BIOLOGICAL STUDIES ON THREE APHIDID^. 



159 



seen that each of the generations might occur in the field much longer 

 than these artificial experiments would indicate. 



On April 15 individuals of the first 2 generations coexisted in 

 the insectary; on May 15, the first 3 generations; on June 15, the 

 first 4 generations; on July 1, 6 generations, from the second to 

 the seventh, inclusive; on August 1, 8 generations, from the third to 

 the tenth ; on September 1, 10 generations, from the fourth to the 

 thirteenth, and between September 6 and October 8 there were 11 

 generations in existence, this being the largest number of generations 

 in existence at any one time. From that date on, the number of 

 generations in existence at any one time rapidly diminished until 

 December 2, at which time all of the aphides were dead. (See fig. 34.) 



The vivaria used in rearing these aphides were simple, each con- 

 sisting of a pot of earth containing a young sorghum plant, over 

 which was placed a lamp chimney closed at the top with a fine-meshed 

 cloth. Individuals were transferred from one plant to another by 

 means of a soft camel's-hair brush, and these would usually remain 

 in the same place, even though the leaf became wilted or dying, and 

 thus it w^as an easy matter to keep track of them and to obtain the 

 numbers of young from day to day. Likewise, in the field this species 

 migrates from one part of the plant to another only to a slight ex- 

 tent. The individuals are usually found on the lower surface of the 

 older and lower leaves, in groups, and the young are almost always 

 found feeding on the leaf around the mother aphis. Another 

 peculiarity of this Sj^ecies is that it is not attended by ants, as are 

 most of the aphides found in the field. 



VIVIPAROUS GENERATION. 



All the following data were obtained in 1906 and 1907, unless other- 

 wise stated. The length of time between the birth of an aphis and 

 that of its first young was between 7 and 31 days, and the average 

 for 79 experiments was 13.3 days. 



Table XI. — Data of iinUrklHal experiments on Sipha flava, viviparous gener- 

 ation, 1906-7. 



Date of birth. 



1906. 



Julyl 



July 9-10 



July 12-15. . . . 



July 18 



July 19 



July 21 



July 27 



Do 



July 31 



August 4 



Date of 



first 

 young. 



1906. 

 July 12 

 July 19 

 July 21 

 July 27 



...do 



July 31 

 Aug. 4 



...do 



Aug. 8 

 Aug. 13 



Produc- 

 tive 

 period. 



Days. 



Number 

 of youiig. 



Total 



length of 



life. 



Days. 

 17 

 15-16 

 12-13 

 29 

 16 

 34 

 43 

 59 

 32 

 46 



