composition and flight phenology of stiletto flies and window flies
occurring along the Kuiseb River, Gobabeb, Namibia
E. Irwin (University of Illinois, 1101 W. Peabody Dr., Urbana,
IL 61801, USA)
and A. H. Kirk-Spriggs (Entomology Centre, National Museum of
P.O. Box 1203, Windhoek, Namibia)
Presented at the IVth International Congress of Dipterology
Oxford, England, September 1998
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.
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.
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 gobabensis Lyneborg 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 genus Scenopinus: 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.
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.
to Ecology Index