Essentially there are three ways in which endothermic insects can regulate heat loss: by convective cooling, using the haemolymph as a heat-exchanging fluid, or evaporative cooling. At thermal equilibrium, MHP equals heat loss:
where C is convective thermal conductance, RHL is radiative heat loss and EHL is evaporative heat loss (Harrison and Fewell 2002). Radiative heat gain is negligible in indoor respirometry experiments (Roberts and Harrison 1999) but can be substantial in the field (Cooper et al. 1985). Convective cooling from a flying insect depends on air speed and the difference between body and air temperatures, so tends to decrease at higher Ta when the temperature excess is less. Convective heat loss is dominant in small insects and most insects fly with Tth close to Ta. Increases in wingbeat frequency with Ta have been measured in small stingless bees (Trigona jaty, 2.5 mg) and in flies (Unwin and Corbet 1984), and this will have a net cooling effect because surface effects predominate in these small insects. Metabolic rate is independent of flight speed in free-flying bumblebees (Ellington et al. 1990), and increasing flight speed has been suggested as a mechanism of increasing forced convection at high Ta (Heinrich and Buchmann 1986).
Convective heat loss may also be varied by altering the distribution of heat within the insect: shifting heat to the abdomen or head increases the surface area for convective and radiative heat loss (Roberts and Harrison 1998). Physiological heat transfer uses the haemolymph as a heat-exchanging fluid between body compartments, and this transfer is minimized during warm-up and maximized during overheating. Countercurrent heat exchange in the bumblebee petiole between thorax and abdomen ensures that heat is sequestered in the thorax, except under conditions of heat stress when the exchanger is bypassed; the circulatory anatomy is described for Bombus vosnesenskii by Heinrich (1976). Active heat transfer to the abdomen is highly variable in bees and not explained by phylogeny (Roberts and Harrison 1998). Although best known for Bombus and possibly Xylocopa (Heinrich and Buchmann 1986), it also occurs at high Ta in the much smaller A. plumipes (Stone 1993), and has been found in other groups such as beetles (Chown and Scholtz 1993).
Evaporative cooling has traditionally been considered negligible, but is important in bees flying at high Ta such as Apis mellifera (Louw and Hadley 1985) and Xylocopa varipuncta (Heinrich and Buchmann 1986). It will be especially effective at the low RH of desert air, and honeybees returning to the hive with a fluid droplet on the tongue can cool themselves down and reduce the load carried at the same time. Foraging honeybees (A. mellifera caucasica) in the Sonoran Desert fly at air temperatures up to 46°C. Pollen foraging decreases at high Ta, and water and nectar foragers extrude droplets and have lower head and thoracic temperatures as a result (Cooper et al. 1985). Figure 6.14 shows that at Ta of 40°C, 40 per cent of nectar gatherers are carrying droplets. The resulting evaporative cooling enables honeybees to achieve head temperatures below Ta during flight at high Ta (Heinrich 1980; Roberts and Harrison 1999). Regurgitation of fluid has also been observed in wasps returning to the nest at high Ta (Coelho and Ross 1996). These hymenopterans can not use the abdomen as a heat radiator as do bumblebees and their abdominal temperature tracks Ta, although some evaporation may occur from the abdominal surface of honeybees at Ta > 40°C (Roberts and Harrison 1999). Evaporative heat loss, while considerable, may be less important to the thermoregulation of flying X. capitata and C. pallida at
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