BIOLOGICAL STUDIES ON THREE APHIDID^. 



127 



On the other hand, beginning with the hist to be borne by the aphis 

 which hatched April 18, and following down the series of the last 

 borne of each generation, there were but 11 generations. From this it 



Genee 

 ation 



Apr. 



May 



June 



July 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



3Sd. 



34.. 



36. 



4fl. 



46. 



4S. 



64. 



rz.. 



77. 



11 



111 



12 



04 



13 



102t 



14 



113 



129. 



92 



Fig. 31. — Periods and succession of generations in Aphis viaidi-radicis, 1906. 



follows that the mean number of complete generations for the year is 

 16^. The first generation extended over a period of 31 days, from 

 April 18 to May 20; the second, 31 days; and the third, 3G days (figs. 



Fig. 32. — Periods and succession of generations in Aphis maidi-radicis, 1906. 



31, 32). The fifteenth generation proved to be the longest, continuing 

 for 129 days. Then the period of each generation diminished gradu- 

 ally. These data, however, Avere taken from only one line of genera- 



