924 



AN INTRODUCTION TO ENTOMOLOGY 



In those species where an alternation of generations exists, one 

 generation consists only of agamic females while the other consists 

 of both males and females, which reproduce sexually. In some cases 

 the galls produced by the two generations are quite similar; but in 

 others they are very different and are found on different parts of the 

 host plant. For an example of this see the account of the hedgehog 

 gall-fly, Andricus erinacei, given below. 



The guest gall-flies or inqui lines. — Some species of this subfamily 

 do not form galls but lay their eggs in the galls made by other species. 

 The larvae of these inquilines feed upon the galls produced by their 

 hosts and in some cases do not discommode the owners of the galls 

 in the least. But some guest gall-flies are parasites as well as guests. 

 For example,- Triggerson in his study of the hedgehog gall-fly ('14) 

 found that the larva of Synergus erindcei, a guest in the hedgehog 

 gall, mines from cavity to cavity of the gall and feeds on the occupant 

 of each in turn. 



Among the more conspicuous of our cynipid galls are the following. 

 The oak hedgehog gall, Andricus erindcei.- — ^A common gall on the 

 leaves of white oak is one known as the oak hedgehog gall. This gall 

 is rounded or oblong, with the surface finely 

 netted with fissures, and more or less densely 

 covered with spines (Fig. 11 61, a). It varies 

 in length from 10 mm. to 15 mm., and occurs 

 on both sides of the leaves. The point of at- 

 tachment is generally on the midrib, though it 

 is often found on the lateral veins. When young 

 the gall is yellowish green, but in autumn it 

 becomes yellowish brown. This gall is poly- 

 thalamous, containing from two to eight larval 

 cells (Fig. 1 161, b). 



Within the hedgehog galls is developed one 

 generation, the agamic one, oi Andricus erinacei. 

 The alternating generation, the sexual one, is 

 developed in very different galls made on the 

 terminal growing points of buds and bud-scales. 

 These are small, thin-walled, elongate, egg- 

 shaped galls, from 2 to 3 mm. in length, and are monothalamous. 

 The life-cycle of this species was very carefully worked out by 

 Triggerson ('14), from whose account the data given here are com- 

 piled. The two generations are distinguished by the use of trinominal 

 names, as follows. 



Andricus erinacei erinacei. — This is the agamic form, which is de- 

 veloped in the oak hedgehog galls. In this form the wings are vestigial 

 not twice the length of the scutellum. The adults emerge in November 

 and deposit their eggs in the leaf- and flower-buds. 



Andricus erinacei bicolens.- — 'This is the sexual form, which is de- 

 veloped in the galls formed in the buds. The larvas hatch in May, 

 from eggs laid by the agamic form the previous autumn and produce 

 the galls known as the soft oak-bud galls. The adults, male and 

 female, emerge early in June, and the females lay their eggs in the 



Fig. 1 161. — The oak 



hedgehog gall: a, 

 gall on leaf; b, sec- 

 tion of gall. 



