Mean CO2 release rates (averaged across all individuals tested) versus PO2 for each species of (A) tenebrionid and (B) scarabaeid. Mean CO2 release rates (averaged across all individuals tested) versus PO2 for each species of (A) tenebrionid and (B) scarabaeid. Bars indicate standard errors; data points with no error bars indicate species with no replication. Mean critical PO2 values are indicated with a red arrow. The species are ordered based on body mass. Hilary M. Lease et al. J Exp Biol 2012;215:2524-2533 © 2012.