Heat Loss and the Body Temperatures of Flying Insects

ABSTRACT The natural internal temperature gradients during flight were reproduced in various medium and large insects by mounting freshly killed specimens in a wind tunnel and heating them with a high-frequency electric current. The heat flow from the flight muscles to other parts of the body and from the body were investigated. Comparison of dead and living insects showed that most of the heat transfer within the body is by conduction; circulation of the haemolymph during flight contributes little to the heat flow. The temperature excess is high throughout the pterothorax in a large insect; where there are no subcutaneous air sacs it is only about 10% less at the surface of the pterothorax than at the centre. Only about 5-15% of the heat generated in the flight muscles is conducted to the prothorax, head, abdomen and appendages, which remain near the temperature of the air. Usually not more than 10-15% of the heat escapes from the pterothorax by long-wave radiation in a large insect flying under a clear sky. Smaller insects lose relatively more of their heat by radiation. Radiation increases with the insect’s temperature but it is never sufficient to give much protection against overheating. Ordinarily 60-80% of the heat is dissipated from the surface of the pterothorax by convection. In convection from a naked insect the relationships between heat loss, the surface temperature excess, size, and wind speed are nearly the same as in convection from a smooth cylinder or sphere, if allowance is made for turbulence in the air flow over the insect. In dragonflies and denuded bees and moths heated in proportion to their pterothoracic volumes in a constant wind, the temperature excess was proportional to the 1·3-1·5 power of the average diameter of the pterothorax. The coats of hair on bumble-bees, hawk moths, and noctuid moths are excellent insulators against convective heat loss. At normal flying speeds they increase the temperature excess by 50-100% or more--in a large hawk moth probably by at least 8 or 9° C. The insulating value of a coat depends mostly on its density and on the size of the insect, and less on the length of the hair. In dragonflies the pterothorax is insulated nearly as effectively by the subcutaneous air sacs.