238 



SUCKING INSECTS. 



Indian species has been traced throng'liont tlie yoav, hut on analogy with 

 species elsewhere, there is probably an alternate generation on another 



Fig. 286. 

 Winged and Wingless Plant Lice. {3IagniJied twelve times.) 



food-plant. These insects injure plants by the enormous quantity o£ sap 

 they extract. They also excrete a sweet li(|uid which, falUng- on the 

 leaves below, makes them sticky and shiny ; this appearance on the plant 

 is generally familiar in India. Ants and other insects are fond, of this 

 liquid and come to the plants to obtain it ; ants obtain it direct from the 

 plant lice, and it is well known that some ants use plant lice as we do 

 cows, not only " milking* ^^ them but preparing shelters for them and 

 caring- for them. 



Plant lice attack plants more often in cloudy weather. This .is 

 due partly to the fact that the winged insects fly in cloudy weather and 

 so spread, and partly to the more unhealthy condition of the plant. 

 As a rule, too, plant lice thrive in damp weather or when a moist wind is 

 blowing ; a dry hot wind often kills them so that their occurrence on the 

 crops is largely a matter of climatic conditions. These conditibns are 

 not as yet fully understood and plant lice sometimes thrive best in the 

 absence of rain. Not only climate, but the condition of the plant 

 affect the occurrence of plant lice, and the latter is a subject that has 

 jiot been investigated. Plant lice attack several crops in India and are 



