Factors Affecting the Temperature Excess of Insects in Sunshine

Abstract

1. The temperature excess developed by insects under known radiation strength equivalent to sunshine has been studied in relation to modifying factors in a wind tunnel in the laboratory. 2. Temperature excess was measured by a thermocouple unit in the larger insects, and by the increase in rate of wing beat in Drosophila. 3. Temperature excess varied directly with radiation strength. 4. For insects of breadth greater than 0.3 cm. spectral composition of radiation over the normal sunshine range was of negligible importance to the temperature excess. 5. The effect of colour on the temperature excess was slight. Absorptivity was estimated by comparing temperatures of normal and black-painted insects. Some insects were hotter in the normal state because of change in the site of absorption of heat. White-painted insects were 64-85 % as hot as when black painted, because the white paint still absorbs infra-red radiation. 6. Temperature excess varied with a power of the size between 0.7 and 0.4 in the plasticine spheres, according to size. Temperature excess of the locust type varied as about the 0.4 power of the size; of the Diptera-Hymenoptera type, as about the 1.0 power. Size for size, compared with the spheres, the locust became between half as hot again and twice as hot, and the Diptera-Hymenoptera type between half as hot again and three times as hot, as the plasticine spheres. 7. Temperature excess varied inversely as the square root of the wind speed above speeds of 20-30 cm./sec. and tended to become independent at lower speeds, with the transition between forced and natural convection. The latter condition will be more important close to the ground. 8. Temperature rise by flight activity is not additive to temperature rise from radiation because it is associated with extra cooling. 9. Evolution of insects from the type of the Orthoptera to the Diptera and Hymenoptera has been associated with the attainment of greater temperature excess for given body size. This is probably of selective significance.