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 («) of Schizoneura ulmi, also the 

 capsular gall (J) of Tetmneuva 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 nigfa), the tree 

 specially affected by these Aphides. Fig. 5 depicts the 

 scroll galls of Pemphigus affinis, showing the early and green 

 state at {d), and the later brilliantly tinted one at (e). The 

 pear-shaped galls of Pemphigus bursarius axe 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 lo-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," whiph 



