Page:Insects - Their Ways and Means of Living.djvu/358



much in form in the moth, and they continue their ex- cretory function during the pupal stage. The silk glands of the caterpillar are greatly reduced in size, and their ducts, as a consequence of the suppression of the spinneret, open at the base of the labium within the entrance to the mouth. lnternal organs that have hot been specially modified in their development for the purposes of the larva, in- c]uding usually the nervous system, the heart, the respira- tory tubes, and the reproductive organs, surfer little if any disntegration in their tissues; they simply grow to the mature form, which may be much more elaborate than that of the larva, by a resumption of the ordinary processes of development. The nervous system, and particularly the tracheal system, however, in some insects undergo much reconstruction between the larval and the adult stages. A most important part of the reconstruction between the larva and the adult has to do with the muscle system. Since, in its two active stages, the insect leads usually two very different lires, the mechanism of locomotion is likely to be radicaliy different in the larva and in the adult; and in such cases the transformation of the insect will involve particularly a thorough reorganization of the musculature. Most larvae have acquired an elaborate system of speciai muscles for their own use because they have adopted a wormlike mode of progression. On the other hand, the adults have need of certain muscles, par- ticularly those of the wings, which would be only an en- cumbrance to a larva. Consequently, muscles needed only by the adult are suppressed in the larval stage, and the special muscles of the larva must be cleared away during the pupal stage. The metamorphosis in the muscle sys- rem, therefore, varies much in different insects according to the mechanical difference between the larva and the aduit. The purely larval muscles that are to be discarded when

[3ool

THE