2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 143 



the same habitat as their parents, have the same kind of mouth parts, 

 and eat the same kind of food. There is no reason why the young of 

 such insects should be radically different from their parents. They 

 are able to develop gradually into the adult form. Many other insects 

 are in the same class and go through life without any metamorphosis. 



Let us suppose now that the adults of some other insects far back 

 in the early days of their evolution adopted a way of feeding that 

 depended on their ability to fly and became structurally adapted to 

 obtaining a special kind of food. If the young of these insects had to 

 inherit the new kind of feeding organs of their parents, being unable 

 to fly, they would be left to starve and the species would die out. To 

 prevent such a calamity the usual laws of heredity have somehow been 

 set aside in such cases, allowing the young insect to undergo an evolu- 

 tion on its own part adapting it structurally to some suitable environ- 

 ment where it could live and grow to maturity. A good example is 

 the dragonfly ; the flightless young insect could not catch mosquitoes in 

 the air as do its parents, so it took to the water where plenty of live 

 food was available, and has become structurally so individualized that 

 it now has no resemblance to its parents. Likewise the young mosquito 

 without wings could not practice bloodsucking as does its mother, or 

 the nectar-feeding of its father, so it also became aquatic and has 

 been equipped for its own way of feeding in the water. Adult fleas 

 are wingless, but they have substituted jumping legs for wings and are 

 able to feed on the blood of vertebrate animals. The larval flea thus 

 left where it was hatched has to make the best of the circumstances by 

 feeding on whatever it can find, but its form and mouth parts are 

 suitable to the life it has to lead. The wasps and the bees take care 

 of their helpless larvae, but the young of insects deserted by their 

 parents receive special attention from nature. 



Most any of the lower insects undergo some changes at the last 

 moult, such as the completion of the wings and the external genital or- 

 gans, or a remodeling of the shape and proportions of the body. Such 

 changes may be called "metamorphosis" in a literal sense, but they are 

 merely the final stage of normal adult development. A true meta- 

 morphosis involves the discarding of specialized larval characters, 

 which allows the completion of adult development, and differs in 

 degree according to the degree of aberration of the young from the 

 adult structure. Various cases might be cited in which the young insect 

 differs from its parents only in some minor character of its own, which 

 is discarded at the last moult. 



A very simple example of metamorphosis due to juvenile specializa- 

 tion is seen in the cicada. The young nymph of the seventeen-year 



