April, '11] HERRICK: CABBAGE APHIS 221 



November when temperatures of 20° F. were not uncommon. By 

 December activity had practically ceased and the aphides began to 

 decrease in numbers. On January 4, with the temperature one degree 

 below zero, a few were still alive but all of them succumbed during the 

 night of the 4th, when the temperature fell to 5° below. 



Reproductive Habits of the Agamic Forms 



The stem-mothers molted at intervals of three or four days and 

 matured in about fourteen days. They gave birth to young that 

 developed into wingless agamic females. The average length of life 

 of a stem-mother determined from the complete records of fourteen 

 different individuals was 45.21 days. The maximum length of life 

 was 50 days, the minimum 41 days. The average number of young 

 produced by a stem-mother, determined from the records of the four- 

 teen individuals, was 42.86. The maximum number was 53, the 

 minimum 27. 



From a record of many of the ordinary wingless agamic forms it 

 was found that the average time required for maturing was 12.81 

 days, the average length of life 46.14 and the average number of young 

 produced 41+. The highest number of young produced in one day 

 was six. 



The length of Ufe of the winged forms is apparently much less than 

 that of the wingless, at least this was true of those reared in the insec- 

 tary. In no case did they hve more than ten days after maturing, and 

 some gave birth to as many as seven, eight, and thirteen young while 

 some died without giving birth to any. 



Reproductive Habits of the Sexual Females 



The sexual forms were first observed October 10 and were in 

 evidence until the latter part of November. In the case of four 

 females whose times of pairing with the males were observed, eggs 

 were deposited from twenty-four to twenty-six hours after fertili- 

 zation. There is good evidence that some females oviposit without 

 fertilization. 



From the records of sixteen fertilized females we found the largest 

 number of eggs deposited was five, the lowest one, with an average for 

 the sixteen of 2.56 eggs. The intervals between the times of ovi- 

 position varied greatly. In some cases oviposition occurred at quite 

 regular intervals while in other cases long intervals elapsed between 

 periods of egg-laying. 



