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 hngitarsus 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 alneum. I have received 

 on more than one occasion Alder leaves bearing patches of 

 this'iirown 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 Eriophyes 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 Eriophyes macrotrichus on Hornbeam 

 leaves is very curious, and deserves comment here. The 



