GALLS INDUCED BY PLANT-LICE 87 
large slit occurs at the base of the neck of the gall at the 
time when the insects are ready to emerge. An Elm leaf 
bearing the scroll gall (a) of Schizoneura ulmi , also the 
capsular gall ( b ) of Tetraneura ulmi , are shown on the pre¬ 
ceding page. 
Some very interesting galls are caused by Aphidae of the 
genus Pemphigus on the leaves and petioles of our native 
Poplars, also the Lombardy Poplar ( Populus fastigiata). 
Three of these galls are shown in Plate XII.; all are in 
connexion with the Black Poplar ( Populus nigra), the tree 
specially affected by these Aphides. Fig. 5 depicts the 
scroll galls of Pemphigus a finis, showing the early and green 
state at ( d ), and the later brilliantly tinted one at (e). The 
pear-shaped galls of Pemphigus hursarius are shown at Fig. 6 
growing from buds. Fig. 7 depicts the apterous viviparous 
female, and Fig. 8 a pupa, both about five times the actual 
size. These galls may occur also on the petioles and 
leaves. On the former they are situated on the upper side 
of the groove, and the insects emerge by a small slit at a 
point remote from the petiole—that is, at the apex of the 
gall. The petiole is also attacked by P. spirothecae , and a 
very distinctive and curious spiral gall results. The edges 
of the grooved petiole swell up and arch over ; at the same 
time the petiole becomes spirally twisted. The swollen edges 
meet, but do not fuse. When the limit of growth is reached, 
they contract and separate, leaving a spiral slit through 
which the insects emerge. Fig. 9 shows a petiole with the 
gall of P. spirothecae , about half the actual size, and Fig. 10 
a magnified representation of the apterous viviparous female. 
We must now pass on to a consideration of some galls 
caused by Psyllids. The Psyllidae, or springing plant-lice, 
are minute insects with three ocelli and 8- to io-jointed thin 
antennae. They differ greatly in the early and adult stages, 
the legs and antennae varying in length and in the number 
of joints. There may be four or five moults. In the early 
stages of some species—for example, Trioza rhamni —the body 
has long, broad, and flat hairs, known as “ wax hairs,” which 
