INSECT METAMORPHOSIS
have rudimentary or shortened wings in the adult stage often have wings larger than those of the adult, indicating that the wings have been reduced in the adult since the time when the pupa was first established. Here, therefore, we see a case of metamorphosis between the pupa and the adult. Adult moths and butterflies have no mandibles or have mere rudiments of them (Fig. 163), but the jaws are often quite visible in the pupae (Fig. 159 H, Md), and the pupa of one moth has long, toothed mandibles which it uses to liberate itself from the cocoon before transforming to the adult.
The structural changes that accompany the transformation of the larva into an adult insect are by no means confined to the outside of the body. Much internal reorganization goes on which involves changes in the tissues themselves. The larva may have built up a highly efficient alimentary canal well adapted for handling its own particular kind of food, but perhaps the adult has adopted an entirely different diet. The alimentary canal, therefore, must be completely remodeled during the pupal stage. The nervous system and the tracheal system are often different in the larval and the adult stages, but the change in these organs is usually in the nature of a greater elaboration for the purposes of the adult, though the larva may have developed special features that are discarded.
It is in the muscles usually that the most radical reconstructive processes of the transformation from larva to adult take place. The muscles of adult insects are attached to the outer cuticular layer of the body wall, which in hard-bodied insects constitutes the "skeleton," and the mechanical differences between the larva and the adult lie in the relation between the muscles and the cuticula. With the change in the external parts between the two active stages of the insect, therefore, the larval muscles are likely to become entirely unsuited to the purposes of the adult. The special larval muscles, then, must be
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