Body size is not critical for critical PO₂ in scarabaeid and tenebrionid beetles

Constraints on oxygen delivery potentially limit animal body size. Because diffusion rates are highly distance dependent, and because tracheal length increases with size, gas exchange was traditionally thought to be more difficult for larger insects. As yet the effect of body size on critical oxygen partial pressure (P_crit) has not been measured for any clade of insect species for which there are interspecific data on tracheal scaling. We addressed this deficiency by measuring P_crit over a 4150-fold mass range (ratio of largest to smallest species mean) of two families of Coleoptera (Tenebrionidae and Scarabaeidae). We exposed adult beetles to progressively lower oxygen levels and measured their ability to maintain CO₂ release rates. Absolute metabolic rates increased hypometrically with beetle body mass (M) at both normoxic (M^0.748) and hypoxic (M0.846) conditions. P_crit, however, was independent of body size. Maximum overall conductances for oxygen from air to mitochondria (GO₂,max) matched metabolic rates as insects became larger, likely enabling the similar P_crit values observed in large and small beetles. These data suggest that current atmospheric oxygen levels do not limit body size of insects because of limitations on gas exchange. However, increasing relative investment in the tracheal system in larger insects may produce trade-offs or meet spatial limits that constrain insect size.