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 completed 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 functional 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 cuticula 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 plausible 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 consequent 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 meaning of metamorphosis; it is quite another to understand how it has come about that an animal undergoes a meta-
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