Wasps, Bees, and Ants 



469 



of the wriggling body. In some species the gall grows around and includes 

 but a single larva, in others around several to many. The larva reaches its 

 full development about coincidently with the 

 full growth or end of the vitality of the gall, 

 this period varying much with different galls. 

 In the galls on deciduous leaves the vitality 

 is shortest, ending in autumn; in twig-galls 

 it may not end until winter or even until the 

 following or indeed the .second winter. When 

 "dead" the gall dries and hardens, thus form- 

 ing a firm protecting chamber in which the larva 

 or larvae pupate. The pupa undergoes its non- 

 food-taking life securely housed in the dry gall, 

 which may fall with the autumn leaves or cling 

 to the bare twigs. From the galls the fully 

 developed flies gnaw their way out when new- 

 leaves and tender shoots are appearing, ready 

 to prick in new eggs for another life cycle. 



But, strange to say, with some species 

 the new eggs may be deposited on plants of 

 another kind and the hatching larva; stimulate 

 the growth of entirely different-shaped galls, 

 and they themselves develop into gall-flies 

 of markedly different appearance from their 

 mothers. These new gall-flies in their turn lay 

 eggs on the first host-plant; the forming galls 

 are like those of the grandparent generation 

 and the fully developed flies are of the grand- 

 parent kind. This alternation of generations — 



a condition in which a single species appears in two forms and produces 

 two kinds of galls, usually on different host-plants — has been long known, 

 but still remains a problem which interferes sadly with a number of popular 

 biological generalizations. One of these generations appears exclusively in 

 only one sex, the female, so that the other generation, composed of both 

 males and females, is produced uniformly from unfertilized eggs. The 

 adults and galls of the two generations were formerly described as belong- 

 ing to two different Cynipid species. Not all gall-flies, however, show this 

 dimorphic condition; some appear habitually in but one form and pro- 

 duce but one kind of gall; in most if not all of these cases the species is 

 represented only by female individuals. 



The great variety of the galls, the extraordinary instinct which leads the 

 adult flies to the right selection of plant and position on twig or leaf for ovi- 



FlG. 657. — Galls made by a 

 C)Tiipid gall-fly. (Natural 

 size.) 



