Species composition and flight phenology of stiletto flies and window flies occurring along the Kuiseb River, Gobabeb, Namibia

M. E. Irwin (University of Illinois, 1101 W. Peabody Dr., Urbana, IL 61801, USA)
and A. H. Kirk-Spriggs (Entomology Centre, National Museum of Namibia,
P.O. Box 1203, Windhoek, Namibia)

Presented at the IVth International Congress of Dipterology
Oxford, England, September 1998

Almost no written information exists on adult cycling and flight timing of stiletto flies (Diptera: Brachycera: Asiloidea: Therevidae) or window flies (Scenopinidae). This fact is disturbing because of the important role stiletto flies, in particular, play in ecosystem dynamics and population regulation of soil-dwelling arthropods. On the one hand, adult stiletto flies are often abundant at certain times of the year and quite vulnerable to larger predaceous invertebrates and vertebrates. Adults can travel considerable distances in search of water in xeric environments; a few species also imbibe nectar and insect secretions. Stiletto fly larvae, on the other hand, are voracious predators of fossorial insects within sandy substrates and leaf litter. Because of their abundance and ravenous habits, these larvae are excellent regulators of most subterranean phytophagous and saprophytic arthropods and are an extremely important component of nearly all arid and semiarid ecosystems.

A Townes style Malaise trap (Santé Inc., 739 Cooper Dr., Lexington, KY, USA 40502) was set up on a sand bench surrounded by low-growing perennial vegetation within a riparian zone of the Kuiseb River bed just south of the Namib Desert Research Station at Gobabeb. This site is within the Namib-Naukluft National Park at about 450 meters above mean sea level. A global positioning system (GPS) fix of 23° 33′ 45″ S, 15° 02′ 38″ E was established for the site. The Malaise trap was kindly maintained and the contents extracted about weekly between November 22, 1996, and November 10, 1997, by Mr. Immanuel Netumbo Kapofi, Research Assistant, Desert Research Foundation of Namibia, Gobabeb, P.O. Box 1592, Swakopmund. The material was packaged and sent to MEI in two separate shipments by Mr. Eugene Marais, The National Museum of Namibia, P.O. Box 1203, Windhoek, where the stiletto flies and other Asiloidea were separated, sexed, counted, tabulated, and then stored in 95% ethanol. Voucher material will be deposited in the National Museum of Namibia.

Four species of Therevidae were represented by 1350 specimens and six species of Scenopinidae were represented by 129 specimens collected during the study. By far the most abundant species was Stenogephyra torrida Lyneborg, a small phycine stiletto fly that seems confined to the riparian environment. The second most abundant species was Phycus niger Kröber, a larger phycine that appears to be closely associated with acacia trees within the riparian zone. The last two species of therevids, Rueppellia basalis (Loew) and Orthactia gobabensisLyneborg are not strict riparian denizens; indeed, O. gobabensis seems to be closely associated with the small shrubs in the dune hummocks to the south of the river, while R. basalis seems to be most closely associated with vegetation in the rocky hillsides to the north and east of the site. The Scenopinidae were not identified, but three species appear to be within the large, cosmopolitan genusScenopinus: one large, robust species with 80 specimens captured during the study, a smaller, robust species with 13 specimens captured, and a large, thin species represented in the samples by one male and one female. A fourth species represented by four specimens was also taken; it is closely related to the genus Scenopinus but is broad and flattened, with dense patches of microtrichia on the wings. Two species of Pseudotrichia were also taken, a large, bright yellow species represented by 17 specimens, and a smaller, dull yellow species represented by 13 specimens. All specimens of Scenopinidae were combined for the purposes of determining flight phenologies.

Collection samples suggest that adults of both the Therevidae and Scenopinidae began appearing in late July or early August and continued through May. The scenopinids appear to begin flight activity slightly later and terminate slightly earlier than the therevids. Of the two therevid species caught in sufficient numbers to analyze, males of S. torrida began flight in late July or early August and terminated in late January, with a possible second generation emerging around the second week of February and terminating in June or July. Females were much less abundantly collected and were present about a week after the males first appeared.P. niger, in contrast, only had a single generation with flight patterns commencing in late November or early December and terminating in mid to late May. Although males were more abundantly captured than females, the same pattern was noted for both sexes. R. basalis was collected mainly during late December through early February, while O. gobabensis was taken mainly during late August through late October.