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Figure 3.2 Means and 95% confidence intervals of metabolic rate (circles) and wing-stroke frequency (squares) as a function of ambient pO2 in flying honeybees.

Source: Physiological Zoology. Joos et al., 70, 167-174. © 1997 by The University of Chicago. All rights reserved. 0031-935X/97/7002-9637$02.00

insects. In both honeybees (Joos et al. 1997) and dragonflies (Harrison and Lighton 1998), flight metabolism is sensitive to ambient oxygen partial pressures below 10 kPa. In honeybees, this is thought to limit their altitudinal distribution to below 3000 m, because above this altitude convective oxygen delivery limits metabolic rates to levels insufficient to meet the aerodynamic power requirements of hovering flight (Fig. 3.2). Indeed, these data show that oxygen supply is not always in excess for insects, as is often supposed based on resting metabolism, and might serve to explain insect gigantism characteristic of the late Paleozoic (Miller 1966). Ambient oxygen concentrations at this time were as much as 35 per cent higher than at present, and gigantism among flying insects (and other terrestrial arthropods) was a characteristic of the Carboniferous (Dudley 1998). Moreover, it appears that during environmental hyperoxia, of both the late Paleozoic and late Mesozoic periods, flapping flight in insects and pterosaurs and birds and bats, respectively, may have evolved (Dudley 1998).

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