Mechanisms of Body-Temperature Regulation in Honeybees, Apis Mellifera II. Regulation of Thoracic Temperature at High Air Temperatures

Abstract

Honeybees could remain in continuous free flight at extremely high air temperatures (up to at least 46 °C). The metabolic rate in free flight, 80–85 ml O₂g body weight⁻¹ h⁻¹, was independent of air temperature (T_A) over a span of at least 22 °C. The bees' ability to fly at high T_A was due to their ability to maintain thoracic temperature (T_Th) near T_A despite prodigious rates of heat production. Mechanisms of preventing T_Th from overheating at high T_A were investigated. Bees in flight at high T_A regurgitated fluid from their honeycrop and large droplets sometimes spread over the anterior portion of the thorax. Bees without the first two sets of legs, or without a ‘tongue’, maintained as low T_H and T_Th as intact bees. The abdomen serves only a minor function as a heat exchanger. In tethered bees, heating of the thorax to 45–50 °C resulted in significant, yet relatively little, temperature increase of the abdomen above that of dead or non heat-stressed animals. Similarly, in free flight abdominal temperatures (T_Ab) were close to T_A at all T_A. Thoracic heating to near lethal temperatures did not result in droplet extrusion from the mouth nor in significant physiologically facilitated heat transfer to the head. Furthermore, it resulted in no, or in relatively small, changes in pulsation of the aorta and the heart. However, the bees prevented the head from overheating, and the head served as a heat sink for excess heat from the thorax. Keeping T_H < T_A resulted in keeping T_Th near T_A. It is concluded that during flight at high T_A regulation of T_H by evaporative cooling is the primary mechanism of reducing T_Th.