160 THE STUDY OF INSECTS. 



(My'sus per'si-cae). The last three are almost invariably 

 attended by ants. 



The Plant-lice of the genus Lachnus (Lach'nus) are 

 usually found on the limbs of trees and shrubs. To this 

 genus belong our largest Aphids. some of 

 them measuring one-fourth inch in length. 

 Figure 199 represents one of these enlarged. 

 Some species of plant-lice live both on 



FIG. 199. Lachnus. 



the roots and on the leaves of plants. One 

 of these is the Grape Phylloxera, Phylloxera vastatrix (Phyl- 

 lox-e'ra vas-ta'trix), which is the most important enemy of 

 the grape. The presence of this insect is manifested by the 

 vines in two ways : first, in the case of certain species of 

 grapes, there appear upon the lower surface of the leaves 

 fleshy swellings, which are more or less wrinkled and hairy ; 

 these are hollow galls, opening upon the upper surface of 

 the leaf, and containing a wingless agamic plant-louse and 

 her eggs ; second, when the fibrous roots of a sickly vine are 

 examined, we find, if the disease is due to this insect, that 

 the minute fibres have become swollen and knotty ; or, if 

 the disease is far advanced, they may be entirely decayed. 

 Upon these root-swellings we also find an agamic, wingless, 

 egg -laying plant-louse, the author of the mischief. The 

 insects found upon the roots differ slightly from those found 

 within the galls, but their specific identity is now generally 

 accepted. 



Owing to the great injury which this species has done to 

 the vineyards of France, hundreds of memoirs have been 

 published regarding it. But as yet no satisfactory means of 

 destroying it has been discovered. The difficulty lies in the 

 fact that the insecticide must be one that can penetrate the 

 ground to the depth of three or four feet, reaching all the 

 fibrous roots infested by the insect. It must be a substance 

 that can be cheaply applied on a large scale, and it must 

 also be something that will kill the insect without injury to 



