Mechanisms of thermal stability during flight in the honeybee Apis mellifera

Thermoregulation of the thorax allo,vs honeybees (Apis mellifera) to mainta in the flight muscle temperatures necessary to meet the power requirements for flight and to remain active outside the hive across a wide range of air temperatures (T-a), To determine the heat-exchange pathways through which flying honeybees achieve thermal stability, we measured body temperatures a nd rates of carbon dioxide production and mater vapor loss between T-a valu es of 21 and 45 degrees C for honeybees flying in a respirometry chamber. B ody temperatures were not significantly affected by continuous flight durat ion in the respirometer, indicating that flying bees were at thermal equili brium. Thorax temperatures (T-th) during flight were relatively stable, wit h a slope of T-th on T-a of 0.39. Metabolic heat production, calculated fro m rates of carbon dioxide production, decreased linearly by 43 % as T-a ros e from 21 to 45 degrees C. Evaporative heat loss increased nonlinearly by o ver se, enfold, with evaporation rising rapidly at T-a values above 33 degr ees C, At T-a values above 43 degrees C, head temperature dropped below T-a by approximately 1-2 degrees C, indicating that substantial evaporation fr om the head was occurring at very high T-a values. The water flux of flying honeybees was positive at T-a values below 31 degrees C, but increasingly negative at higher T-a values. At all T-a values, flying honeybees experien ced a net radiative heat loss. Since the honeybees were in thermal equilibr ium, convective heat loss was calculated as the amount of heat necessary to balance metabolic heat gain against evaporative and radiative heat loss. C onvective heat loss decreased strongly as T-a rose because of the decrease in the elevation of body temperature above T-a rather than the variation in the convection coefficient. In conclusion, variation in metabolic heat pro duction is the dominant mechanism of maintaining thermal stability during f light between T-a values of 21 and 33 degrees C, but variations in metaboli c heat production and evaporative heat loss are equally important to the pr evention of overheating during flight at T-a values between 33 and 45 degre es C.