The Gall-Flies 



that the little grubs found within it were the result of spontaneous 

 generation. Later it was supposed that galls were caused by the 

 punctures of insects and the injection of a poisonous liquid. 



With the true gall-flies, 

 however, the gall appar- 

 ently does not commence 

 to form until after the egg 

 hatches. It is supposed 

 that the larva secretes a 

 liquid which causes the 

 abnormal growth of the 

 plant, the plant cells 

 which are most active in 

 growth and subdivision 

 being directly affected. 



The egg of the gall- 

 fly is slender, and has a 

 very long petiole which 

 is six to ten times the 

 length of the egg body, 

 and this is inserted by 

 means of a very long 

 curiously formed ovi- 

 positor. A good account 

 of the method of ovipo- 

 sition reported by Riley 

 from observations made 

 by Pergande will be 

 found in the Proceedings 

 of the Entomological Society of Washington (Vol. III. pp. 

 260-263). 



Most of the Cynipoids make galls upon oak. Others, how- 

 ever, are found upon rose bushes. All parts of the plant are 

 affected — roots, stems, twigs and leaves, as well as leaf petioles. 

 In some galls but a single larva develops, while in others very 

 many develop. The oak galls of commerce are European galls, 

 but some of our native galls would undoubtedly be found to 

 possess commercial value through the quantities of tannin they 

 possess. 



The origin of tannin in galls has been the subject of investi- 



54 



Fig. 28. — Diastrophus nebulosus. 

 fA/Wr Riley.) 



