MALE PERCH SELECTION AND THE MATING SYSTEM OF THE ROBBER FLY, PROMACHUS-ALBIFACIES (DIPTERA, ASILIDAE)

We studied the activity and spatial distribution of the robber fly, Pr omachus albifacies, in a desert grassland habitat in central New Mexic o. Late in the season males spent most of the daytime on or near choll a and yucca plants that had dead stems or dead flower stalks at least 1 m high. Of the three hypotheses (thermoregulation, foraging, mate en counter site) considered as explanations for this distribution, the ma te-encounter-site hypothesis was best supported. Plants used by female s as oviposition sites were the focus of male activity. Males perched within or near these plants and attempted copulations with females det ected nearby. Most matings were initiated at these locations. Seasonal changes in male and female activity also supported the mate-encounter -site hypothesis. Early in the season, females spent little time ovipo siting, and predictably, males spent little time on or near these plan ts. Such a mating system may be described as resource defense polygyny , since males acted aggressively toward one another at oviposition sit es even when females were not present. However, the short tenure of ma les at these sites is suggestive of scramble competition polygyny. We discuss possible reasons why this particular mating system has evolved .