Hilara sp. (Diptera: Empididae; Empidinae): Mating System, Swarm Movements, and Inbreeding Avoidance
In a study spanning parts of nine years, an undescribed species of Hilara Meigen was observed to form mating swarms displaying complex behaviors. Typically, swarms were shaped like a flattened torus rotating rapidly about a horizontal axis. Many swarms also moved up and down and turned slowly back-and-forth about a vertical axis. Both up-and-down and turning movements were random in extent and direction, suggesting that they might arise as random, asymmetric density fluctuations within the swarms themselves. A rotating secondary swarm appeared intermittently inside one end of some primary swarms. Swarm membership changed continually as flies left one swarm to join another and as entire swarms coalesced. At one site the set of all swarms displayed properties not found in the swarms individually: spatial extension, daily dissipation and reconstitution over a period of weeks or months, reproductive potential, and gene flow. Such emergent properties qualify the set as a multicomponent swarm, an object heretofore known only in computer models. Hilara sp. appears to be protandrous, univoltine, and promiscuous. Generally, males paired preferentially with somewhat smaller females, but some small and medium-sized males paired with much larger females. Although males of nearly all known Hilara species present nuptial gifts of prey or other items to females, nuptial gifts were not observed at any time during the present study. Many characteristics of swarms of Hilara sp. can be understood as adaptations that reduce inbreeding.
KeywordsHilara rotating swarms multicomponent
I thank Jeffrey Cumming, Agriculture and Agri-Food Canada, for determining that submitted specimens represent an undescribed species of Hilara and for contributing numerous helpful comments on early drafts of the manuscript. Thanks are also due to Brian Brown, Natural History Museum of Los Angeles County, for his helpful comments on an early draft; to Richard E. Davis, University of San Francisco (ret.), for help with statistical questions; and to Linda McLaughlin for assistance in preparing the figures.
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Conflicts of interest
The author declares that he has no conflicts of interest.
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