ORDER HYMENOPTERA. 



167 



sibly a slight amount of poison conveyed by the ovipositor, causes 

 an abnormal growth of the plant-cells, until a tumor or gall is formed 

 within which the larva hatches and lives. Each species of fly 

 produces a peculiar gall, which is charac- 

 teristic of that species. Great numbers of 

 female gall-flies are agamous, i.e., are pro- 

 duced from unfertilized eggs, there being 

 no males in existence. Thus Cynips quer- 

 cus-aciculata O. Sack., which produces a 

 large gall in the autumn, in the spring of 

 the year succeeding lays eggs which pro- 

 duce^ galls disclosing i dimorphic form, Fia. 210.-Gall-fly of oak. 

 originally thought to be a distinct species, and named G. q. spongi- 

 flca by Osten Sacken. The autumn brood of this Cynips consists 

 entirely of agamous females, while the spring brood consists of both 

 males and females; there is thus an alternation of generations, and 

 this is the case in many species. In some cases the two generations 

 belong to what were originally described as separate genera. Thus 

 Adler observed that the" European Neuroterus lenticularis produces 

 galls of a certain form on the under surface of oak-leaves, the galls 

 falling off in the autumn, and the fly appearing in the early spring. 

 It then deposits its eggs on the buds of the oak, which produce 

 upon the leaves and stalks of the male flowers galls unlike those of 

 the preceding autumn. Moreover, the fly which emerges from 

 them has been referred to a separate genus as Spathegaster baccarum. 

 This in turn lays eggs which produce the original Neuroterus form; 

 and the Neuroterus generation consists of females alone, while the 

 Spathegasters are of both sexes. 

 The gall-flies fall into two sections, the first containing the true 



FIG. 2U.Evania Icevigata, male, and pupa. 



gall-flies (Pseuides) and the guest gall-flies (Inquilines, which are 

 commensals or boarders living at the expense of the true gall-flies), 

 with the peculiar genus Ibalia, while the second includes the Figi- 

 tides, which are parasitic. In the sub-family Figitinoe the abdomen 

 is elongate-ovate, compressed, with the apex more or less pointed. 



