INSECT METAMORPHOSIS 



new muscles are formed in an insect a new cuticula must 

 also be produced in order that the muscle fibers may 

 become attached to the skeleton. New muscles com- 

 pleted at the time of a molt may be anchored into the new 

 cuticula formed at this time; but if the completion of the 

 muscle tissue is delayed, the new fibers can become func- 

 tional only by attaching themselves at the following molt. 

 Conversely, if the new muscles are not perfected at the 

 time of the last normal molt, the insect must have an 

 extra molt later in order to give the muscles a functional 

 connection with the body wall. 



Thus Poyarkoff would explain the origin of the pupal 

 stage in the life cycle of the insect. His theory has much 

 to commend it, for, as Poyarkoff shows in an analysis of 

 the various processes accompanying metamorphosis, none 

 of the changes in any of the organs other than the muscles 

 would seem to necessitate the production of a new cutic- 

 ula and thus involve an added molt. If insects with 

 incomplete metamorphosis add new muscles for the 

 adult stage, such muscles must be ready-formed at the 

 time of the last nymphal molt; but it is probable that 

 there are few such cases in this class of insects. 



Adopting Poyarkoff's theory, then, as the most plausi- 

 ble explanation of why a pupal stage has become separated 

 by a molt from the fully-matured adult stage, we may 

 say that the reason for the pupa is probably to be found in 

 the delayed growth of the adult muscles and in the conse- 

 quent need of a new cuticula for their attachment. 



With a pupal stage once established, however, the pupa 

 has undergone an evolution of its own, as has the larva 

 and the adult, though to a smaller degree than either 

 of these two active stages. The pupa is characteristically 

 different in each of the orders of insects, and many of its 

 features are clearly adaptations to its own mode of life. 



It is one thing to know the facts and to see the mean- 

 ing of metamorphosis; it is quite another to understand 

 how it has come about that an animal undergoes a meta- 



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