Natural History of Three Riparian Species of Robber Flies in a Panamanian Forest (Diptera: Asilidae)

Abstract

Seasonal abundance, microhabitat preference, and diet were determined over 3 field seasons for adults of 3 riparian species of robber flies on Barro Colorado Island, Panama. Adults of all species were rare during the dry season. During the wet season, Eumecosoma gibbum and Eumecosoma shropshirei were relatively common for approximately 2 months, and Psilonyx sp. had a flight period of only 3-4 wk. All 3 species perched in deep shade and at similar heights above ground. Both E. gibbum and E. shropshirei displayed pronounced specificity for a particular type of perching substrate, whereas Psilonyx sp. utilized several different substrates for perches. Although the species fed upon prey of similar size, they varied in the types of prey consumed. E. gibbum and E. shropshirei fed primarily upon nematocerous Diptera and Coleoptera, whereas Psilonyx sp. consumed greater proportions of acalypterate Diptera and Psocoptera. ADULTS OF MOST ROBBER FLY SPECIES ARE VISUAL, sit-andwait predators that attempt aerial capture of flying insects. Individuals typically wait for prey from conspicuous perches and make rapid attack flights ranging from 0.1 to 2.0 m in distance (Lavigne & Holland 1969, Dennis & Lavigne 1975, Shelly 1984). Following a successful capture, the robber fly returns to its perch and consumes the liquid contents of the prey with its heavily sclerotized hypopharynx. Because of their conspicuous habits and their method of feeding, adult robber flies have been the subject of numerous ethological investigations (e.g., Hespenheide & Rubke 1977, Dennis 1979, Scarbrough & Sraver 1979). In a previous paper (Shelly 1985), I described ecological relationships among 15 species of robber flies coexisting in the understory of a lowland Neotropical forest. The present study complements this earlier paper by providing data on seasonal occurrence, microhabitat preference, and diet for 3 riparian species of robber flies inhabiting the same forest. Other studies that make ecological comparisons among syntopic species of robber flies include Adamovic (1966), Rogers and Lavigne (1972), Dennis and Lavigne (1975), Scarbrough (1982), and Lavigne and Dennis (1985). STUDY SITE AND METHODS STUDY SITE.-Field work was conducted on Barro Colorado Island (BCI), Panama, during February-August 1979, April-August 1980, and April-July 1981. BCI is covered by a lowland tropical forest (Holdridge et al. 1971), sections of which vary in age between 60 and 130 yr (Knight 1975). All data were collected along the 0.7-km Received 21 May 1985, revision accepted 24 October 1985. 180 BIOTROPICA 19(2): 180-184 1987 stretch of Lutz Creek between its entry into the Panama Canal and the Weir Trail crossing (see Croat 1978 for a detailed map of this area). This section of the creek has a fairly steep gradient (t 25 m/km), and the streambed is primarily exposed bedrock overlain with large boulders. Steep hills on either side shade the creek during most of the day. Rainfall on BCI is highly seasonal. The wet season usually extends from April to mid-December, and the average total rainfall during these months is approximately 250 cm (Croat 1978). During these months, Lutz Creek generally (i.e., not immediately post-storm) consists of a series of shallow pools connected by a narrow (1-2 m) flow of running water. During the dry season, total rainfall averages only 20 cm (Croat 1978), and the streambed is usually dry. Figure 1 provides rainfall data for 19781980. THE SPECIES ASSEMBLAGE.-Eumecosoma gibbum Curran (9.4 mg), E. shropshirei Curran (9.5 mg), and Psilonyx sp. (9.2 mg; values represent mean wet weights for 5 Y and 5 d of each species) comprise a distinct ecological assemblage in central Panama. All 3 species commonly were found along streams on BCI and on the mainland as well. Moreover, they were only rarely found away (>2-3 m) from stream channels. In more than 600 census hours over much of BCI, I saw only 21 E. gibbum, 9 E. shropshirei, and 6 Psilonyx sp. perching in the understory far from any stream. [A second undescribed species of Psilonyx, distinguishable only by the male genitalia (E. Fisher, pers. comm.), occurs within the forest well away from streams. The 6 individuals of the riparian Psilonyx sp. seen in the forest represent 6 males of approximately 50 Psilonyx males captured in the forest and examined in the laboratory.] Likewise, species of robber flies inhabiting the forest understory were rarely, if ever, found perching near This content downloaded from 157.55.39.173 on Thu, 19 May 2016 05:11:56 UTC All use subject to http://about.jstor.org/terms