94 BRITISH GALLS 



have not sown. The greatest living authority on the Vermi- 

 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 speciiss of British gall-causing mites ; 

 the great majority belong to the genus Eriophyes. Of the 

 species belonging to the four other British genera, Monochetus 

 sukatus gives rise to pod-like galls on leaf buds of the Beech ; 

 Epitnmerus 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 Eriophyes. 



E. similis commonly galls Blackthorn leaves ; the margin 

 of the leaf becomes more or less swollen (Plate XXL, Fig. i). 

 An individual gall is piijiple-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 Eriophyes goniothorax. 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. centaweae, causing pustules, which are 

 greenish at first (Plate XXL, Fig. 4), becoming violet-black 



