Summary
Thoracic flight temperatures (Tth) of workers of three species of honeybees (genus Apis) in Nepal were measured at relatively low ambient temperatures (Ta). At Ta = 18–24 °C, A. dorsata workers arriving at feeders to collect concentrated (⩾=40%) sugar syrup maintained higher temperature gradients (Tth — Ta) than workers arriving at feeders with dilute (6–17%) syrup. Temperature gradients were inversely related to Ta, indicating thermoregulation at low Ta. Similarly, temperature gradients varied inversely with Ta in A. cerana and A. laboriosa workers arriving at feeders at Ta = 12–20 °C. Temperature data suggest that honeybees have the ability to regulate heat production in flight and that they may vary their flight efforts according to expected gains and associated costs. Temperature gradients of A. laboriosa workers in flight are apparently about the same as those of A. mellifera workers, whose body mass is only half that of A. laboriosa. The circulatory systems of A. laboriosa workers show no striking differences from those of other species of Apis and are therefore probably equally effective at retaining heat in the thorax. This suggests that the relatively low (in relation to the body size) Tth— Ta maintained by A. laboriosa may be an indication of a relatively low metabolic rate and consequent low heat production. This is supported by an analysis of mass1,/3-specific wing-loading and, in turn, suggests that A. laboriosa may be grouped with A. dorsata and A. florea as a relatively low-powered, open-nesting honeybee, in contrast to the more high-powered cavity-nesters, A. cerana and A. mellifera.
