GALLS CAUSED BY FLIES 69 
The cylindrical hairy 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. 
of the Larvae of Oligotrophus bursarius. ( 1 / 1 .) 
Fig. 17 Ground-Ivy Leaf after the Galls have fallen 
AWAY. (i/l.) 
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 covni 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 corni , 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 
