INSECTS IN RELATION TO PLANTS 215 



which serves as food for the larva; this transforms within the gall and. 

 escapes as a winged insect. The physiology of gall-formation is far 

 from being understood. It has been found that the mechanical irrita- 

 tion from the ovipositor is not the initial stimulus to the development 

 of a gall; neither is the fluid which is injected by the female during 

 oviposition, this fluid being probably a lubricant; if the egg is removed, 

 the gall does not appear. Ordinarily the gall does not begin to grow 

 until the egg has hatched, and then the gall grows along with the larva; 

 exceptions to this are found in some Tenthredinidae in which the egg it- 

 self increases in volume, when the gall may grow with the egg. It 

 appears that the larva exudes some fluid which acts upon the proto- 

 plasm of certain plant cells (the cambium and other cells capable of 

 further growth and multiplication) in such a way as to stimulate their 

 increase in size and number. The following observations on this subject 

 by A. Cosens are important. The cells of the plant that immediately 

 surround the larva are known as nutritive cells. In Cynipidae the 

 larva gradually withdraws the contents of these cells, by means of the 

 mouth and not by absorption, and the cells gradually collapse. The 

 proportion of sugar to starch decreases from the inside of the nutritive 

 zone (nearest the larva) to the outside. This is owing to an enzyme 

 that changes starch into sugar, the source of this enzyme being probably 

 a pair of salivary glands that open externally on each side just below the 

 mouth of the larva. The larva by accelerating the rate of change from 

 starch to sugar renders available to the plant more food than usual and 

 therefore stimulates the activity of the protoplasm toward greater cell- 

 growth and more rapid cell-reproduction. Thus the gall as well as the 

 larva draws food from the nutritive zone. 



Why the gall should have a distinctive, or specific, form, it is not yet 

 known. There is no evidence that the form is of any adaptive impor- 

 tance, and the subject probably admits of a purely mechanical explana- 

 tion. One factor in determining the form of the gall is the direction in 

 which the stimulus is applied; a spherical cynipid gall arising when the 

 influence is about equally distributed in all directions (Cosens) . 



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 Cynipidae, 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 in- 

 terrelations of these insects hard to determine. Many species of insects 



