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., Erineum and PhylUvium. We may instance the 

 rusty-brown patches so frequently seen on the under surface 

 of Alder leaves. Persoon christened them Erineum alneum ; 

 about a century later Fockeu gave the name of Eriophyes 

 hrevitarsus 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 



