Dec. IS. 1919 A p pie-Grain A phis 317 



MIGRATION 



Summer migration, of course, is carried on mainly by the alate insects. 

 The apterous mothers, however, frequently wander from plant to plant. 

 They seldom deposit more than two or three young without changing 

 their position, and frequently the young from one mother will be found 

 on four or five different plants. In one case a mother deposited young 

 on four different plants in less than 48 hours. 



The young themselves, by the time they are two or three days old, 

 wander considerably. Usually they do not leave the plant on which 

 they were born, but occasionally they will. This is especially true in 

 case such plants furnish insufficient nourishment. 



As was found to be true with Aphis pomi,^ the alate insects occurring 

 during the summer evinced no particular tendency to leave the plants 

 on which they developed. Occasionally one was found wandering about 

 on the inner surface of the cage, but usually they remained on the plants, 

 fed, and reproduced without any more restlessness, certainly, than was 

 shown by the apterous insects. 



SUMMER APTEROUS FORM 

 DURATION OF LARVAL STAGES 



The immature stages of the summer apterous form covered periods 

 varying from 6 to 12 days, the length of the period being almost wholly 

 controlled by temperature conditions. Out of several hundred experi- 

 ments with this form, evidence of an effect on the duration of the 

 nymphal period could be traced to food conditions in only two cases. 

 In both of these experiments the young aphids were living on very 

 poor plants, and in both they required about 2 more days to attain 

 maturity than did the insects born on the same day in other experiments. 



The chart in figure i shows how important the effect of the temperature 

 is upon the length of the larval period. The figures for temperature read 

 down and those for number of days read up, since the length of the 

 period varies inversely with some factor of the temperature, and this 

 method allows the curves to parallel instead of cross and so facilitates 

 the reading of the chart. It must be remembered that this chart shows 

 the temperature effect only in a general v/ay, since the length of the 

 periods, while listed in whole number of days, actually varies from the 

 number by a few or possibly many hours in many cases. Since observa- 

 tions were made only once a day it is impossible to give more accurate 

 figures than are given here. Moreover, in certain individual cases in 

 which the mean temperatures varied greatly during a given period, the 

 retardation caused by low temperatures during one period of the life of 

 the immature aphid was found to be not wholly compensated for by 



1 Baker. A. C, and Turner, W. F. op. cit. 



