GALLS CAUSED BY FLIES 65 



is one of the few felt-like galls caused by the presence of 

 dipterous larvae; the majority arise from the attacks of 

 mites, and the felt often appears on the surface of a leaf 

 which is normally quite smooth. 



The galls of Perrisia veronicae occur throughout the summer 

 and autumn. The pouch in large specimens may be 

 ^ inch in diameter ; the larvae are gregarious, usually seven 

 or eight together in a gall. They are of a gorgeous orange 

 hue, which deepens as they approach maturity. The pupae 

 may be found in August, but larvae are present in some 

 galls as late as the end of September. The gall laid open 

 is a most attractive object to view with a binocular micro- 

 scope, using a 2-inch objective and transmitted light. The 

 lower margins of the two leaves forming the pouch are 

 joined together; the upper are closely adpressed only. 

 Sometimes the felting extends to the base and petiole of the 

 second pair of leaves. The apex of all attacked leaves 

 remains normal. A similar gall occurs on the Ground 

 Ivy, caused by Penisia gkckomae. Perrisia veronicae occurs 

 throughout Europe and the British Isles. On the Continent 

 it attacks Veronica officinalis and V. tnontana, causing similar 

 galls, but I am not aware that they have been observed on 

 these plants in Britain. 



The greenish gall induced by Perrisia urticae on the 

 Common Nettle affords another instance of hypertrophy of 

 hairs on the galled parts of a plant. Its European range 

 equals that of the preceding. It is unilocular, containing 

 but a single cavity, and multilarval, the cavity containing 

 two or more larvae. Seldom solitary, these galls usually 

 occur on the lower part of the leaf, with an elongated 

 opening on the upper surface ; but they appear occasionally 

 on the stem and flower stalks, and are often tinted violet. 

 The white larvae leave the gall in autumn, and pupate in 

 the earth. 



The margins of the pinnules of the Common Bracken 

 are often rolle|i inwards and greatly hypertrophied 

 (Plate XVIII.) consequent upon the attack of Perrisia 

 5 



