BIOLOGICAL STUDIES ON THREE APHIDID^. 131 



The immature stage was found to be quite variable in length, 

 covering from 6 to 15 days, with an average of 8.1-)- days. This is 

 approximately the same as that obtained from experiments of 1905, 

 in which, from 97 records, the average length of this nymphal period 

 was 8.3-f- days. During the first few generations and also the last 

 generation the time from the birth of an aphis until the birth of its 

 first young was nmch longer than it was during the warmer summer 

 months. The time between the birth of the first and that of the last 

 young likewise varied considerably, being from 3 to 10 days, the 

 period being noticeably longer in early spring and in the fall. The 

 average for the entire year was lO.G-f- days. Usually the female 

 Avould live several days after the production of her last 3^oung. The 

 entire length of life of the aphis varied between 11 and 50 days, with 

 an average for the year of "20.1-1- days. During the summer months 

 the aphides, as a rule, had a shorter life and produced more young 

 than in the cooler days of the year. The average number of young per 

 female for the year was 11.1+- The variation in the number of young 

 was from 20 to 96, the latter number being the largest number pro- 

 duced by a single female. For the year the average number of young 

 brought forth by an individual female in a single day was 4.1-|-, the 

 largest number being 11. However, in 1905 as many as 12 were born 

 in one day from one female. The average number of young from 

 April 18 until September 1 was, in 102 experiments, 4.9-|- ; from 

 September 1 to October 3 the average for 26 experiments was 2.4-[- 

 young per day, or one-half as many. It may be noted, in passing, 

 that, as the records made in 97 experiments in 1905 vary only slightly 

 from those obtained in the 128 experiments of 1906, the figures here 

 given are probably sufficiently accurate for any year. 



In 1905 a very interesting incident was observed. A wingless aphis 

 taken in the field June 23 was placed in an insectary cage, and Avithin 

 the next few days gave birth to 6 young. It then discontinued the 

 production of young for several days, then molted, became winged, 

 and produced 21 more 3'oung. 



Buckton, in his " Monograph of British Aphides," Volume I, page 

 87, says: 



Several early observers have erroneously stated that the female aphis is at 

 different periods of her life both viviparous aud oviparous. The acuteness of 

 Xev^rport failed him when he concluded " that aphides " — meaning the same in- 

 dividual — " deposit at one time true ova and at others produce living young." 

 * * * It may be pretty certainly asserted that the viviparous aphis is never 

 oviparous, and that the converse also is true. 



In one experiment in 1906 an aphis born October 6 became adult 

 October 24 and gave birth to a single young November 2, but did not 

 produce any more young, and soon died. Upon an examination of 

 her body only eggs were found. 



