2l6 ENTOMOLOGY 



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

 different kinds of insects, belonging to almost all the orders, as- having 

 been reared from a single species of gall. 



Parthenogenesis and Alternation of Generations. Parthenogenesis 

 has long been known to occur among Cynipidae. 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 generations, 

 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 com- 

 paratively 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 catchfly (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 purpose of catching insects. The stout hairs of these leaves end 

 each in a globular knob, which secretes a sticky fluid. When a fly 

 alights on one of these leaves the hairs bend over and hold the insect; 

 then a fluid analogous to the gastric juice of the human stomach exudes, 

 digests the albuminoid substances of the insect and these are absorbed 

 into the tissues of the leaf; after which the tentacles unfold and are 

 ready for the next insect visitor. The Venus's flytrap is another well 

 known example; the trap, formed from the terminal portion of a leaf, 

 consists of two valves, each of which bears three trigger-like bristles, 

 and when these are touched by an insect the valves snap together and 

 frequently imprison the insect, which is eventually digested, as before. 

 In the common pitcher-plants, the pitcher, fashioned from a leaf, is lined 

 with downward pointing bristles, which allow an insect to enter but 

 prevent its escape. The bottom of the pitcher contains water, in 

 which may be found the remains of a great variety of insects which 

 have drowned. There are even nectar glands and conspicuous colors, 

 presumably to attract insects into these traps, where their decomposi- 

 tion products are more or less useful to the plant. In Pinguicula the 

 margin of a leaf rolls over and envelops insects that have been caught 

 by the glandular hairs of the upper surface of the leaf, a copious secretion 



