Scaling of insect metabolic rate is inconsistent with the nutrient supply network model

1. The nutrient supply network model of the metabolic theory of ecology predicts that metabolic rate scales as mass(0.75) at all hierarchical levels. 2. An alternative, cell size, model suggests that the scaling of metabolic rate is a by-product of the way in which body size changes, by cell size or number, or some combination thereof. It predicts a scaling exponent of mass(0.75) at the widest interspecific level, but values of mass(0.67-1.0) for lower taxonomic groups or within species. 3. Here these predictions are tested in insects using 391 species for the interspecific analysis, and the size-polymorphic workers of eight ant species at the intraspecific level. In the latter, the contribution of ommatidium size and number to variation in body length, which is closely related to eye size, is used to assess the relative contributions of changes in cell size and number to variation in body size. 4. Before controlling for phylogeny, metabolic rate scaled interspecifically as mass(0.82). Following phylogenetic correction, metabolic rate scaled as mass(0.75). 5. By contrast, the intraspecific scaling exponents varied from 0.67 to 1.0. Moreover, in the species where metabolic rate scaled as mass(1.0), cell size did not contribute significantly to models of body size variation, only cell number was significant. Where the scaling exponent was < 1.0, cell size played an increasingly important role in accounting for size variation. 6. Data for one of the largest groups of organisms on earth are therefore inconsistent with the nutrient supply network model, but provide support for the cell size alternative.