Metabolism fuels fundamental biological processes and commonly scales with body mass with an exponent, b, between 2/3 and 1. The Metabolic-level Boundaries Hypothesis (MLBH) predicts that increased activity steepens b. We test this hypothesis by comparing metabolic rates during flight, non-flight locomotion, and rest in winged insects (n = 344), wingless insects (n = 354), and spiders (n = 131). After accounting for phylogenetic relatedness and wing presence, we find interspecific b values vary with activity only in winged insects (resting: 0.78; non-flight: 1.03; flight: 1.06), but not in wingless insects or spiders. Although all arthropods are expected to increase b during activity, this increase occurs only in winged insects, likely due to increased body temperature from muscle energy expenditure. Spiders show a shallower metabolic scaling exponent, potentially due to slowed life history with increasing size. These differences offer new insights into the evolutionary dynamics of arthropod energetics.

Metabolism underpins all life-sustaining processes and varies profoundly with body size, temperature, and locomotor activity. A current theory explains some of the size-dependence of metabolic rate (its mass exponent, b) through changes in metabolic level (L). We propose two predictive advances that: (a) combine the above theory with the evolved avoidance of oxygen limitation in water-breathers experiencing warming, and (b) quantify the overall magnitude of combined temperatures and degrees of locomotion on metabolic scaling across air- and water-breathers. We use intraspecific metabolic scaling responses to temperature (523 regressions) and activity (281 regressions) in diverse ectothermic vertebrates (fish, reptiles and amphibians) to show that b decreases with temperature-increased L in water-breathers, supporting surface area-related avoidance of oxygen limitation, whereas b increases with activity-increased L in air-breathers, following volume-related influences. This new theoretical integration quantitatively incorporates different influences (warming, locomotion) and respiration modes (aquatic, terrestrial) on animal energetics.