324 Experimental Zoology 
process continues throughout the summer, but even during this 
time, especially if the food plant becomes crowded, winged indi- 
viduals appear, and these are also parthenogenetic. They 
usually fly away when the sunlight falls on them, and if they are 
fortunate enough to find a new plant of the right kind they start 
there a new generation of parthenogenetic forms. In the au- 
tumn the males and females appear. The males are winged, the 
females are wingless. The parents of the males are also wing- 
less, but the parents of the sexual females are winged, these 
conditions varying, however, in different species of aphids. In 
some both the males and sexual females are winged. More- 
over, according to Lichtenstein, while it is usual for the mother 
of the sexual forms to produce only males or only females, in 
certain species both sexes are produced by the same parent. 
The males and females pair, and the sexual eggs are deposited 
on the food plant, where they remain over winter. In the spring 
only parthenogenetic females hatch from these eggs. Even 
if kept in a warm place, the winter eggs will not develop until 
several months have passed. 
In cases like this one of the rose aphid, the whole cycle com- 
pletes itself on the same plant. The rose aphid will also, under 
compulsion, live on other plants and even multiply there with 
great rapidity, as on the dock, for example. I have tried in a 
few cases to produce the sexual forms by changing the food plant, 
but so far without success. 
Balbiani has discovered the important fact that the same 
female may give birth to parthenogenetic individuals, as well as 
to sexual females and males. This result removes all grounds 
for the assumption that there are two lines of parthenogenetic 
individuals, — one culminating with males, the other with sexual 
females. Balbiani isolated the females of Centaurea jacea at 
the time of year when the transition from the parthenogenetic 
to the sexual mode of reproduction was taking place. Each day 
the number and the kind of young produced was noted. The 
number varied from one to seven per diem. After a day of great 
productiveness there followed one or more days when no young 
