1. The times for which adult Drosophila subobscura survived at high temperatures in dry and in saturated air were measured at different temperatures, over a range of survival times from 20 to 400 min. There is a linear relationship between the logarithm of the survival time and the temperature in both cases, the values of Q10 being approximately 350 in dry air and 10,000 in wet air.
2. Survival times in dry air were increased in individuals previously kept at a high temperature (25°C.). Two kinds of acclimatization can take place, a longlasting ‘developmental acclimatization’ in individuals kept at 25°C. during pre-adult life, and a transitory ‘physiological acclimatization’ in adults kept at 25°C.
3. Survival times in saturated air were only slightly increased in individuals previously kept at 25°C.
4. Although physiological acclimatization increased the resistance of flies to desiccation rather than to high temperature as such, it is nevertheless a response to previous exposure to high temperatures, and not to high saturation deficiencies.
5. The reversibility of changes occurring at high temperatures was studied by exposing individuals for 50 min. to a temperature which would kill them in 100 min. and then retesting them after an intervening period at 20°C. Flies exposed to dry air recovered fully if they were allowed to drink; flies exposed to saturated air recovered in 3 hr. at 20°C., but complete recovery in this period required the presence of food and water.