POTATO PLANT- LOUSE. 239 



migrant generation and with many species the migrants desert 

 the host plant upon which they have been feeding and seek a 

 plant of an entirely different species. Thus the plant louse 

 destructive to hops passes part of its life cycle upon plum trees. 

 This alternation of hosts is a point in the life history of Aphi- 

 didse of great economic significance, for it sometimes happens 

 that a species can be controlled on one plant and thus its attack 

 upon the alternate host be prevented. 



After spending a few weeks or a few months upon the second 

 host plant, winged individuals called fall migrants appear and 

 return to the same kind of plant, the winter host, upon which 

 the stem mother and the spring generations had lived, and 

 there continue the series of generations. Up to this time no 

 males have appeared and all of the forms, whether winged or 

 wingless, have been females giving birth to living young as 

 was the case with the "stem mother." But after the fall migra- 

 tion they are likely to develop the true sexes, males and egg- 

 laying females. These oviparous females deposit a few com- 

 paratively large eggs, in which stage the insect winters and 

 from which the stem mothers hatch in the spring. 



I,t is some such outline as the foregoing to which a species 

 whose life history is unknown must be referred as a working 

 basis. Any variation of the general life cycle of the plant lice, 

 however, is never a fair cause for surprise. One is quite likely 

 to find, for instance, that a certain species does not pass the 

 winter in the egg stage but as a subterranean form at the roots 

 of some plant. 



The difficulties as to life history studies presented by the 

 alternation of host plants common among the Aphididse are 

 augmented by the fact that certain differences in structure, 

 great enough to count as specific if occurring in other families 

 of insects, are common in different generations of a single 

 species of plant louse. It not infrequently happens, therefore, 

 that the same species may, when found upon different host 

 plants, be recorded as two or more distinct species and their 

 identity not suspected for years. Also, 2 actually distinct spe- 

 cies may resemble each other so closely in certain forms that 

 they are easily mistaken for one species. Moreover the specific 

 characters of the genus Nectarophora have not been system- 

 atically determined. 



