THE DOWNY PLANT-LICE. 241 



the peach-tree (Aphis Persicca 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. 

 31 



