The Influence of Atmospheric Humidity on the Thermal Death Point of a Number of Insects

An account is given of a technique suitable for exposing small insects to high temperature and air of controlled humidity. Data of survival points obtained from a number of species are given, for 1-hour and 24-hour experiments. In the 1-hour experiments, the humidity of the air had no effect on the death point, except in the case of large meal-worms, which died at 1° C. higher in dry air than in moist. The temperature which any species can stand for 1 hour is sharply defined, but there is a range of 7° C. between the species of insects worked with. In 24-hour experiments, in moist air, all the species died between 36 and 39.5° C. Their death was presumably caused by the heat. In dry air, those insects not able to conserve their water died at low temperatures--22° C. in the case of flea larvae: this was attributed to desiccation. There seem to be two main causes of death of insects when they are killed at high temperatures: (1) When the temperature is over 40° C., they die from the effects of the heat. (2) Below 36° C. all the insects experimented with were able to survive at least 24 hours in moist air, but in dry air insects unable to conserve their water may die of desiccation. In hot air, over 40° C., certain large insects are better able to survive in dry air, as they keep their bodies cool by evaporating water. I am grateful to Mr H. S. Leeson for the supply of X. cheopis, and to Dr R. P. Hobson for the Lucilia adults. And I am indebted to Dr P. A. Buxton and Dr V. B. Wigglesworth, who made many helpful suggestions when the work was in progress and who read through the typescript.