Critical PO2 is size-independent in insects: implications for the metabolic theory of ecology

Insects, and all animals, exhibit hypometric scaling of metabolic rate, with larger species having lower mass-specific metabolic rates. The metabolic theory of ecology (MTE) is based on models ascribing hypometric scaling of metabolic rate to constrained O-2 supply systems in larger animals. We compiled critical PO2 of metabolic and growth rates for more than 40 insect species with a size range spanning four orders of magnitude. Critical PO2 values vary from far below 21 kPa for resting animals to near 21 kPa for growing or flying animals and are size-independent, demonstrating that supply capacity matches oxygen demand. These data suggest that hypometric scaling of resting metabolic rate in insects is not driven by constraints on oxygen availability.