Rate of Ageing in Drosophila suhohscura 273 



temperatures may be different from those acting below about 

 31°. This suggestion is confirmed by the finding that the 

 changes which occur at high temperatures are, wholly or in 

 part, reversible, whereas the changes which occur at 30-5° are 

 irreversible. 



The reversibility of a change is judged by exposing in- 

 dividuals to high temperatures intermittently, with interven- 

 ing periods at a lower temperature. Thus if flies are exposed 

 to a high temperature (33-5° in dry air or 34-3° in saturated 

 air) for 50 minutes (i.e. for about half their expectation of life 

 at that temperature) and are then kept for three hours at 

 20°, their survival times when they are again exposed to the 

 high temperature are as great or greater than the survival 

 times of flies not previously exposed. Thus death in these 

 conditions is due to changes which can be reversed at a 

 lower temperature; therefore the changes responsible for 

 death at high temperatures are not regarded as processes of 

 senescence. Experiments in which flies were kept in food vials 

 alternately for eight hours at 33° and for 16 hours at 20° 

 showed that the changes responsible for death at 33° in food 

 vials are also in part reversible. 



In contrast, as is shown in Table II, the changes responsible 

 for death at 30-5° are not to any appreciable extent reversed 

 at lower temperatures. There is evidence for a small degree of 

 recovery in males, since the first eight-day interruption at 

 20° did slightly increase the further expectation of life at 

 30-5°, although the second interruption did not. Females 

 which were exposed intermittently had total survival times 

 which were if anything slightly shorter than those of flies 

 exposed continuously. 



Since the changes responsible for death at 30-5° are, at 

 least in the females, irreversible, and since they take an 

 appreciable time to reach completion (mean of 17-6 days for 

 females), it seems reasonable to regard them as processes of 

 ageing. The question then arises, are they the same processes 

 as are responsible for ageing at 20°? If the processes of ageing 



