Population Structure and Mating System of a Desert Bee Fly (Lordotus Pulchrissimus; Diptera: Bombyliidae). 1. Male Demography and Interactions

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Abstract
In the Mono Lake Basin of California [USA] the bee fly Lordotus pulchrissimus occurs on stabilized sand dunes dominated by the rabbit brush Chrysothamnus nauseosus, on which adults of both sexes feed. Adult activity was synchronous with the bloom of C. nauseosus for four or five weeks beginning the first week of August, 1981 to 1987. In 1986, males occurred at densities of four to six per hectare during peak abundance, at the end of the 10- to 15-d period of emergence. During this time mortality was approximately 15 to 30% per day; adult life expectancy of males was between seven and eight days, after correction for emigration. Males did not occur uniformly in dune habitat but gathered daily for one to one-and-one-half hours to engage in aerial chases with other males. These aerial swarms occurred in the same locations each year. There were eleven swarm areas in ca. 28 ha of dune habitat; nine of these were studied and followed for three to seven years. Swarm areas were uniformly distributed in suitable habitat, averaging 120 m to the next nearest swarm. Swarms in the nine areas were roughly the same relative size each year. The size of swarms was determined most proximately by differential immigration and especially emigration of males at each area, with larger swarms having lower emigration rates than smaller ones. The number of males may be determined by an ideal-free process, with costs of competition among males setting an upper limit on swarm size and with equal payoffs for individuals (presumably opportunity to mate) in swarms of different sizes. More information on the mating system and on characteristics of the swarm sites that may attract males is provided in the next paper.