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 O2 g body weight-1 h-1, was independent of air temperature (TA) over a span of at least 22 °C. The bees’ ability to fly at high TA was due to their ability to maintain thoracic temperature (TTh) near TA despite prodigious rates of heat production. Mechanisms of preventing TTh from overheating at high TA were investigated. Bees in flight at high TA 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 TH and TTh 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 (TAb) were close to TA at all TA. 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 TH TA resulted in keeping TTh near TA. It is concluded that during flight at high TA regulation of TH by evaporative cooling is the primary mechanism of reducing TTh.