Mating and Sexual Selection in Empidine Dance Flies (Empididae)

SIMPLE SUMMARY: In many species of Empis, Rhamphomyia and Hilara dance flies, females rather than males display ornaments prior to mating. These ornaments appear to have evolved by Darwinian sexual selection in which females compete for access to choosy males that supply courtship (nuptial) gifts, which in many species appear to be the main or only source of dietary protein for adult females. We review the diversity of mating in this group of flies, including the different types of ornaments and aspects of behaviour and morphology thought to influence sexual selection on each sex, including nuptial gifts, and the sex ratios of aerial swarms where pairing takes place. ABSTRACT: Species whose behaviour or morphology diverges from typical patterns can provide unique insights on the evolutionary forces that promote diversity. Darwin recognised that while elaborate sexually selected traits mostly occurred among males, in a few species females possess such traits. Some species from the subfamily Empidinae (Diptera: Empididae) are among the animals that are often invoked to illustrate female ornaments. Empidines include taxa that exhibit varying levels of female ornament expression; some species possess multiple, elaborate female-specific ornaments while others have fewer and more modest adornments, and many species are altogether lacking discernible sexual ornamentation. This continuous variation in display traits in the Empidinae provides unique opportunities to explore the causes and consequences of sexually selected ornament expression. Here, we review the literature on sexual selection and mating systems in these flies and synthesise the evidence for various evolutionary forces that could conceivably create this impressive morphological and behavioural diversity, despite evolutionary constraints on female ornament exaggeration that help to explain its general rarity among animals. We also suggest some aspects of diversity that remain relatively unexplored or poorly understood, and close by offering suggestions for future research progress in the evolutionary ecology of mating behaviour among empidine flies.