INSECTS 



much in form in the moth, and they continue their ex- 

 cretory function during the pupal stage. The silk glands 

 oi 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. 



Internal organs that have not been specially modified 

 in their development for the purposes of the larva, in- 

 cluding usually the nervous system, the heart, the respira- 

 tory tubes, and the reproductive organs, suffer little if any 

 disintegration 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 lives, the mechanism of locomotion is 

 likely to be radically 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 special 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- 

 tem, therefore, varies much in different insects according 

 to the mechanical difference between the larva and the 

 adult. 



The purely larval muscles that are to be discarded when 



[3°o] 



