Gonin has found that the thoracic legs of the caterpillar correspond only to the tarsi of the imago (Fig. 608). It results, he says, from this fact that in accordance with the observations of Réaumur (which were wrongly interpreted by Newport and Künckel D’Herculais) that the amputation of the legs of the larva does not involve the entire leg, but only the extremity of the leg of the imago.
Formation of the cocoon.—While the larvæ of many insects, as those of the butterflies, suspend themselves before transforming, and spin no cocoon, or dig into the earth for protection and to secure an immunity from too great changes of temperature, a large proportion of the larvæ of metabolous insects which lead an inactive pupal life, line their earthen cells with silk, or spin a more or less elaborate case of silk, called the cocoon. We have seen that the inactive pupa of the male scale-insects is covered by the scale itself, or even in one case the insect forms a true cocoon of fibres of wax. The aquatic larvæ of the Neuroptera and Coleoptera creep out of the water, and by the movements of their bodies make a rude earthen cell in the bank, while that of Donacia spins a dense, leathery cocoon (Fig. 567) in the earth. The larvæ of the Embiidæ are protected by a cocoon, which they renew at each moult. Coniopteryx spins an orbicular cocoon, the Hemerobiidæ a spherical, dense, whitish one. The Trichoptera transform within their larval cases, which thus serve as cocoons, as do certain case-bearing Lepidoptera, notably the Psychidæ.
Fig. 567.—Cocoon (natural size) of Donacia proxima.
Fig. 568.—Cocoon and larva of Lucanus dama.
The pupa of certain leaf-eating beetles (Chrysomelidæ), as well as the Coccinellidæ, Dermestidæ, Hister, etc., are usually protected by the cast larval skin, which is retained, forming a rude shelter. While many beetles spin an oval cocoon (Gyrinus, Silphidæ), the wood-boring species make one of chips glued together, and that of Lucanus, which feeds on decayed wood, is lined with silk (Fig. 568). Anobium constructs a silken cocoon, interweaving the fine particles of its thin castings; the larvæ of weevils also usually spin silken cocoons.
Fig. 569.—Larva (a), puparium (b), and imago (c) of Sarcophaga, enlarged.
