Feb. 21, 1916 
Green Apple Aphis 
987 
which they occurred in the experiments. 1 Yet some viviparous insects of 
the sixteenth generation had been born as early as August 17, indicating 
very clearly that the season is of great importance in determining the 
production of sexual forms. 
The evidence supporting the other factor is not quite so direct. The 
first sexes in the eleventh generation were bom on September 2, in the 
twelfth and thirteenth, on September 5; in the fourteenth, on Sep¬ 
tember 8; in the fifteenth, on September 22; in the sixteenth and seven¬ 
teenth, on September 24. In all the generations up to and including the 
fifteenth, viviparous young were bom on or before September 3. 2 In 
the sixteenth generation no young were produced between September 
3 and 9, when viviparous young were bom. The earliest vivipara in 
the seventeenth gener¬ 
ation were produced on 
September 19. 
The accompanying 
diagram (fig. 3) gives 
the curves for percent¬ 
age of experiments con¬ 
taining sexes, by dates. 
Each date summarizes 
the production for 
seven days, the record¬ 
ed date being the mid¬ 
dle one of the seven. 
The writers can not 
give the exact percent¬ 
age of sexes in each 
generation, since all of the progeny were not reared. However, of the 
generations occurring wholly after September 1, the sixteenth contained 
sexes in 51 per cent of the experiments, the seventeenth in 80 per cent, 
and the eighteenth in 100 per cent. In the nineteenth generation all 
the insects produced were oviparous females or males. 
The most striking points brought out by these figures are that, besides 
the fact that each generation first occurs at a later period than its prede¬ 
cessor, an additional period is required (to and including the seventeenth 
generation) for the first appearance of sexes, and that in general the 
earlier generations are producing sexes in every experiment at a time 
when later generations are producing them in a very small percentage of 
experiments. This would indicate that, while seasonal climatic eon- 
1 It seems probable that they may occur as early as the eighth generation under some conditions. See 
page 982. 
* The insects bom in the sixteenth generation before August 17 are not included in this discussion, since 
they failed to reach maturity, and it was necessary to go back two generations for new material. 
22534°—16-3 
the green apple aphis in which the sexes appeared. 
