Sexual morphology of male Sepsis cynipsea (Diptera: Sepsidae): lack of support for lock-and-key and sexually antagonistic morphological coevolution hypotheses

Traits that function in male–female sexual interactions tend to diverge rapidly compared with other traits. Several hypotheses attempt to explain this evolutionary pattern. Predictions of two of these hypotheses, lock-and-key and sexually antagonistic morphological co-evolution, were tested by examining how two sets of species-specific male structures fit with female structures during courtship and copulation in the fly Sepsis cynipsea (L., 1758). Contrary to predictions of both hypotheses, neither the species-specific modifications of the male's front legs nor those of his genitalic surstyli were matched by modifications of the female structures with which they meshed (wing bases, 6th abdominal sternite); males damaged small patches of microtrichia on the female's wings with their legs, but the morphology of the female's wings and abdomen showed no sign of the defensive designs expected under antagonistic morphological co-evolution. Data regarding the alternative hypothesis of sexually antagonistic behavioral co-evolution by females in response to male morphology were less conclusive, but this hypothesis failed to explain the sustained, apparently stimulatory rhythmic squeezing by the male genitalia and the lack of female defensive responses to this squeezing. These movements of the male surstyli during copulation suggest that they function to stimulate the female. The wing base of the female has apparent sense organs near the sites contacted by the male, as expected under the alternative hypothesis of traditional female choice to explain rapid divergent evolution. The male's genitalic surstyli were also used in novel precopulatory interactions. A pair of previously undescribed processes at the bases of the surstyli probably grasp and may also rhythmically squeeze the female during copulation.