486 Studies in the Theory of Descent. 



limbs of typical importance entirely vanish in the 

 one group, but in the other attain to complete 

 development. 



In the Hymenoptera there exists therefore a 

 very considerable incongruence in the systems 

 based morphologically, i. e. on the pure form- 

 relationships of the larvae and of the imagines. 

 The reason of this is not difficult to find : the 

 conditions of life differ much less in tlie case of 

 the imagines than in that of the larvce. In the 

 former the conditions of life are similar in their 

 broad features. Hymenoptera live chiefly in the 

 air and fly by day, and in their mode of 

 obtaining food do not present any considerable 

 differences. Their larvae, on the other hand, live 

 under almost diametrically opposite conditions. 

 Those of the saw-flies live after the manner of 

 caterpillars upon or in plants, in both cases their 

 peculiar locomotion being adapted for the acquisi- 

 tion and their masticatory organs for the reduction 

 of food. The larvae of the other Hymenoptera, 

 however, do not as a rule require any means of 

 locomotion for reaching nor any organs of mastica- 

 tion for swallowing their food, since they are fed in 

 cells, like the bees and wasps, or grow up in plant 

 galls of which they suck the juice, or are parasitic 

 on other insects by whose blood they are nourished. 

 We can readily comprehend that in the whole of 

 this last group the legs should disappear,' that the 

 jaws should likewise vanish or should become 



