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 w^ill also, under 

 compulsion, Hve 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 Hnes 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 



