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 
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 Cynipide, 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 parasitic Hymenoptera, making the interrelations 
of these insects hard 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 
a problem for the 
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 Cynipide. 
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 
