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water loss of Bombus terrestris workers could be measured in a flow-through system only after the anus was sealed with dental wax (Nichol 2000). Water balance of flying bees is a function of body mass (and thus metabolic rate), nectar intake and its concentration, and ambient temperature and water vapour activity (Roberts et al. 1998; Roberts and Harrison 1999). The effect of air temperature on water flux (metabolic water production minus evaporative water loss) of honeybees is illustrated in Fig. 4.5: water flux during flight is neutral at ambient temperature values near 30° C (Louw and Hadley 1985; Roberts and Harrison 1999).

It has been argued that flying insects have significantly higher resting metabolic rates than flightless species (Reinhold 1999; Davis et al. 2000) and this is supported by the large-scale data of Addo-Bediako et al. (2002). Selection to reduce resting metabolism will be less intense when it constitutes a smaller proportion of the daily metabolic costs (Reinhold 1999), so that even during rest we might expect flying insects to have higher respiratory transpiration.

Air temperature (°C)

Figure 4.5 Water flux as a function of air temperature for honeybee workers flying in a respirometry chamber.

Air temperature (°C)

Figure 4.5 Water flux as a function of air temperature for honeybee workers flying in a respirometry chamber.

Note: Water flux is calculated as the difference between metabolic water production and evaporative water loss. Symbols represent honeybees from three different colonies.

Source: Roberts and Harrison (1999); data for Xylocopa capitata are from Nicolson and Louw (1982).

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