Do insects vary their flight performance for thermoregulatory as well as aerodynamic purposes? Metabolic rate in flight has been shown to be independent of Ta in various large endothermic insects: for example, sphinx moths Hyles lineata (Casey 1976), bumblebees Bombus spp. (Heinrich 1975), carpenter bees Xylocopa capitata (Nicolson and Louw 1982). If VO2 in flight is unchanged with Ta, heat production must be constant and regulation of Tth can only be achieved by heat loss. This generally accepted picture (Heinrich 1993) changed when Harrison et al. (1996) demonstrated that free-flying honeybees subjected to a rise in Ta from 20 to 40°C decrease their wingbeat frequency and MHP (calculated from CO2 production). Variation in MHP accounts for most of the thermoregulation of flying honeybees between Ta of 21 and 33°C, whereas evaporative heat loss is also important between 33 and 45°C (see the negative water balance at high Ta in Fig. 4.5) (Roberts and Harrison 1999).
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