LIFE CYCLE OF HYDATINA SENTA 163 



paper for Hydatina, namely, that the accumulation of substances 

 in the water towards the end of a family may be conducive to 

 parthenogenesis, may possibly apply, mutatis mutandis, to daph- 

 nians. Until we are assured that the water into which a late 

 brood was hatched was of the same chemical composition as at the 

 beginning of the family, we must still consider the possibility that 

 chemical substances, and not a factor connected with the age of 

 the parent, are responsible for the difference in the sexual tendency 

 in different parts of the family. In view of these considerations 

 it is highly important that attempts be made to find single sub- 

 stances, or definitely known combinations of substances, which 

 will modify the life cycle of daphnians. 



Among the aphids the question is somewhat different. The 

 medium in which these insects live is not suitable for conveying 

 many chemical substances. Nevertheless, the character of their 

 food plant may change chemically, and it isposs ble that this chemi- 

 cal change works large changes in the cycle of the aphids. Such 

 chemical changes doubtless occur in autumn with the aging of the 

 food plants. If the plants are reared under such conditions as 

 fail to bring about the chemical changes which usually occur at 

 that season, what influence may this not exert upon the life cycle 

 of the insects? 



I desire not to be misunderstood here because of certain terms 

 I have used. If we are to ascribe the influence of external condi- 

 tions on the cycle of the aphids to the chemical nature of their 

 food plants, it might seem that this were a question of nutrition, 

 whereas we have discarded starvation as a probable factor in- 

 fluencing the cycle in Hydatina. It need only be pointed out in 

 tills connection that those who have advocated the view that nutri- 

 tion influences any of these alternating cycles have referred to 

 quantity, not chemical quality, of food. It may even be suggested 

 by some that, in rotifers, where chemical substances have been 

 proven to be effective agents, these substances act only indirectly 

 by stimulating or otherwise affecting nutrition. Even if this be 

 true, the real agent is not quantity of food available, as was for- 

 m.erly held to be the case. 



