Preflight thermogenesis involves simultaneous contractions of opposing flight muscles, so that all the energy is degraded to heat, and the metabolic power output during warm-up is as high as during flight (Bartholomew et al. 1981; Casey and Hegel-Little 1987). Stone and Willmer (1989b) investigated the importance of body size and phylogeny in the warm-up rates of a wide range of female bees from six families. They demonstrated a positive correlation between warm-up rates and body mass after controlling for phylogeny and thermal environment (characterized by the minimum Ta for foraging of each species). Bees from cooler environments fly at lower Ta and have greater endothermic abilities, shown by their higher flight temperatures and warm-up rates. An extreme example is the small solitary bee Anthophora plumipes (Anthophoridae) with a mean warm-up rate of 12°Cmin—1 and this species has been the subject of detailed study (Stone 1993, 1994a; Stone et al. 1995). Body mass ranges from 100 to 240 mg, females being larger, but males and females do not differ in warm-up rates after allowance is made for the effects of mass. Larger bees warm faster than small bees but also warm to higher Tth than small bees, and these two opposing influences may cancel out effects of mass on the duration of warm-up (Stone 1993). Correlations between body mass and warm-up rate have been demonstrated within this species (Stone 1993), among 19 species of the genus Anthophora (Stone 1994b), and across the Apoidea as a whole (Stone and Willmer 1989b). Even the smallest species investigated, a 10 mg halictid bee, elevated its Tth by 2-3°C before flight (Stone and Willmer 1989b).
Measurement of metabolic rate during preflight warm-up in M. americanum (Lasiocampidae) (Casey and Hegel-Little 1987) shows the importance of
Ta: heat production is related to Tth, passive heat loss is proportional to Tth — Ta (the temperature excess), and so the cost of warm-up is inversely related to Ta. Stone (1993) calculated the energetic cost of warm-up in A. plumipes under different conditions. At Ta of 9 and 21°C and with a temperature excess of 6°C in each case, passive cooling accounts for 70 and 30 per cent, respectively, of the heat generated. It is not surprising that declining warm-up performance in A. plumipes in the laboratory is dramatically improved by the provision of sucrose solution. Endothermic warm-up is often supplemented by basking, especially at low Ta, leading to considerable savings in costs and time required. For example, a male A. plumipes basking at Ta of 9°C can achieve Tth of 20°C without metabolic cost (Stone et al. 1995). Male carpenter bees, Xylocopa capitata (Anthophoridae), use a combination of basking and wing buzzing to maintain Tth at a mean of 41°C during stationary intervals in territorial patrolling (Louw and Nicolson 1983). This buzzing with outstretched wings is not the same as preflight warm-up (in which the wings are motionless and folded dorsally) but serves to maintain readiness for immediate flight.
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