INSECTS
cleared away, and a new muscle system must be built up suitable to the adult mechanism. Most of the other organs are transformed by a gradual replacement of cells in their tissues, with the result that each organ itself remains intact during the whole period of its alteration—the insect is never without a complete alimentary canal, its body wall always maintains a continuous surface. This condition, however, is not entirely true of the muscles, for with some insects undergoing a high degree of metamorphosis in external structure, the muscular system may suffer a complete disorganization, the fibers of the larval system being in a state of dissolution while those of the adult are in the process of development.
The muscles of adult insects, as we have just said, are
Fig. 142. Diagram of the attachment of a muscle to the body wall of an adult insect by means of the terminal fibrillae (Tfbl)
BM, basement membrane; Enct, endocuticula; Epct, epicuticula; Epd, epidermis; Exct, exocuticula; Mcl, muscle; Tfbl, terminal fibrillae of the muscle anchored in the cuticula
attached to the outer layer of the body wall (Fig. 142). This layer is composed partly of a substance called chitin formed by the cellular layer of the body wall beneath it, and constitutes the cuticular skin that is shed when the insect molts. The newly-formed cuticula is soft and takes the contour of the cellular layer producing it.
The muscles of the larva that go over into the adult stage and the new muscles of the adult must become fastened to the new cuticula, and this is possible only when the cuticula is in the soft formative stage. It has been pointed out by Poyarkoff that, for this reason, whenever
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