This paper investigates mechanisms of sperm competition by comparing reproductive characteristics across 74 butterfly species. Testis size scales with body size and, after controlling for this allometry, relative testis size increases with risk of sperm competition, as defined by female mating frequency. Both eupyrene (fertilizing) and apyrene (non-fertile) sperm lengths correlate positively with body size. After controlling for body size, relative eupyrene sperm lengths are greater in species where males experience higher risks of sperm competition. These results suggest that sperm competition in butterflies selects for increased investment in spermatogenesis, and specifically longer fertilizing sperm. Because longer sperm may be faster and more powerful, eupyrene sperm may therefore compete energetically, and are not selected to be minimally sized to maximize numbers for a purely raffle-based sperm competition mode. Apyrene sperm lengths are not affected directly by risk of encountering rival sperm. Instead, apyrene sperm show closer associations with body size which, if female tract morphometry correlates with body size, is consistent with the hypothesis that apyrene sperm retard female sexual receptivity by moving while in storage.