THE INSECT WORLD. 



377 



out devouring any part of its vegetable envelope, changes to a 

 pupa, and emerges from the gall, leaving it intact, except for 

 the hole through which it emerged. It seems, therefore, as if 

 certain abnormal plant ex- 

 udations secreted in the ^^^- 433- 

 gall form the actual food 

 of the larva, and it is a 

 curious and interesting fact 

 that the same species in- 

 variably produces in the 

 same place the same kind 

 of gall. In other words, 

 the appearance of the gall 

 is an unfailing index for An oak gaii-fly. 

 the special student to the 



species that made it. Many interesting problems are connected 

 with the study of these gall-flies ; for instance, in some species 

 both sexes are present early in the season, but in the second or 

 midsummer brood females alone make their appearance. The 



Fig. 434. 



^<5^£^ 



Gall made by the larva of Cynips q. spongijica.—a, larva in its cell ; *, point 

 of exit of adult. 



progeny of this female form, which may or may not resemble 

 the spring form, in turn produces males as well as females. Of 

 other species no males are yet known, and, so far as we have 



