther carried out by the presence of a pair of prothoracic spiracles, none existing on the other two thoracic segments.
