The front legs of Sepsis flies (Diptera: Sepsidae) are stimulation devices.

Male genital and non-genital structures that are specialized to contact the female during copulation have diverged rapidly in many animal taxa. Three hypotheses that provide different explanations for this trend are reproductive isolation, sexual conflict, and female choice. We tested these hypotheses in sepsid flies, which have been considered case studies of sexual conflict, as males grasp and hold the females' wings with their species-specific front legs before and during mating, and females seem to actively resist this hold. The reproductive isolation and the sexual conflict hypotheses predict that the species-specific modifications of the male's front legs function as devices to mechanically improve their hold on the female's wings. The female choice hypothesis predicts that the male front legs function as stimulatory courtship devices. We tested these predictions by observing mating interactions, morphological contact, and the distribution of female sense organs in two species, Sepsis barbata and S. thoracica. Male front legs performed rhythmic, stereotypic, and species-specific movements, providing tactile stimulation of zones of the female wing that contained numerous sense organs but not improving their mechanical grip. We conclude that male front legs are stimulation devices rather than species-specific mechanical clamps, supporting the female choice hypothesis.