GALLS CAUSED BY MITES 
99 
entirely due to the stimulus of the irritating presence of 
these minute creatures. Yet it is so. Were the mites not 
present upon the young leaves in spring, these hyper¬ 
trophies would not arise. Not all the occupants are the 
true causers. Epitremerus longitarsus is a commensal therein. 
This felt-gall was quite misunderstood by the older botanists. 
Persoon thought it was of fungoid origin, and described the 
fungus under the name of Erineum alneutn. I have received 
on more than one occasion Alder leaves bearing patches of 
this brown felt, with the request to name the fungus infest¬ 
ing them. The blunder is not an egregious one after all, for 
we often find patches on the leaves in autumn without even 
a solitary mite in occupancy. These growths are the 
summer residences of the mites, and they not infrequently 
quit them in early autumn, long before the supply of chloro¬ 
phyll has been cut off from the leaf, or even much diminished. 
The last to be described of our trio of Alder-leaf mite-galls 
is perhaps the best known. It is caused by Eriopliyes laevis. 
An individual gall is a little spherical pimple, about 2 mm. 
high, on the upper surface of the leaf. It is green at first, 
then various shades of yellow and brown, becoming either 
bright red or purple at maturity. These galls are usually 
densely gregarious (Plate XXL, Fig. 2), often occurring in 
hundreds on a single leaf. They are at their best in August, 
and not infrequently Alder bushes around ponds have the 
majority of their leaves attacked by the mites. A spray of 
leaves gathered from such a bush probably harbours millions 
of these mites! The orifice of the gall follows the rule in 
being on the under side of the leaf, and thus protected from 
rain. It is on a slight swelling surrounded by a circular 
canal; the interior of the gall is lined with cylindrical hairs, 
none being visible externally. 
The patches of brown hairs often occurring on the axils 
of the larger veins of the leaf are also caused by an 
Eriophyid, a species at present without a name. 
The influence of Eriopliyes macrotrichus on Hornbeam 
leaves is very curious, and deserves comment here. The 
