DEVELOPMENT 147 



Even when considerable difference exists between the nymphal and 

 imaginal environments, as in Ephemerida and Odonata, the activity of 

 the individual may still be continuous, even if somewhat lessened as the 

 period of transformation approaches. 



With Neuroptera, the pupal stage appears. In these and all other 

 holometabolous insects the larva accumulates a surplus of nutriment 

 sufficient for the further development, which becomes condensed into 

 a single pupal stage, during which external activity ceases temporarily. 



With the increasing contrast between the organization of the larva 

 and that of the imago, the pupal stage gradually becomes a necessity. 

 Metamorphosis now means more than the mere acquisition of wings, for 

 the larva and the imago have become adapted to widely different en- 

 vironments, chiefly as regards food. The caterpillar has biting mouth 

 parts for eating leaves, while the adult has sucking organs for obtaining 

 liquid nourishment; the maggot, surrounded by food that may be ob- 

 tained almost without exertion, has but minimum sensory and locomotor 

 powers and for mouth parts only a pair of simple jaws; as contrasted 

 with the fly, which has wings, highly developed mouth parts and sense 

 organs, and many other adaptations for an environment which is strik- 

 ingly unlike that of the larva; so also in the case of the higher Hymen- 

 optera, where maternal or family care is responsible for the helpless con- 

 dition of the larva. 



Thus it is evident that the change from larval to imaginal adapta- 

 tions is no longer congruous with continuous external activity; a quies- 

 cent period of reconstruction becomes inevitable. 



As was said, the cruciform type of larva has been derived from the 

 thysanuriform type, the strongest evidence of this being the fact that 

 among hypermetamorphic insects, the change from the one to the other 

 takes place during the lifetime of the individual. Furthermore, the 

 cruciform condition is plainly an adaptive one, brought about by an 

 abundant and easily obtainable supply of food. The lack of a thysanuri- 

 form stage in the development of the most specialized cruciform larvae, 

 as those of flies and bees, is regarded by Hyatt and Arms as an illustra- 

 tion of the general principle known as "acceleration of cfevelopment,'' 

 according to which newer and useful adaptive characters tend to appear 

 earlier and earlier in the development, gradually crowding upon and 

 forcing out older and useless characters. In connection with this sub- 

 ject, the appearance of temporary abdominal legs in embryo bees is 

 significant, as indicating an ancestral active condition. In accounting for 



