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AN ECONOMIC ENTOMOLOGY. 



observed within a period of twenty-four hours from one specimen, 

 and it is not unusual to find, early in the season, a single large 

 louse surrounded by a group of anywhere from a dozen to 

 twenty or even more small specimens. The rate of growth also 

 varies, depending upon the weather ; indeed weather conditions, 

 early in the season, frequently determine the question of whether 

 or not certain species are to become injurious later on. A warm, 

 moist temperature favors their development, and reproduction 

 goes on at a rapid rate. Correspondingly, cold, wet weather 

 checks development, and may even destroy a large number, 

 especially of the young. Plant-lice, in their younger stages, are 

 exceedingly susceptible to sudden changes of temperature, and 

 at almost any time in the season a sudden drop of from fifteen to 

 twenty degrees, accompanied by a rain, will prove fatal to a great 

 proportion of them. But assuming that all is favorable, the 

 young that were first brought forth are in turn ready to repro- 

 duce in five or six days, and they also form little colonies ; this 

 method of reproduction continuing as long as food is plenty and 

 the weather mild. Experimentally, reproduction of this kind 

 has been continued for several years in succession, without any 

 tendency to develop sexed individuals or to produce eggs. At 

 almost any time after the first generation, specimens may become 

 winged, and these fly to other localities, forming new colonies 

 wherever suitable food is found. In this way they spread, and, 

 though they may have started from a single favorable locality, 

 yet in the course of a few weeks they may cover many hundreds 

 of acres. Exactly what determines the formation of wings in 

 some specimens and not in others is not known. We do know, 

 however, that the progeny of a single individual is variable, and 

 that while some become winged others do not ; but whether 

 winged or wingless, the specimens are equally without sex, and 

 all are viviparous, or bring forth living young. As the summer 

 advances, reproduction becomes less rapid. Plants tend to dry, 

 the supply of sap is not so plenty, and these features become 

 more marked through the autumn months until, with the ap- 

 proach of cold weather, plant growth entirely ceases. It becomes 

 necessary now to provide for the continuation of the species 

 during winter, and sexed forms are developed. The males are 

 winged and usually appear a short time before the females, which 



