NO. 2 ENCYCLOPEDIA OF INSECT ANATOMY — SNODGRASS I3 



adult epidermis formed from islands of imaginal cells in the larval 

 epidermis. The alimentary canal goes into dissolution, and the adult 

 food tract is generated from replacement cells in the wall of the 

 larval canal. The larval musculature may be completely destroyed 

 and new muscles for the adult formed in the pupa. Some organs 

 such as the tracheal and nervous systems may be simply remodeled 

 to serve the needs of the adult. How much reuse versus remodeling 

 versus replacement is involved for the cells within the nervous sys- 

 tem has not yet been determined, but it is clear that the nervous 

 system is not replaced in toto as some other systems are. Clearly, 

 most of this process of change is not a metamorphosis of larval 

 tissues into adult tissues but a replacement of larval organs by newly 

 formed adult organs. The result is an entire transformation in the 

 appearance of the insect between larva and adult. This is because 

 the two stages are really two different animals — one stage is not 

 transformed into the other. The &gg simply has the potentiality of 

 forming first the larva and then the adult, as was clearly expressed 

 by Janet long ago (1909). 



The term metamorphosis has become so firmly established in 

 entomological nomenclature that undoubtedly it will persist even if 

 its erroneous implications become generally recognized. Insects have 

 become famous for their metamorphoses. 



If the young insect in the form of a larva does not grow into the 

 adult of its species, it may be of interest to speculate on its nature and 

 how it came to dift'er from its parents. We must suppose that primi- 

 tively the young of all insects resembled their parents except in mat- 

 ters of immaturity, as do the young of a modern cockroach or grass- 

 hopper. With most of the higher insects, however, the winged adults 

 have become specialized for a life and ways of feeding that the 

 flightless young could not follow. The young left behind were forced 

 to adopt ways of living suitable to themselves and so have undergone 

 a juvenile evolution quite independent of their elders ; they have be- 

 come specialized for their own various habitats and ways of feeding. 

 Thus the lar^^al stages have acquired many diverse forms in the 

 several orders and have become as distinctive of their species as 

 their parents (this is shown by the fact that taxonomists have been 

 able to construct keys to larvae as well as to adults, and in some 

 groups it is easier to identify larvae than the adults). Insect larvae, 

 therefore, are not ancestral forms though many of them have taken 

 on a wormlike shape. Structurally they remain insects. It must be 

 clear that the evolution of young insects into their specialized modern 



