50 



Experimental evidence indicates that the sequence of winged 

 and apterous agamic females is largely due to some internal in- 

 herent tendency. w.v. tend to produce a. v. and a.v. may 

 produce entirely a.v. or a mixed progeny, consisting of a 

 variable percentage of winged forms. The apterous condition is 

 to be regarded as an adaptation, over a long period of time, to 

 seasonal food and temperature conditions. 



The appearance of sexual forms in the experiments — especially 

 having regard to the cytological investigations in Aphids — shows 

 that the change from the \ iviparous parthenogenetic phase to the 

 sexual phase is doubtless associated with the chromozome complex, 

 and not primarily due to food and temperature changes. 



The agamic generations appear to be interpolated between the 

 winter egg and the sexual generations as an adaptation to 

 seasonal conditions. 



Certain a.v. may produce agamic forms as well as sexml 

 forms. In some cultures which were kept in a greenhouse, a.v. 

 and sexuparce alatco (mothers of the oviparous females) developed 

 in every generation throughout winter from September to April. 



The degree of infestation for different species of plants varies 

 considerably. Thus, experimenting with several plants of the 

 same kind, the maximum total number of aphids produced from 

 one a.v. over a 14-day period, for each kind of plant, is shown in 

 the table below : — 



The higher figures obtained in Germany on Sugar Beet, Red 

 Beet and Alangolds, suggests a local adaptation of the species to 

 these food plants. Owing to other factors however, especially 

 temperature, it is diflficult to draw fine conclusions from any two 

 series of experiments not carried out under the same experimental 

 conditions. 



The relative susceptibility of different varieties of Broad Beans 

 was tested in 1920. Ten varieties were taken and 5 plants of each 

 variety were infected with one a.v. 



The average numbers of aphids produced from one a.v. on 

 the 5 plants of each variety over a 14-day period were : — 



