1921] Uichanco — Reproduction in the Aphididce 101 



The Nature of Influence of Extremes of Temperature and 



Lessening of Food Supply in the Determination of 



Amphigony. 



It may be inferred from the foregoing considerations that 

 winter, with its attendant low temperatures and inadequate food 

 supply, plays a very important, if not an exclusive, part in the 

 determination of amphigony. Amphigony correspondingly be- 

 comes of less frequent occurrence as winter becomes less severe in 

 a locality, until in tropical regions where the temperature and, 

 incidentally, the food supply also are more equably maintained 

 throughout the year, amphigonous froms occur very rarely, if at all. 



In this connection, it is interesting to note that, although heter- 

 ogony has been brought about evidently as an adaptation to un- 

 favorable environmental factors, the effects of low temperatures do 

 not seem to be a necessary immediate stimulus in the production 

 of amphigonous individuals and that aphid strains normally under- 

 going heterogony may continue to produce an amphigonous gener- 

 ation and lay eggs at least in the fall following the spring of the 

 same year in which the insects from the open are taken into the 

 greenhouse. The following experimental evidence, which has led 

 me tentatively to arrive at this conclusion, is based on preliminary 

 observations, which will have to be confirmed by further investiga- 

 tions under more adequate control: 



In July, 1920, seedlings of Tanacetum vulgare Linna?us were 

 transferred to the greenhouse of the Bussey Institution. At about 

 the middle of August, after the plants had been well started, they 

 were inoculated with their common aphid pest, Macrosiphum tan- 

 aceti Linna?us. The host plants appeared to grow normally and the 

 aphids continually reproduced parthenogenetically until about the 

 end of October, 1920. In October the temperature outdoors began 

 to fall every now and then, especially at night, and at this time 

 the greenhouse was kept heated to an average of about 65 °F. This 

 temperature had been observed previously by Ewing (1916) to be 

 the optimum for aphids, in that it is least stimulating to the pro- 

 duction of wings in the viviparous forms; and this behavior may 

 be interpreted as an indication that the insects were under more 

 favorable environmental conditions than the individuals which 

 showed greater tendency to produce wings when subjected to other 



