AbstractHandicap theory explains that exaggeratedly developed sexual traits become handicaps but serve as honest signals of quality. Because very weak signals are less likely to provide benefits than to simply incur costs, it is interesting to elucidate how sexual traits are generated and developed during evolution. Many stalk-eyed fly species belonging to tribe Diopsini exhibit marked sexual dimorphism in their eye spans, and males with larger eye spans have larger bodies and reproductive capacities, which are more advantageous in terms of contests between males and acceptance for mating by females. In this study, we investigated the role of eye span in a more primitive species, Sphyracephala detrahens, in tribe Sphyracephalini with less pronounced sexual dimorphism. Male-male, female-female, and male–female pairs showed similar contests influenced by eye span, which was correlated with nutrition and reproductive ability in both sexes. During mating, males did not distinguish between sexes and chose individuals with larger eye spans, whereas females did not choose males. However, males with larger eye spans copulated repeatedly. These results indicate that, in this species, eye span with a small sexual difference does not function in sex recognition but affects contest and reproductive outcomes, suggesting the primitive state of sexual dimorphism.