146 PLANT LICE. 
“The number of agamic generations that may follow with- 
out the intervention of sexual forms varies with different species, 
and, in some cases at least, varies in the same species, depend- 
ing upon temperature and other conditions. Thus Kyber, in the 
early part of this century, succeeded, by keeping the insects in a 
warm room, in raising a series of agamic generations of two 
species of Aphids, which extended through four years without 
the intervention of sexual forms. 
“As already indicated, the agamic generations are of two 
forms, wingless and winged. Each of these has a peculiar func- 
tion in the economy of the species. The wingless generations, 
which are usually the more numerous, by their great fecundity 
provide for the enormous and rapid multiplication of individuals, 
which is so characteristic of these insects. But this great in- 
crease of individuals would be disastrous to the species, by the 
destruction of the infested plants and the consequent starving 
of the insects, were it not supplemented by other powers. We 
find, therefore, interspersed among these wingless sedentary gen- 
erations, generations which are winged and migrating. Thus the 
spread of the species is provided for. 
“Generally on the setting in of cold weather, or in some 
cases on the failure of nourishment, the weather being still warm, 
there is produced a generation including individuals of both 
sexes. The males may be either winged or wingless; but, so 
far as is known, the females that pair with the males are always 
wingless. These females, after becoming impregnated, produce 
the winter eggs; thus is completed the cycle of changes through 
which the species passes. In many cases, at least, the individuals 
of the agamic generation that immediately precedes the sexual 
one produce but few pseudova; from these pseudova the sexual 
individuals emerge, not as larve, but as fully developed indi- 
viduals, ready to pair and reproduce; in fact in the cases referred 
to, the sexual individuals have the mouth-parts in a rudimentary 
state, and take no nourishment. In many species the impreg- 
nated female produces a single egg, which is nearly as large as 
the insect herself; frequently this egg is not laid, but remains 
throughout the winter in the dry skin of the dead parent. 
“Agamic Aphids may hibernate, and may coexist with the 
sexual generation of the same species. 
