Swarming in a Balloon-Carrying Empidid (Empididae: Hilara)

Swarms are perhaps the most spectacular of insect mating aggregations. Usually groups of males fly at particular stations or markers where females come to locate mates (Sullivan 1981). Swarming behavior is particularly common in Diptera, the flies. In one family, the Empididae, it is especially well known because swarming males often carry gifts of prey which are presented to females during copulation (i.e. nuptial feeding, Downes 1969). The gift-giving has become ritualized in some empidids and "balloons" made of glandular secretions produced by males are transferred to females at coupling (Kessel 1955). Balloon flies are found among three genera of empidids (Empimorpha, Empis, Hilara). Though a large and diverse group of flies, few species of empidids have been extensively studied in the field (Alcock 1973, Chvala 1976). In North America only five balloon-carrying species have been studied (Kessel 1959) and all occur in western United States and Canada. This report is the most extensive of the field studies of the swarming and mating behavior of a single species of balloon fly and is the first detailed study of a balloon fly from the eastern U.S.