Mating and Sexual Selection in Empidine Dance Flies (Empididae)

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.

Keywords: female ornamentation; mating system; nuptial gift; sexual dimorphism.

Figure 1

Rhamphomyia longicauda mating swarm. (A) A female-biased mating swarm showing 13 females displaying multiple ornaments and one male (centre) carrying a nuptial gift. (B) Two female R. longicauda individuals in a mating swarm displaying their ornaments—extended abdominal sacs and three pairs of legs with extensive pinnate leg scales. Reprinted with permission from John Alcock.

Figure 2

Mating Empis tessellata; the pair lands on a substrate and the male supports the feeding female during copulation. Top: male; centre: female; bottom: nuptial gift. Note that the nuptial gift here is another empidine, Rhamphomyia crassisrostris. Reprinted with permission from Tom Houslay.