Hexapoda, the insects and their relatives, includes over half of all described species. Because large proportions of this diversity cluster within a small set of phytophagous groups, dietary-substrates have been proposed to shape patterns of richness within the clade through antagonistic co-evolution and zones of ecological opportunity. Here we explore these processes in the context of a recent dated phylogeny of Hexapod families. Our results indicate phylogenetic clustering of specialized ecologies such as phytophagy and parasitism, but reveal no consistent associations between the use of particular dietary substrates and clade richness. We also find no evidence that diets expected to promote antagonistic co-evolution are consistently associated with elevated species richness, nor that sister clades differing in dietary state are associated with greater-than-expected differences in richness. We do, however, identify variation in the age of, and transition rates among, dietary states that are likely to play a role in the observed heterogeneity in richness among dietary classes. Based on these findings we suggest remaining circumspect about the generality of adaptive zones based on broad dietary groupings as an explanation for hexapod richness, and suggest that richness heterogeneity may be better explained by origination and transitions rates, and variation within dietary categories.