Figure 3.17 The relationship between body mass and oxygen consumption rates in resting and hovering heterothermic moths.

Source: Bartholomew and Casey (1978).

instantaneous increase in VO2 (Fig. 3.18), which is generally higher than that required for maintenance of flight temperature (Bartholomew et al. 1981; Nicolson and Louw 1982). The metabolic costs of warm-up also scale positively with body mass at the interspecific level (Bartholomew and Casey 1978), although the slope of the regression (0.9-1.0) varies with thoracic temperature in dragonflies (May 1979a). In endothermic species that closely regulate thoracic temperature, regulation is largely due to manipulation of convective, or sometimes evaporative, heat loss (Dudley 2000, Chapter 6). However, it is now clear that in several bee species thermoregulation can take place by variation of metabolic heat production (Chapter 6).

During flight, metabolic rate is independent of speed, at least between forward speeds of 0 (hovering) and 4 ms_1, which are within the normal range of free-flying individuals (maximum speed of 5 m s_1), in the only species examined to date, the bumblebee Bombus lucorum (Ellington et al. 1990). In consequence, estimates of metabolic costs for forward flight can simply be estimated from those measured during hovering (a more straightforward procedure—see Joos et al. 1997; Roberts and Harrison 1999). It does seem likely though that metabolic costs of flight will increase at very high airspeeds (Harrison and Roberts 2000), resulting in a J-shaped power curve, owing to increases in parasite power requirements (the power required to move the body through the also varies with age, reward rate (Moffatt and Nunez 1997), allele frequencies of malate dehydrogenase, type of load carried and subspecies (Harrison and Fewell 2002; Feuerbacher et al. 2003) (Fig. 3.19).

Constructing, maintaining and fuelling the flight apparatus are clearly all energetically expensive. In consequence, it should come as no surprise that there is an environmentally dependent trade-off between maintaining flight machinery (and wings) and survival, development rate and fecundity (Roff 1990; Zera and Denno 1997, Chapter 2). This tradeoff arises because of the energetic costs of flight and the flight apparatus, which can be diverted to other fitness components, and probably also because of architectural costs of maintaining a flight apparatus (Marden 1995). Nonetheless, flight clearly has many ecological benefits, to which the diversity of the insects (Harrison and Roberts 2000), and redevelopment of flight following its loss in some stick insects (Whiting et al. 2003) clearly testify.

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