THE PEA APHIS WITH BELATION TO FOEAGE CEOPS. 49 



An average of all experiments for the two years, carried through 

 out of doors and apparently under normal natural conditions, was 

 12.1 + days. As would be anticipated, and as is shown in Tables IV 

 and V, the age at which females began reproducing was shortest 

 during the warmer parts of the year and longest during the early 

 spring and late fall months. In the case where immaturity lasted 56 

 days the aphis would doubtless have remained immature for a much 

 longer time, possibly all winter, had it been left out of doors under 

 natural conditions. (See footnote 1, Table VI.) From 16 individual 

 experiments, between March 23 and September 21, Mr. R. L. Webster 

 (Folsom, 1909) found that a female begins to reproduce 11 days after 

 birth, on an average. 



REPRODUCTIVE PERIOD. 



The reproductive period— that is, the time or period during which 

 the insect gave birth to young — varied from 2 to 68 days in 1912 and 

 from 2 to 61 days in 1913, or an average for the two years (53 exam- 

 ples) of 22 days. In computing these averages some records, where 

 the aphis was known to have died from other than natural causes, 

 were not considered. Except in cases where death was due to other 

 than natural causes, the female almost invariably lived several days 

 after the birth of its last young, the length of time varying from 2 

 or 3 to 28 days. 



LONGEVITY. 



As is the rule with all of the Aphididae, the pea aphis lives for a 

 much longer time in the spring and fall, especially in the fall, than in 

 the summer. In our records the total length of life — that is, from 

 date of birth to date of death — of individuals varied from 10 to 85 

 days in 1912 and from 10 to 78 days in 1913, or an average of 39.1 + 

 days from 51 records made during the two years. Tables IV and V 

 may be referred to for the variation in lengths of life at the different 

 times of the year. In Mr. Webster's experiments reported by Dr. 

 Folsom (1909) the length of life of 16 individuals from as many con- 

 secutive generations varied from 13 to 50 days, with an average of 

 25.4 days. 



FECUNDITY OF VIVIPAROUS FEMALES. 



The aphis under discussion is one of the most prolific of all plant- 

 lice. We find that a female may give birth to as many as 14 young 

 in a period of 24 consecutive hours. The average number of young per 

 female for all individuals where records were kept in 1912 and 1913 

 (59 examples) was 3.7+ per day, but this is the average for the 

 entire reproductive period of each female, and as we learned, the 

 birth rate greatly diminishes toward the latter days of its life, often 

 not averaging one young a day. Taking this into consideration 

 we find the average birth rate during the active life of the mother 



