HYMENOPTERA OAK GALLS. 151 



to the more upright twig. The portion thus affected seldom does any- 

 more good, hut, when this occurs on minor twigs, the tree is not 

 appreciably injured (fig. 145). When, however, several galls are on 

 one leaf and very abundant on a young tree, the leaves are arrested in 

 their development, and the result in the aggregate would diminish 

 growth and cause disfiguration. The gall itself is an irregular swell- 

 ing, of a green colour, and poddike in structure. When opened, the 

 central cavity is comparatively large, and contains a small brownish 

 seeddike pod, which is the inner gall, wherein is found the larval form 



Fig. 14d. Galls of Aphilothrix quadnlineata onjluwtrs of oak. 



of the gall-fiy. This inner gall is not attached to the wall of the outer 

 gall. The galls may be looked for in May, and the flies appear in June. 

 The fly is about 1.5 to 2 mm. in size. Body and antenna? shining 

 black ; legs lighter in colour ; wings more or less hyaline. 



The two following species of the Aphilothrix group are not 

 associated by Adler as connected with the alternation of generations, 

 but are nevertheless more or less common. 



Aphilothrix quadrilineata (Htg.) 



The gall of this species, fig. 146, is found on the flowering catkins 

 about the end of May or beginning of June. It is very small in size, 



