While investigating the mating strategy of wasps in the genus Philanthus we have discovered that the major determinant of mating success in males is body size (i.e., larger males are more successful at obtaining matings; this will be summarized further at the end of this report). In vertebrates that display sexual dimorphism and where there is male-male combat involved in the mating strategy, males are usually the larger sex. This is not the case in the Sphecidae where, in the vast majority of species (including Philanthus), the males are smaller, on the average, than females. This is puzzling, given the superiority of larger males in intrasexual competition. Even though larger males show a higher reproductive success than smaller males, there must be stronger selection pressures acting on female body size which promotes larger size in most species of digger wasps.