Davidson — Biological Studies of Aphis rumicis L. 315 



later. In the loss of the wmgs, with the resulting change in body form, and 

 in the loss of the sensoria on the antennae, is indicated a degeneration 

 correlated with the more localized parasitic habits of the apterous form. 



It seems to me therefore that external factors can only be considered as 

 likely to exert an influence in restricting or favouring what is already present 

 as an inherent tendency, and not in produciug two complex dimorphic forms. 

 Doubtless, external factors — food and climatic seasonal conditions — extending 

 over long periods of time, have influenced the winged and apterous condition 

 in the agamic generations. 



(B). — Appearance of Sexual Forms. 



The early conception of the alternation of sexual and parthenogenetic 

 generations in the life cycle of Aphids was that external conditions — food 

 and temperature — were the chief factors accounting for the appearance of the 

 sexual forms and for bringing the parthenogenetic generations to an end. 

 The experiments of several early workers, such as De Geer, Kyber, and 

 others, whereby, with certain species of Aphids, parthenogenetic reproduction 

 was continued for a great number of generations, under favourable food and 

 temperature conditions, seemed to support this conception. Further, the 

 view readily fitted in with general observations on Aphids in the field, namely, 

 the widespread distribution of agamic females in the summer months and 

 the occurrence of sexual forms in the autumn. Comparatively little experi- 

 mental work on the Aphididae has been carried out, however ; and the views 

 held regarding Aphids are largely influenced by the results of the numerous 

 researches on other animals, particularly the Cladocera and Botif era, although 

 in these cases sex appears to be largely determined by the chromosome 

 complex. 



The literature on this subject has been so frequently reviewed that it is 

 not proposed to deal with it here. 



With the development of the cytological aspect of the life cycle of the 

 Aphididae, notably by Morgan, Stevens, Tanreuther, and von Baehr, it 

 became clear that internal factors, closely associated with the chromosomes, 

 must be considered of first importance. Whatever influence external factors 

 may exert, they can hardly be considered as determining the sexual forms, 

 although in the progress of adaptation and selection these factors have, 

 doubtless, played an important part in favouring or restricting the 

 parthenogenetic generations. 



The more recent results obtained by breeding species of Aphids through 

 several generations support the view that internal factors are the important 

 ones concerned. 



