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Air temperature (°C)

Figure 6.13 Wingbeat frequency as a function of air temperature in Centris pallida (Anthophoridae) flying in a respirometer (circles) and hovering in the field (stars).

Note: The negative relationship between wingbeat frequency and Ta was evident by the fourth minute of flight in the respirometer (open circles), but not during the first minute (closed circles).

Source: Roberts and Harrison (1999).

Similarly, field and laboratory measurements show that Centris pallida (Anthophoridae) decreases wingbeat frequency (Fig. 6.13) and MHP at high Ta, thereby achieving very precise thoracic thermoregulation (Roberts and Harrison 1998; Roberts et al. 1998). Differences between field and respirometer measurements of wingbeat frequency are attributed by these authors to difficulties with maintaining continuous flight in closed-system respirometry. It seems likely that thermoregulation via modulation of MHP may be more widespread than is currently appreciated (Harrison and Roberts 2000). It is not clear whether the reduced metabolic rate at high Ta is a secondary consequence of greater efficiency of flight at high Ta, or a regulated response (Roberts and Harrison 1998).

The optimal temperature for force production during tethered flight of honeybees is 38°C (Coelho 1991), and flight metabolic rates of honeybees may decrease at Tth above or below 38°C, which can account for some of the discrepancies between different studies (Harrison and Fewell 2002). Sources of individual variation in metabolic rates of flying honeybees are illustrated in Fig. 3.19 (Chapter 3) and bees flying at high Ta (45°C) have among the lowest measured values. It follows that high air temperatures must contribute to an increase in food collecting efficiency and colony performance (Harrison and Fewell 2002).

Power output is also altered during flight in the green darner dragonfly Anax junius (Odonata, Aeschnidae). Field measurements under various conditions show increased wingbeat frequency and inferred heat production at low Ta, together with a reduction in the proportion of time spent gliding (May 1995). Giant Palaeozoic dragonflies may have used gliding to reduce endogenous heat production (Dudley 2000).

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