Robert A. Cannings. 1998. Robber Flies (Insecta: Diptera: Asilidae) in Smith, I.M., and G.G.E. Scudder, eds. Assessment of species diversity in the Montane Cordillera Ecozone. Burlington: Ecological Monitoring and Assessment Network, 1998.
ROBBER FLIES
(INSECTA: DIPTERA: ASILIDAE)
Robert A. Cannings
SYSTEMATIC REVIEW
A systematic checklist of the 101 species (8 of which are listed as potentially endangered, threatened or vulnerable) including their zoogeographic elements, is included as Appendix 1. A brief review of the major taxa of the ecozone with notes on habits is presented below.
SUBFAMILY LEPTOGASTRINAE (2 species, 2%)
These delicate little flies are represented by two species, Leptogaster arida and L. fornicata. Both are seldom collected, and their distributions are poorly known. L. arida is the more widespread, living across the southern part of the province from Vancouver Island eastward; L. fornicata is restricted to Interior valleys. They are extremely slender, long-legged, almost bare species that hover among the grasses of grasslands and dry forests in the southern valleys of the Ecozone.
SUBFAMILY DASYPOGONINAE (9 species, 9%)
The species of Dasypogoninae are distinguished by an enlarged or twisted spine at the apex of the fore tibia. The nine species in the Montane Cordillera are small to medium-sized flies. Comantella pacifica, a species known only from mesic grasslands in the Okanagan Valley, is unique in its flight period. For an asilid in Canada, it flies unusually late into the autumn (late October) and is the first species to appear in the spring (late March). There is evidence that adults overwinter in protected places. Cophura albosetosa, C. brevicornis, and C. vitripennis are small, dark species active on the forest floor of open woods in the southern part of the Ecozone. The silvery little Lestomyia sabulona sits on the bare ground in grasslands of the southern Okanagan. The genus needs revision, and it appears that the Canadian species is actually undescribed. Nicocles is a genus of beautiful flies with brown-spotted wings and in the male, with brilliant silver terminal abdominal segments. N. canadensis, N. dives, N. pollinosus and N. utahensis are all species of western montane forests and grassland edges.
SUBFAMILY LAPHRIINAE (23 species, 23%)
The 23 robber flies of the Laphriinae in the Montane Cordillera are forest species whose larvae develop in rotting wood. Many of them are large and colourful and mimic bees and wasps. Adults perch on leaves or logs, stumps and tree trunks and wait for prey to fly by. Beetles are a favorite prey of some species. Andrenosoma fulvicaudum is a black and orange fly that ranges across the continent south of the northern forests. It is attracted to forest fires; the females lay eggs in burned trees where the larvae prey on metallic woodboring beetle (Buprestidae) larvae (Fisher 1986). Pogonosoma ridingsi, another black species, is a widespread cordilleran asilid Laphria is the largest genus in the Montane Cordillera with 21 species. Laphria s.str. is a group of bumblebee mimics with bright fuzzy yellow and black or yellow, red and black bodies. Cordilleran species include L. astur, L. asturina, L. columbica, L. fernaldi, L. partitor, L. sackeni. L. insignis and L. posticata are boreal species. The rest of the genus can be divided into three other distinctive groups based on genitalic and other characters.
Laphria asackeni and L. vultur are spectacular, large flies clothed with glowing orange-gold pile. L. asackeni is the more common, widespread species, but both are restricted to the western mountains. L. janus is a common Boreal species.
A group in which the males bear paired protuberances on the sixth abdominal segment contains the widespread black-bodied Laphria franciscana, a Cordilleran species. Two much rarer species, both with golden yellow abdomens, are L. index, an Austral species and L. scorpio, the only species in the Transitional element in the Ecozone.
A third group (Choerades) has distinctive lamellae formed from fused bristles in the male genitalia. L. aimatus, L. felis, L. ferox, L. milvina, and L. vivax are restricted to the mountain forests of western North America; L. sadales and L. gilva are boreal. The latter species is one of two holarctic asilids; it ranges from Scotland to Siberia, from Alaska to Labrador.
