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 O₂ 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.