Aim: Interspecific variation among metazoans often follows a latitudinal pattern, with species at higher latitudes being larger-bodied than related species from lower latitudes (Bergmann’s rule). For parasitic species, body sizes within any higher taxon often correlate with the body sizes of their hosts (Harrison’s rule). Whether temperature-driven latitudinal effects or host-driven resource constraints act independently or additively to shape interspecific variation in parasite body sizes remains unknown. We use a comparative approach to test the...