94 
BRITISH GALLS 
have not sown. The greatest living authority on the Vevmi- 
formia is Dr. Alfred Nalepa, the distinguished Viennese 
zoologist. Notwithstanding his patient and long-continued 
investigations, the field of research is so vast that practically 
only a fragment of it has been explored. 
There are about fifty species of British gall-causing mites ; 
the great majority belong to the genus Eviophyes. Of the 
species belonging to the four other British genera, Monochetus 
sulcatus gives rise to pod-like galls on leaf buds of the Beech ; 
Epitrimevus trilobus causes the leaf margins of the Common 
Elder to roll upwards, forming a pouch ; Tarsonemus spirifex 
is probably responsible for the looped swellings sometimes 
seen on stems of the Mat Grass; Phyllocoptes acericola causes 
a slight swelling on the upper surface of Sycamore leaves, 
the depression on the underside being clothed with hairs; and 
Phyllocoptes fraxini attacks Ash leaves. The margins become 
tightly rolled towards the lower surface; the interior of the 
roll is lined with hairs. 
We must now comment upon some galls caused by 
species of Eviophyes. 
E. similis commonly galls Blackthorn leaves; the margin 
of the leaf becomes more or less swollen (Plate XXI., Fig. i). 
An individual gall is pimple-like, light green at first, 
becoming red or brownish at maturity. The galls are usually 
present in great numbers, becoming confluent, when they 
considerably distort the leaves. They are abundant in June 
and July. 
Another common mite-gall occurs on Hawthorn leaves, 
probably often escaping observation though its unattractive 
appearance; it is caused by Eviophyes goniothovax. Here, 
again, the margin is the part usually attacked; it becomes 
involute and slightly thickened; sometimes the incurvature 
extends to the midrib. The affected part is always a lighter 
tint than the rest of the leaf. 
The radical leaves of the Great Knapweed are sometimes 
infested with E. centauveae , causing pustules, which are 
greenish at first (Plate XXI., Fig. 4), becoming violet-black 
