INSECTS IN RELATION TO PLANTS 1 99 



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

 different kinds of insects, belong to almost all the orders, as having been 

 reared from a single species of gall. 



Parthenogenesis and Alternation of Generations. Partheno- 

 genesis has long been known to occur among Cynipidae. It has repeat- 

 edly 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 Dar- 

 win'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 bott'om 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 in- 

 sects into these traps, where their decomposition 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 digests the softer portions of 



