1. Moths belonging to the family Sphingidae are not capable of controlled flight until the temperature of the flight muscle has been raised by a preliminary period of vibrating the wings.
2. The flight-temperature of forty-five specimens of Deilephila nerii varied between 34 and 45° C., but individuals always flew at the same temperature.
3. The temperature inside the thorax rose at a mean rate of 4.2° C./min.
4. Alteration of the ambient temperature affects the duration of the warming period but not the flight-temperature.
5. The flight-temperature shows a positive correlation with the wing loading. In Deilephila and two other genera of similar dimensions, an increase of 50 mg. in the wing loading corresponds to a rise of 5.75° C. in the flight-temperature.
6. A method of measuring the rise in wing-beat frequency during the warming period is described. The thoracic temperature increases linearly with the frequency.
7. It is concluded that the frequency of the wing beat is determined principally by the wing loading, whilst variations in the other parameters of the wing stroke provide the ‘fine control’ of flight regulation required during flight and whilst hovering.