148 : PLANT LICE. 
below the surface of the plants they infest, they are not injured 
by the application of poisons to the plants.” 
The presence of plant-lice is made manifest in various ways. 
Sometimes the plant is sapped of its vitality and dies, or is im- 
poverished simply by loss of its sap. At other times an abnormal 
growth is produced in the leaf or stem on which the louse feeds. 
In the case of the currant-louse (Fig. 134, plate IX), as well 
Fic. 137a—Antenna of wingless forms of Fic. 137—Beak of plant louse; a, 
cabbage lice; b, antenna of winged proboscis of wheat-louse; b, lan- 
forms;c, beak of younglice; d, beak of cets contaimed in it, c, antenna 
winged form; e,claw. After Smith. showing the sensory pits; d, tar- 
sus. After Smith. 
as in that of the plum-louse, the twisted and bladder-like leaves 
present a shelter in which the young lice can be raised in safety. 
At other times again the abnormal growth extends to the forma- 
tion of a hollow gall, in which the family of lice live very much 
as in a small house. Our cottonwood trees produce a familiar 
example in the form of large terminal galls; such galls are always 
open to allow for the exit of their inmates. Fig. 135 shows a 
se 
