60 



ARTHROPODA 



fifteen hundred species lay their eggs in 

 some part of a plant, and the develop- 

 ing larvaB by their excretions cause 

 an overgrowth of tissue producing an 

 enlargement called a gall. The oaks 

 are most frequently affected. Each 

 species forms its characteristic gall 

 FIG. 43. Photograph an d always on the same kind of plant, 

 of oak gaiifly en- ^ g^i opened hi May or June will 



larged twice. . ..,.", 



usually contain a single wormlike white 

 larva. The flies may be secured from many of the galls 

 by tying a piece of cheesecloth over them in early June 



FIG. 44. On the left three galls or oak apples, of which two are cut open, a, 

 larva. On the right an oak gall about one month old. Photograph by Overton. 



or by merely collecting them about the middle of June 

 and placing in a tight box until the flies appear. Some- 



