LIFE CYCLE OF HYDATINA SENTA 161 



he calls the period of "transitioD from parthenogenesis to sexu- 

 ality," characterized in Moina by violet-blue eggs, during which 

 warmth induces parthenogenesis, and cold sexuality. 



The phenomena in aphids have not been so thoroughly studied, 

 probably because the external agents acting on them are not so 

 easy to manipulate. The chief experiments that have been per- 

 formed have consisted in keeping the aphids, and of course their 

 food plants, in a warm place when cold weather advanced. Thus 

 Kyber ('15) reared aphids parthenogenetically during a period of 

 several years; and much more recently Slingerland ('93) has re- 

 peated the experiment with the same result. Slingerland 's pub- 

 lished data include 62 generations in 33 months, but I learned from 

 Prof. Slingerland that the experiment was continued until more 

 than ninety generations had been secured, without a single sexual 

 form. Other experimenters and observers, some of them earlier 

 even than Kyber, have agreed with these conclusions ; but none 

 have worked more extensively, hence it is needless to refer to them 

 individually. Mention may also be made of the work on the corn 

 root-aphis by Davis ('09), who has shown that the sexual forms 

 are not associated with given generations. He was inclined to 

 attribute the occurrence of sexual forms in his experiments to 

 sea onal changes of temperature, though his experiments did not 

 bear directly on this point. It is thus clear that external agents of 

 some kind influence the cycle of some aphids. On the other hand, 

 there are aphids in which the cycle is a closed one, the sexual 

 females and males appearing in nature in a given generation, and 

 attempts made to alter the cycle have so far proven fruitless. In 

 the aphids, therefore, as in the rotifers and daphnians, it seems 

 that both external and internal agents are effective in producing 

 the life cycle. 



This brief review of the work on parthenogenesis and sexual 

 reproduction in daphnians and aphids is given merely that we may 

 compare these two groups with the rotifers, and suggest that some 

 light may be thrown upon the phenomena in the first two groups 

 by our findings in the last Too little is as yet known of the inter- 

 nal factors to make a comparison profitable; but regarding exter- 

 nal agents there is something to be said. The discovery that 



