178 EN TOMOLOGY 
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 1s 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 eruciform 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 eruciform 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 
eruciform larve, 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 
