GALLS CAUSED BY MITES 
93 
Mites are rarely found in some galls with which they are 
known to be associated, e.g ., the red, pimple-like galls on 
Sycamore leaves, and the nail-like galls so frequently seen 
on the leaves of the Lime. This may probably be explained 
to some extent by their migratory habits. There is some¬ 
times great similarity in size and shape between certain galls 
caused by flies and mites. They may be distinguished 
easily, however, by macroscopic characters. Mite-galls 
always have a hole leading into them ; those caused by gall- 
gnats have no such opening, the larva being completely 
enclosed. 
The chief distinguishing feature of mite-galls is the felt of 
abnormal hairs which, with but few exceptions, covers the 
attacked part. These hairs arise from altered growth of the 
flat epidermal cells, which are stimulated, probably by a 
secretion from the mites, into outward growth. They assume 
various forms—in some galls they are quite simple and 
filiform, in others they are strongly clubbed at the apex, and 
resemble a miniature agaric. The pubescence is usually 
very dense, and the creatures are not easily distinguished, 
being so very minute, amidst the mass of tangled hairs in 
which they reside. They were quite overlooked by early 
botanists, who gave various names to the galls without 
being aware of the true cause of their production ; for 
instance, the red pustules on the upper surface of Alder 
leaves were designated Cephaloneon pustulatum by Bremi. 
Within recent years the mites which infest the hairs received 
the name of Eriophyes laevis Nalepa. The flat and often 
conspicuously coloured patches of pubescence on leaves were 
supposed to be caused by fungi, and were assigned to various 
genera— e.g. t Erineum and Phyllerium. We may instance the 
rusty-brown patches so frequently seen on the under surface 
of Alder leaves. Persoon christened them Erineum cilneum ; 
about a century later Fockeu gave the name of Eriophyes 
brevitarsus to the inhabitants of these miniature forests. The 
study of gall-causing mites is difficult, because various other 
mites associate with them, reaping a harvest where they 
