Biological diversity, population size and body size are interdependent, but there is little consensus on the nature or causes of these relations. Here we analyse the most thoroughly sampled ecological community to date, a grassland insect community sample containing 89,596 individuals of 1,167 species. Each taxonomic order had a distinct body size at which both species richness and number of individuals were highest, but these peak sizes varied more than 100-fold among five major orders. These results suggest that there may be fewer undiscovered small insect species than previously thought. Moreover, we found a surprisingly strong, simple, but unreported, relation between species richness (S) and the number of individuals (I) within size classes, S = I0.5. Because this held across numerous body types and a 100,000-fold body-size range, there may be a general rule that is independent of body size for the relations among interspecific resource division, abundance and diversity.