104 Fmjcl^e [June 



formulate definite conclusions. The present observations, how- 

 ever, suggest the following preliminary deductions: 



1. Heterogon}' in certain aphids of temperate climates has 

 probably become a rhythmic process, occurring regularly at definite 

 periods in their yearly cycle of generations and independently, for 

 a period at least, of the immediate stimulus brought about by 

 adverse temperature conditions. 



2. Amphigonous reproduction - in these aphids, although evi- 

 dently maintained as an adaptation to, and under the influence of 

 adverse climatic conditions, continues to occur at these definite 

 cyclical intervals for some time after the causative factors have 

 been removed. 



The foregoing views find additional support in the fact that in 

 nature in temperate climates amphigonous and parthenogenetic 

 individuals of identical species in the same locality and feeding 

 on the same parts of a host plant live side by side for considerable 

 periods of time, even weeks, on the onset of the fall. I have 

 noticed such a condition in Boston, and other workers have observed 

 it elsewhere. This failure of all the individuals to respond simul- 

 taneously in the same manner to a given condition of the environ- 

 ment tends to show that environmental factors do not furnish the 

 immediate or adequate stimulus in the determination of amphigony. 



In 1907 Tannreuther (1907), in a paper on Melanoxantlius sali- 

 cis Weed, M. saUcicola Thomas, and several other species of aphids, 

 announced somewhat similar views. He said in part that "exter- 

 nal conditions, whether severe or normal, would not bring about the 

 production of sexual generation before a definite number of par- 

 thenogenetic generations had intervened." He further noted, after 

 two seasons of experimentation, that "if a stem mother and off- 

 spring were kept in favorable conditions in the greenhouse on 

 the same species of hosts as out of doors, the time and length of 

 period for each succeeding generation was approximately the same 

 as out of doors, and that in both instances the sexual females and 

 males appeared after the intervention of six parthenogenetic gener- 

 ations." 



It may be necessarv, at this juncture, to call jiarticular atten- 

 tion to the fact that in the foregoing discussion the role of tem- 

 perature in influencing the form of aphid reproduction is by no 



