178 ENTOMOLOGY 



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 environments, 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 obtained 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 strikingly un- 

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

 optera, where maternal or family care is responsible for the 

 helpless condition of the larva. 



Thus it is evident that the change from larval to imaginal 

 adaptations is no longer congruous with continuous external 

 activity; a quiescent period of reconstruction becomes in- 

 evitable. 



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 life- 

 time of the individual. Furthermore, the cruciform condi- 

 tion is plainly an adaptive one, brought about by an abundant 

 and easily obtainable supply of food. The lack of a thysa- 

 nuriform stage in the development of the most specialized 

 cruciform larvae, as those of flies and bees, is regarded by 

 Hyatt and Arms as an illustration of the general principle 

 known as " acceleration of development," 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 subject, the appearance of temporary abdominal legs in 

 embryo bees is significant, as indicating an ancestral active 



