The adult is the sexually capable instar, whose responsibility is to find a mate and reproduce, thus perpetuating the species. Development of the internal and external sexual organs completes growth and usually molting, although in some noninsect terrestrial arthropods, molting may continue throughout life. Wings in the insects also become fully grown and functional at this time, the one exception being the mayflies, which have a winged instar (subimago) preceding the full imago.

Some aberrant conditions occur in insects, such as neoteny, in which the adult retains its outward larval body form but completes development of the internal reproductive organs. Neotenic adults mate and parent offspring while continuing to feed and live as immatures. This is a common condition in railroad worms and other beetles and in some primitive flies.

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