Davidson — Biological Studies of Aphis rumicis L. 317 



In this respecL it is interesting to refer to the life cycle of Ap/iis saliceti, 

 a species in which the number of parthenogenetic generations is very limited, 

 the sexual forms appearing in early summer. As shown by Klodnitzki, the 

 ova hatch out in April and May. Some of the Fundatrices produce S 6 and 

 sexual + ° ; others produce w. v. ° +, which migrate to other willow trees, 

 and produce ^ (^ and sexual + +. 



Passing now to Aphis rumicis, it is seen that the agamic generations are 

 further extended over the favourable seasons of the year. Normally in 

 nature, the approach of winter conditions will be the important factor in 

 bringing these agamic generations to an end. 



In view of the eytological investigations in Aphids, it seems evident that 

 some factor or factors associated with an adaptation of Aphids to seasonal 

 conditions cause a radical change in ehromozome segregation, resulting in 

 the development of sexual forms, and the production of ova at a period which 

 ensures the continuation of the species. 



It seems feasible to expect that with an extension of favourable seasonal 

 conditions, such as would obtain in a warmer climate, there would be a 

 corresponding extension (due to adaptation] of the agamic generations. This 

 appears to be the case in Aphis avenae Fabr., and Toxoptera graminum 

 Eond. in America. 



It is clear from my experiments that certain of the apterous, 

 parthenogenetic females may carry on the parthenogenetic strain throughout 

 winter if given favourable food and temperature conditions. This cannot, 

 however, be considered as wholly due to these two factors, because sexual 

 forms and agamic forms appear together in each generation under the same 

 environmental conditions. It is, I think, just a case of parthenogenetic + °, 

 which normally on the secondary host-plants would have died off on the 

 approach of unfavourable weather conditions, being saved by keeping them 

 in a suitable environment. 



One cannot say at present how far these cases occur in the Aphididae. 

 It is highly probable that in breeding experiments, considering the very 

 small percentage of the offspring used in each generation, it is only by 

 chance that individuals are selected which have the continuing partheno- 

 genetic tendency. 



It is seen in the series Broad Beans B, B,, Bj, &c., as illustrated in 

 fig. 3, that parthenogenetic individuals were carried on Beans from May 

 onwards throughout the winter. In winter the plants were kept in a warm 

 green-house, and fresh Bean plants were raised in succession for each genera- 

 tion. Towards October, sexual forms appeared, and in some lines of the 

 cultures the cycle closed by the production of these forms. Some of the 



