Davidson — Biological Studies of Aphis rumicis L. 313 



plaut, winged forms always appear in due course. Although at the time that 

 winged forms appear in great numbers the plant may be heavily infested, 

 the appearance of the winged forms is not necessarily connected with the 

 condition of the plant, but due to the innate tendency of the apterous females 

 to produce winged forms as described above. It would seem that the 

 overlooking of this fact has clouded the observations of earlier observers, 

 who correlated the conditions of the plant with the appearance of winged 

 forms. 



The sequence of the development of winged foims on Eumex, Euomjmus 

 and Beans is well shown in iigs. 1 and 3. 



The percentage of winged forms which may develop in any generation 

 as offspring of a. v. ? ? , varies considerably. 



The tendency for a higher percentage of winged forms to develop in some 

 cases on Sugar Beet, lied Beet, and Poppies, and also on some varieties of 

 Broad Beans, has been observed in my experiments.^ It is not at all clear, 

 however, that this can be considered as due to the nature of the cell sap of 

 the different plants acting on the Aphid metabolism. Owing to the wide 

 variation in the numbers of winged forms produced, it seems that factors 

 other than food and temperatui-e must be looked for. 



In these experiments temperature and humidity varied, but the conditions 

 were approximately the same for any series of experiments extending over 

 the same period. 



Prom my observations one is led to the view that the appearance of 

 winged forms is determined by internal factors. It seems probable, however, 

 that environmental conditions may exert an influence by restricting or 

 favouring metabolism, and thus affecting tlie production of winged forms ; 

 but further experimental investigations in relation to temperature, humidity, 

 and food factors and further cytologieal studies are necessary. 



It seems to the author that the methods adopted for breeding Aphids 

 may largely account for the conflicting results obtained by various workers. 

 When one considers the number of offspring that an Aphid mother can 

 produce, it is obvious that only an extremely small percentage of the Aphids 

 in any generation can be isolated to carry on further generations. 



From general observations on the progress of Aphis rumicis in colonies, 

 there is a clearly indicated tendency for apterous viviparous females to 

 eventually produce some winged forms. This inherent tendency may be more 

 developed in some strains than in others, and a big element of chance occurs 

 in the selection of Aphid mothers to carry on the generations. 



1 Vide Davidson, .J. (1921 "). 



