GALLS CAUSED BY FLIES 
65 
is one of the few felt-like galls caused by the presence of 
dipterous larvae; the majority arise from the attacks of 
mites, and the felt often appears on the surface of a leaf 
which is normally quite smooth. 
The galls of Pervisia veronicae occur throughout the summer 
and autumn. The pouch in large specimens may be 
\ inch in diameter; the larvae are gregarious, usually seven 
or eight together in a gall. They are of a gorgeous orange 
hue, which deepens as they approach maturity. The pupae 
may be found in August, but larvae are present in some 
galls as late as the end of September. The gall laid open 
is a most attractive object to view with a binocular micro¬ 
scope, using a 2-inch objective and transmitted light. The 
lower margins of the two leaves forming the pouch are 
joined together; the upper are closely adpressed only. 
Sometimes the felting extends to the base and petiole of the 
second pair of leaves. The apex of all attacked leaves 
remains normal. A similar gall occurs on the Ground 
Ivy, caused by Pervisia glechomae. Pervisia veronicae occurs 
throughout Europe and the British Isles. On the Continent 
it attacks Veronica officinalis and V. Montana , causing similar 
galls, but I am not aware that they have been observed on 
these plants in Britain. 
The greenish gall induced by Pervisia urticae on the 
Common Nettle affords another instance of hypertrophy of 
hairs on the galled parts of a plant. Its European range 
equals that of the preceding. It is unilocular, containing 
but a single cavity, and multilarval, the cavity containing 
two or more larvae. Seldom solitary, these galls usually 
occur on the lower part of the leaf, with an elongated 
opening on the upper surface; but they appear occasionally 
on the stem and flower stalks, and are often tinted violet. 
The white larvae leave the gall in autumn, and pupate in 
the earth. 
The margins of the pinnules of the Common Bracken 
are often rolled inwards and greatly hypertrophied 
(Plate XVIII.) consequent upon the attack of Pervisia 
5 
