Reproductive output (fecundity) is one of the core life history (LH) parameters, because it determines an individual’s fitness and, ultimately, the potential for populations to grow or recover from catastrophic disturbance. Intra-specifically, phenotypic plasticity in LH parameters such as fecundity can co-vary with environmental factors (e.g. temperature) and other LH parameters (e.g. biomass), often in “ecological trade-offs”. Inter-specifically, a species’ fecundity is fixed (genetically determined), but can differ between species as a result of evolutionary processes and “intrinsic trade-offs” or covariance with other LH traits. It is well established that fecundity increases with body size within species; this study tested whether a similar relationship occurs between species of caddisfly. Caddisflies are capital breeders (resources for reproduction are acquired during larval life) so I also tested whether a fecundity–body size relationship co-varied with feeding habit, another LH trait. Using data from the literature and my own, empirically determined estimates of caddisfly fecundity, I demonstrate a positive relationship between fecundity and body size between species. The variance explained however was low. Model fit improved markedly when feeding group was included as a co-variate. Slopes of the fecundity–body size relationships differed with feeding group: the rank order according to decreasing slope was predators > filter-feeders > detritivores > algivores. Although some caddisfly families are associated with particular feeding habits, phylogenetic relationships did not underpin these relationships. The mechanism by which diet influences fecundity in species with different alimentary systems is unclear, but may reflect different strategies of resource allocation to food acquisition vs reproduction. Overall, this evidence suggests that fecundity, and therefore potential rates of population growth, vary between taxa in unexpected ways that have not been considered previously.