472 Keport op the Entomologists of the 



thrust slightly into it The beak incloses the thread-like mouth 

 parts which the louse forces from the apex into the tissue, pro- 

 ducing a wound which causes the sap to flow. This liquid food 

 is pumped through the beak into the insect's body by means of 

 suctorial muscles. Thus each plant louse is literally a minute 

 pump, which robs the host plant of a portion of its sustenance. 



Nature op the Injury Caused by Plant Lice. 



Although all plant lice suck the sap of the host plant, the 

 direct injury which they do may be manifest in different ways. 

 Thus in some cases galls or pit-like depressions are formed on 

 the parts attacked, as is the case with the woolly aphis, and cer- 

 tain species which produce galls on the leaves. Most species, 

 however, which attack the leaves of cultivated fruits, cause them 

 to curl, and, if the lice are very abundant, to v. ither and finally 

 drop off. 



An indirect injury caused by these insects is through the honey 

 dew which they secrete. On badly infested trees the branches, 

 leaves and fruit become coated with this sweet liquid, which, in 

 the process of drying, becomes sticky. This sticky coating soon 

 turns black, because of a black fungus which readily grows in it. 

 Thus both the trees and fruit soon become unsightly, and, not 

 infrequently, the latter is made unfit for market. 



Life History. 



As the life history of these interesting insects is not usually 

 well understood by those not especially interested in the study of 

 insects, it may be well to state briefly the main points in their 

 development. The life history of different species varies, but 

 the following will serve as a general illustration: 



If a colony of plant lice is examined in the late spring or 

 earlv summer it will be found to consist of winged and 

 wingless females in all stages of development, from the newly 

 born larvae to the full grown individuals. The mature females 

 give birth to living young with astonishing frequency. 

 All of the young of these broods are females which mature 



