10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



assume that at some time in the past history of the insects the young, 

 as those of most other animal groups, resembled their parents except 

 for immaturity, as does a modern young grasshopper or a young cock- 

 roach. The question then is : Why have the young of some groups 

 departed from the parental form along their own lines of evolution ? 

 The question is not so difficult to answer as it might seem, since some 

 larvae are very similar to the adults and others depart in varying de- 

 grees until they have lost all resemblance to the adults that produce 

 them. 



As long as the young insect can live and feed in the same environ- 

 ment as its parents, as the young grasshoppers and cockroaches do, 

 there is no need of it having a special structure of its own. The 

 adults of many insects, however, have taken advantage of their wings 

 to explore other habitats for new sources of food, and in most cases 

 they have been structurally modified for life on the wing and for 

 feeding on some special kind of food. The flightless young, there- 

 fore, could not possibly keep up with their parents. So, to insure 

 the survival of the young, nature has fitted them for a way of living 

 and feeding of their own. The young cicada affords a very simple 

 example of juvenile metamorphosis since it is adapted merely for 

 burrowing in the earth. The young mayfly and stonefly are supplied 

 with gills for an aquatic life. More extreme cases are seen in the 

 young of Lepidoptera, Diptera, and Hymenoptera. Caterpillars are 

 adapted for climbing and feeding on vegetation, whereas the adults 

 fly around and usually suck nectar. The young mosquito would 

 starve if it had to feed on blood as does its mother or on nectar as 

 does its father. Hence it has become strictly adapted to an aquatic 

 life and equipped with a special feeding apparatus of its own. Young 

 muscoid flies could not live the life of their winged parents and have 

 become transformed into maggots fitted for other ways of living. 

 The grubs of many Hymenoptera are fitted for living in cells where 

 they would be completely helpless if not fed by the adult. 



In no case can the larva go over directly into the adult. It must 

 at least discard its specialized larval structures, and the more it has 

 departed from the parental form the more it has to discard. In ex- 

 treme cases the larva is almost completely destroyed at the end of 

 larval life. The modern adult represents the last stage of phylo- 

 genetic evolution of its species; the larva is a temporary specialized 

 form of the young insect. In ontogeny the larva develops first, but 

 it must at last give way to development of the adult. (See Pupa.) 



Though the process of the destruction of the larval tissues and the 

 resumption of imaginal development has commonly been called the 



