Summary

Temperatures have been measured by means of thermojunctions at several points in the body of insects preparing for flight.

In butterflies (Vanessa), moths and bumble bees (Bombus), preparation consists in vibratory movements of the wing raising the temperature of the wing muscles usually above 30°C. In lamellicorn beetles (Geotrupes) there are no visible movements of the wings, but the vibrations can be demonstrated by leading off action potentials from the muscles. The heat production takes place always in the wing muscles, but there is a gradual and much slower increase in the temperature of the rest of the body.

The muscle temperature necessary for flight is high (above 32°) and nearly constant in bad fliers (Geotrupes), while the good fliers can fly at different temperatures, Vanessa from about 20 up to 42°. Even in the latter type a high rate of flight can be attained only when the wing muscles have become heated above 35°.

The heat production in the vibrating muscles can be estimated from the temperature increment during the heating process combined with the decrement during subsequent cooling. It increases rapidly with increasing temperature. The final value found in Vanessa just before flight at 34° corresponds to a metabolism of 30 l. O₂/kg./hr. The metabolism in acutal flight reaches much higher values.

It is suggested that a definite relationship may exist between temperature and the maximum work of which a muscle is capable.