Sex-role reversed courtship behaviour, sexual dimorphism and nuptial gifts in the dance fly, Empis borealis (L.)

Abstract

Dance fly swarms usually consisted of five to ten females (range 1-40). Females moved between swarm-sites, which were the same during the whole flight-period and over several years. Usually a single male with a nuptial gift approached a swarm. He either mated with one of the females or left, still with his gift, for another swarm. Males coming to swarm sites activated females resting at the swarm-marker to swarm. At the end of the flight-period males were very few, while females still swarmed. Males hunted for prey, which were presented to the females prior to mating. Females did not hunt and were only seen to consume prey received from males. The most frequent nuptial gifts were Diptera, including conspecifics. Females, and presumably males, mate several times. Swarming females had no ma ture eggs. The number of mated females in the population rose rapidly to 80% during the first six days of the flight-period, and on day 20 all females were mated. No sexual difference was found regarding wet-weight, but wings were longer and broader in the females, whose wing-area on average was about 60% larger than in males. Femur I was about 20% longer in males than in females. We suggest that the sex-role reversed courtship behaviour, the offering of nuptial gifts and the sexual size dimorphism in E. borealis have evolved because males limit fe male reproductive success, and males therefore should become choosy and females should compete for mates.