256 ENTOMOLOGY 



and number. Why the gall should have a distinctive, or spe- 

 cific, form, it is not yet known. There is no evidence that the 

 form is of any adaptive importance, and the subject probably 

 admits of a purely mechanical explanation a problem for the 

 future. 



Gall Insects. The study of gall insects is in many respects 

 difficult. It is not at all certain that an insect which emerges 

 from a gall is the species that made it ; for many species, even 

 of Cynipidse, make no galls themselves but lay their eggs in 

 galls made by other species. Such guest-insects are termed 

 inquilines. Furthermore, both gall-makers and inquilines are 

 attacked by paras.itic Hymenoptera, making the interrelations 

 of these insects ha^rd to determine. Many species of insects 

 feed upon the substance of galls; thus Sharp speaks of as 

 many as thirty different kinds of insects, belonging to nearly 

 all the orders, as having been reared from a single species of 

 gall. 



Parthenogenesis and Alternation of Generations. Par- 

 thenogenesis has long been known to occur among Cynipidse. 

 It has repeatedly been found that of thousands of insects 

 emerging from galls of the same kind, all were females. In 

 one such instance the females were induced by Adler to lay eggs 

 on potted oaks, when it was found that the resulting galls were 

 quite unlike the original ones, and produced both sexes of an 

 insect which had up to that time been regarded as another 

 species. Besides parthenogenesis and this alternation of gene- 

 rations, many other complications occur, making the study of 

 gall-insects an intricate and highly interesting subject. 



Plant-Enemies of Insects. Most of the flowering plants 

 are comparatively helpless against the attacks of insects, though 

 there are many devices which prevent "unwelcome " insects 

 from entering flowers, for instance the sticky calyx of the catch- 

 fly (Silene virginica), which entangles ants and small flies. A 

 few plants, however, actually feed upon insects themselves. 

 Thus the species of Drosera, as described in Darwin's classic 

 volume on insectivorous plants, have specialized leaves for the 



