THE DOWNY PLANT-LICE. 241 
the peach-tree ( Aphis Per&icce of Sulzer) I cannot determine, 
for the want of a proper description of the latter. 
The injuries occasioned by plant-lice are much greater 
than would at first be expected from the small size and 
extreme weakness of the insects ; but these make up by 
their numbers what they want in strength individually, and 
thus become formidable enemies to vegetation. By their 
punctures, and the quantity of sap which they draw from 
the leaves, the functions of these important organs are de- 
ranged or interrupted, the food of the plant, which is there 
elaborated to nourish the stem and mature the fruit, is with- 
drawn, before it can reach its proper destination, or is con- 
taminated and left in a state unfitted to supply the wants 
of vegetation. 
Plants are differently affected by these insects. Some 
wither and cease to grow, their leaves and stems put on a 
sickly appearance, and soon die from exhaustion. Others, 
though not killed, are greatly impeded in their growth, and 
their tender parts, which are attacked, become stunted, 
curled, or warped. 
The punctures of these lice seem to poison some plants, 
and affect others in a most singular manner, producing 
warts or swellings, which are sometimes solid and some- 
times hollow, and contain in their interior a swarm of lice, 
the descendants of a single individual, whose punctures were 
the original cause of the tumor. I have seen reddish tumors 
of this kind, as big as a pigeon’s egg, growing upon leaves, 
to which they were attached by a slender neck, and con- 
taining thousands of small lice in their interior. Naturalists 
call these tumors galls, because they seem to be formed in 
the same way as the oak-galls which are used in the making 
of ink. The lice which inhabit or produce them generally 
differ from the others, in having shorter antennae, being 
without honey-tubes, and in frequently being clothed with 
a kind of white down, which, however, disappears when 
the insects become winged. 
81 
