Thermoregulation and Control of Head Temperature in the Sphinx Moth, Manduca Sexta

Abstract

Head and thorax temperatures (Th and Tth were tightly coupled during pre-flight warm-up over a range of ambient temperatures (Ta). At Ta = 21 °C, the head reached a significantly higher temperature (Tth = 32 °C) than during exogenous heating of dead moths to the same thoracic temperature (Tth = 26 °C). In free-flying moths, slopes of linear regressions for both Tth and TTh versus Ta were about 0.4. At any Ta, Th remained only about 7 °C below Tth Abdominal temperature varied with Ta with a slope of 1.2 and remained 2–5 degrees above Ta. Cooling constants (min−1) for the head, calculated from cooling experiments with the thorax either at Ta or maintained constant at about 40 °C, yielded similar values. During free flight, as Ta increased from 17 to 32 °C, thoracic heat loss decreased from 36 to 20% of the total heat production, head heat loss decreased from 27 to 8%, and abdominal heat loss increased from 17 to 37%. During warm-up at all Ta's the largest component of energy expenditure was the heat storage and heat loss that occurred from the thorax, followed by that from the abdomen and then by that from the head. Exogenous heating of some live moths resulted in cyclic temperature fluctuations in which Th and Tth changed simultaneously and in opposite directions. During each cycle, the heart beat pattern changed, becoming irregular when Tth was increasing and returning to a regular pattern when Tth was decreasing. Smaller temperature fluctuations in the head occurred at the same rate as the heart beat. These data suggest that the head temperatures observed during pre-flight warm-up and flight are the result of active heat transfer from the thorax via the blood circulation.