128 MISCELLANEOUS PAPERS. 



tions — that is, the generations obtained from a single stem-mother, 

 isolated in the spring. If we take into consideration the time during 

 which eggs have been found hatching in the field — from April 8 

 until May 22, a period of 4-4 days — it will be seen that each of the 

 generations might occur in the field much longer than my insectary 

 experiments would indicate. On May 1 individuals of the first 2 

 generations coexisted in the insectary; on June 1, 4 generations, from 

 the second to the fifth, inclusive; on July 1, G generations, from the 

 fourth to the ninth; on August 1, 7 generations, from the seventh to 

 the thirteenth; on September 1, 10 generations, from the eighth to 

 the seventeenth; on September 12, 11 generations, from the ninth to 

 the nineteenth ; and between September 30 and October 24 there were 

 12 generations in existence, from the tenth to the twenty-first, this 

 being the largest number of generations in existence at anj^ one time. 

 (See figs. 31, 32.) From that date on, the number of generations in 

 existence at any one time rapidly diminished until December 21, at 

 which time all of the aphides were dead. The latest date of birth in a 

 viviparous generation was October 7, and the last survivor of this 

 generation died November 28. The first record of the bisexual ovip- 

 arous generation in the insectary, in 1906, was October 2, and eggs 

 were found a few days later. Young of this generation were born as 

 late as November 4, and aphides were still alive December 21. How- 

 ever, in 1905 I found individuals of this oviparous generation as early 

 as September 5; also, they were observed in copula, and eggs Avere 

 found as early as September 30. Bisexual forms may appear in any 

 generation, providing the environmental conditions are such as to 

 favor their development. Thus, in the insectary sexual forms ap- 

 peared in October and November from 12 different generations, vary- 

 ing from the eleventh to the twenty-second, inclusive, thus indicating 

 that the appearance of the sexes may be conditioned by the tempera- 

 ture. This is illustrated by the occurrence of sexual forms on Sep- 

 tember 5, 1905, at which time the weather was quite cool for that time 

 of the year, although in 1906 the sexual forms did not appear until 

 October 2, the weather up to that time being milder than in 1905. 

 Between April 2, 1890, and January 17, 1893, Prof. M. V. Slinger- 

 land carried Myzus achyrantes Monell through 62 generations by 

 keeping the temperature uniform. Although further exj)eriments 

 would be necessary for positive proof, still, from what is now known, 

 it appears that with the necessary conditions for the development of 

 young— food and heat — the aphides would be able to reproduce par- 

 thenogenetically for an indefinite period. Numerous records were 

 made by me of instances in which the first young were viviparous and 

 the last oviparous. In these cases it was noticed that after the pro- 

 duction of viviparous forms the aphis would rest a few days before 

 beginning to produce the sexual forms. 



