192 Annals Entomological Society of America [Vol. XIII, 



vegetable tissue? That it fed externally in the manner of a 

 modern sawfiy seems in the highest degree improbable. 



It is obvious that parasitism has arisen over and over again 

 among the Hymenoptera, as Wheeler* has recently shown in 

 some detail. The parasitic aculeata have nothing to do with 

 the Ichneumonoidea or Chalcidoidea, and the inception of 

 their parasitic habits' was entirely different. Among the bees, 

 the extraordinary case of Androgynella is very suggestive. f 

 This type, of which two species are known, is a Megachile in 

 which the ventral scopa is wholly lacking in the female, which 

 has 13-jointed antennae and spined anterior coxae, as in a male, 

 but still retains the sting. In the light of modern genetics, 

 we can form some idea of the origin of such a condition, but 

 that it should be able to survive is surprising. It could survive 

 ■only by becoming a guest or parasite of the other or normal bees, 

 and this is evidently what has taken place. Thus parasitism 

 among bees may owe its origin to- a species of necessity, con- 

 fronting insects which no longer possessed the power to function 

 in the ordinary way. Among the Ichneumonoidea it was rather 

 the development of an advantage, leading to a great increase 

 in the number of genera, species and individuals. The parasitic 

 aculeata have never been dominant like the ichneumons; they 

 are mostly comparatively rare or sporadic in their occurrence. 



These considerations suggest interesting possibilities of 

 experiment. How far is it possible to modify the reactions of 

 insects by changing their conditions of life? But when the 

 modification is internal, is due to some alteration of the germ- 

 plasm, it may well affect instincts as well as structure. Modern 

 breeding, such as that of Drosophila, has revealed the fact 

 that, at least in certain insects, mutations are relatively fre- 

 quent. In nature, no doubt, the vast majority perish, but an 

 occasional one survives and enters upon a new course of life. 

 Thus a process which is detrimental to many individuals, is 

 advantageous to the group, being in the long run a potent 

 cause of specific and generic differentiation. The outcome is 

 diversity of adaptation, purposeful in the sense that the existing 

 powers of variation and response are capable of leading to the 

 results we see. 



*Proc. Amer. Philosophical Society, LVIII (1919), pp. 1-40. 



tFor details, see Philippine Journal of Science, XIII, D. 4 (1918), p. 140. 



