GALLS CAUSED BY FLIES 



69 



The cylindrical liairy outgrowths so frequently seen, in 

 shady situations, on the upper surface of leaves of the 

 Ground Ivy arise from the presence of the larvae of 

 Oligotrophus bursarius. The eggs are deposited in spring. 

 The gall is green at first, becoming reddish or purple at 

 maturity. It is about 4 mm. high, and contains a single 

 larva. The interior is smooth, but the aperture is sur- 

 rounded with hairs. These galls are usually gregarious, 

 falling away after the larvae have left them to pupate in 

 the earth, and leaving circular holes in the leaf. The 

 gall is sparsely covered with hairs. 



16 17 18 



Fig. 16 — Ground-Ivy Leaf with Galls cadskd by the prese nc 



OF THE Larvae of Oligotrophus bursarivs. (i/i.) 

 Fig. 17 — GRonND-IvY Leaf after the Galls have fallen 



AWAY, (i/i.) 

 Fig, 18 — Section of a Gall. (5/1.) 



The Dogwood is not attacked by many gall-causing 

 parasites. On the Continent, an Aphis, three mites, and 

 the dipteron Oligotrophus comi attack the leaves, and the 

 Neuropteron Lestes viridis Van der Lind causes little swell- 

 ings on the stem. The galls caused by these parasites 

 are obscure and of local distribution, excepting that of 

 Oligotrophus comi, which occurs throughout Europe, and is 

 the only insect which causes galls on the Dogwood in 

 this country. Its galls are large and very distinctive 

 (Plate IX., Fig. 7). They take the form of a truncated 

 cone, divided into two or three lobes at the apex, which is 



