FOREST LIFE. 



213 



Fig. lEo.— a gall-fly. 



galls which are much more abundant than those of the 



gall-flies ; these are the galls made by plant-lice and 



by mites. The galls 



produced by plant-lice, 



and also those produced 



by mites, differ from the 



galls of the gall-flies in 



tliat each gall has an 



open mouth. 



The conical galls 

 which are so common 

 on the leaves of witch- 

 hazel (Fig. 181) are good 

 illustrations of this type 

 of gall. These galls project from the upper side of 

 the leaf, but each has an opening on the lower side of 

 the leaf. The plant-louse that produces this gall is 

 an agamic female (see page 177); when this female 

 is mature, she gives birth to numerous young, which 



escape from the mouth 

 of the gall, scatter over 

 the leaf, and each in 

 turn produces a gall. 



In most localities 

 there are so many kinds 

 of galls that it would be 

 unwise for a student to 

 attempt to study them 

 all. A better plan is to 

 select some one species 

 or genus of trees and to study the galls made on 

 these trees by one family of insects. Thus, if oaks 

 occur in the locality, a good subject is the oak galls 



Fig. 181. 



