Rate of Ageing in Drosophila subobscura 275 



them at 20° until they died. The exposure, so far from 

 decreasing their expectation of Hfe, in fact increased it, by as 

 much as 50 per cent in females exposed for eight days. In a 

 similar experiment, a group of males were exposed to 30-5° 

 for five days, or two-thirds of their expectation of life at that 

 temperature. The further expectation of life of these males 

 at 20° did not differ from that of a group of unexposed con- 

 trols. 



56 64 72 80 



Age in days 



96 104 112 120 128 136 



Fig. 2. Survival time at 20° of females previously exposed to 30-5°. 

 A, unexposed; B, exposed for 5 days; C, exposed for 8 days; D, ex- 

 posed for 12 days. 



It follows that, both for males and females, different pro- 

 cesses are responsible for death at the two temperatures; we 

 are therefore obliged to accept a multiple theory of ageing for 

 this species. The situation is further complicated by the 

 different response of males and females to exposure to 30 • 5°, 

 w^hich prolonged the life of females but left that of males 

 unaltered. The clue to this difference was found when it was 

 observed that exposure to a high temperature caused a 

 partial regression of the ovaries of females, which sub- 

 sequently laid eggs at only about half the rate of unexposed 

 females. This suggested that the process of egg-laying might 

 accelerate ageing in females, and that the exposure to a high 



