INTRODUCTION 
9 
inwards and thickened; each roll contains the larva of a 
fly, Pevrisia 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 Tetrancnva) 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 annulipes (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 Perrisia nrticae. 
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 Biorrhiza apteva 
(Plate IV.) are perhaps the best known. 
