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8 10 12 14 16 18 20 22 24 26 28 30 Ambient temperature (°C)

Figure 6.15 Voluntary (VFT) and stable flight temperatures (SFT) as functions of Ta for male and female Anthophora plumipes.

Note: VFT is the temperature at which the bee initiates tethered flight and SFT is the temperature after a period of continuous flight. Curves are fitted to the means for each sex.

Source: Stone (1993).

with body mass within the genus Anthophora and across the Apoidea (Stone 1994b).

Thermal constraints on flight activity limit the effectiveness of bees as pollinators (Corbet et al. 1993). Diurnal changes in microclimate define a thermal window suitable for flight, but floral resources also vary throughout the day. The activity patterns of female bees are determined by interactions between their physiology, changing floral resources, and surrounding microclimatic changes, as well as by nest construction in solitary species (Stone et al. 1999). Temperature and water relations of desert bees have been compared with those of temperate species by Willmer and Stone (1997), who make the interesting suggestion that endo-thermy in bees may have evolved in arid zones. Unlike ectothermic ants and beetles, bees of desert habitats show similar warm-up rates, cooling mechanisms, and upper critical temperatures to their temperate relatives. (Because bee activity is accompanied by heat generation, there has to be a safety margin.) Size strongly influences the activity of bees in arid environments, either on a daily basis (examples in Willmer and Stone 1997) or seasonally, as when mean body size of solitary bees decreases through spring and summer in Mediterranean scrublands of Israel (Shmida and Dukas 1990). Activity patterns of desert bees show avoidance of midday heat, and the endothermy needed for cold desert mornings and late afternoons could have pre-adapted bees for invasion of cool-temperate areas. Arid environments are centres of diversity for bees, including the genus Anthophora, and patterns of nectar secretion in these environments may have shaped the high levels of endothermy in this genus (Stone 1994b). Flowers pollinated by Anthophora species have deep, tubular corollas and peaks in nectar secretion early or late in the day, and activity patterns of the bees are strongly bimodal. Morning and evening peaks in nectar secretion may minimize water stress to desert plants, but may also be a response to the thermal tolerances of their pollinators.

Honeybees of the genus Apis are all of tropical origin and only A. mellifera occurs in northern climates. Dyer and Seeley (1987) measured flight temperatures in three Asian species of honeybees and compared them with data for A. mellifera.

Although there was a fivefold range of body mass in foragers of the four species, the temperature excesses did not increase with body size as expected. Apis mellifera and A. cerana, the two species of intermediate body mass, have disproportionately high wing loading, flight speed, and MHP (calculated from Tth — Ta in flight and thoracic conductance). These two species build cavity nests. Slower physiology is apparent in A. florea and A. dorsata, the smallest and largest species respectively, which build exposed combs protected by curtains of bees which are effective through sheer numbers rather than high activity levels (Dyer and Seeley 1987). In a follow-up study, Underwood (1991) studied the thermoregulation of the largest honeybee, A. laboriosa, in Nepal, and it appears that this species can be grouped with A. florea and A. dorsata as a relatively low-powered, open-nesting honeybee.

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