INTRODUCTION 9 



inwards and thickened; each roll contains the larva of a 

 fly, Pemsia filicina. 



(b) Pocket Galls 

 Hollow protuberances arising from the tissue of the leaf 

 lamina and forming an excavated chamber. These galls 

 exhibit great variety of form and internal structure. Types 

 caused by aphides {Schizoneura and Tetraneura) on Elm 

 leaves are illustrated in Chapter VI. ; others on leaves of 

 Alder and Mealy Guelder Rose arising from stimulus by 

 mites are shown in Plate XXI. See also the familiar nail- 

 gall on the upper surface of Beech leaves caused by the 

 dipteron Oligotrophus annuUpes (Plate IX.). 



(c) Covering Galls 



Hollow protuberances rising around the animal, the up- 

 ward growth and over-arching of the tissue finally forming 

 a roofed-in chamber; the tissues, however, do not fuse 

 when they meet, and they ultimately contract and shrivel, 

 leaving a slit for the exit of the insects. A well-known 

 example is the gall on the leaves and petioles of the 

 common Nettle, caused by the gall-gnat Pemsia uriicae. 



3. Solid or Tubercular Galls 



Hypertrophy takes place around the larva, which is 

 hatched from an egg deposited in the tissues; hence the 

 chamber is never lined with part of the original outer 

 surface of the afflicted area. Amongst the numerous galls 

 of this group may be mentioned those caused by gall-wasps 

 on leaves of Oak, Willow, and Briar (Plates II., III. and V.). 



4. Root Galls 



Spherical outgrowths of variable size on the roots of 

 many trees and plants. Some result from the attacks of 

 eelworms, others from insect irritation. The galls on the 

 root of the common Oak caused by Bionhiza aptera 

 (Plate IV.) are perhaps the best known. 



