198 ENTOMOLOGY 



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 irritation 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 itself increases 

 in volume, when the gall may grow with the egg. It appears that the 

 larva exudes some fluid which acts upon the protoplasm of certain plant 

 cells (the cambium and other cells capable of further growth and multi- 

 plication) in such a way as to stimulate their increase in size and number. 

 The following recent observations on this subject by A. Cosens are im- 

 portant. The cells of the plant that immediately surround the larva 

 are known as nutritive cells. In Cynipidae the larva gradually with- 

 draws 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 