SUBFAMILY STENOPOGONINAE (42 species, 41%)
Species of this subfamily lay their eggs in the soil and the larvae develop there. The group is dominated in the Montane Cordillera by the large holarctic genus Cyrtopogon, with 19 species in the forests of the region. This is 27% of the Nearctic Cyrtopogon fauna, which is overwhelmingly western in distribution. Indeed, all but one of the Montane Cordillera species is Cordilleran in origin. The exception is C. bimacula, a very common Boreal species with striking dark spots on the wings; in the western mountains it is largely a subalpine species. With their fuzzy yellow and black abdomens, C. dasyllis and C. dasylloides look a bit like Laphria species; the wings of the males have dark brown patches. The mostly subalpine C. auratus and C. aurifex also have thick, tufted, golden hair on the abdomen; the former is widespread in the south, the latter is restricted to the Cascade Mountains in the extreme southwestern corner of the Ecozone. The beautiful chocolate-winged C. princeps has a similar range in the Cascades. C. banksi, a small black and grey species of low and mid elevation forests, is a widespread species in the Ecozone. C. montanus, densely black haired, is the most often encountered species in the dry forests of low and medium elevations throughout the region; the closely related C. inversus, with its striking white-haired hind femora, appears in Ponderosa Pine and Douglas-fir woods in the spring. C. willistoni is common in the moister, higher elevation grasslands across the Montane Cordillera. The male has beautifully decorated legs that it uses in mating displays; the fore tarsi are densely silver- haired and the mid tarsi are tipped with a fan-shaped tuft of black setae.
Eucyrtopogon contains 12 named grassland and forest species restricted to the West, seven of which live in the Montane Cordillera. It is badly in need of revision and there are certainly a number of undescribed species in the Ecozone. The wings are brown-spotted in both sexes. Some species are notable for late or early seasonal activity. For example, E. calcaratus has been collected on 23 November in Vernon and 23 January in Penticton. Coleomyia hinei, C. setigera, Heteropogon senilis, Holopogon albipilosus, H. stellatus and Callinicus pollenius are species of forest openings and grassland edges. Callinicus is an uncommon, beautifully yellow-banded wasp mimic.
Myelaphus lobicornis lives in intermontane grasslands from southern British Columbia to California and Utah. It is known from only two grassland sites in the Montane Cordillera, one at Penticton, the other at Dutch Creek in the Rocky Mountain Trench. At the Penticton site it flies only around Common Rabbit-brush (Chrysothamnus nauseosus) in June. With its almost hairless body, dark head, thorax and wings, elongate antennae, red abdomen and yellow legs, it resembles a wasp. Dicolonus nigricentrum and Dioctria henshawi are grassland or dry forest edge species at the northern limit of their range and rarely collected in the Ecozone. They are known only from the Okanagan Valley. Dioctria pusio and Eudioctria sackeni are tiny species that hunt from leaves and twigs in forest openings and edges at low and mid elevations. The latter is particularly common and widespread across the southern part of the Montane Cordillera and ranges south to California. It has two striking colour morphs; in the male of the more common one the wings are orange basally and gray apically.
Stenopogon inquinatus, big and usually red, is one of the most common and noticeable species in the Montane Cordillera. Hunting a variety of prey in habitats ranging from arid sagebrush steppe to open Douglas-fir forests, it is always an impressive sight. I have frequently seen it feeding on grasshoppers larger than itself, and in one case, a dragonfly, Gomphus graslinellus, with a mass many times that of the robber fly, had been killed on the ground. A close relative, S. rufibarbis, is smaller and more rufous in colour; it is common and widespread in the moister, upper grasslands. Scleropogon neglectus, grey, and similar in size and shape, is also common on a wide range of grasslands. Much rarer, and known from only a handful of localities in the Okanagan and Thompson valleys, Ospriocerus aeacus is a red- abdomened, black-winged, grassland denizen that ranges over western North America and into Mexico. Willistonina bilineata inhabits open woods and grassland borders but has been collected only two times in the region.
SUBFAMILY STICHOPOGONINAE (6 species, 6%)
The Stichopogoninae, characterized by a broadened face above the antennae, is dominated in the Montane Cordillera by the genus Lasiopogon. These are small grey or brown flies that hunt from the bare ground or from rocks and logs. Larvae develop in the soil. All but one of the species in the region are Cordilleran in origin, and all are at home in the mountains of the Ecozone, although just to the south in the Columbia Basin several species live in grasslands. L. prima is an East Beringian species, being most closely related to the Asian fauna. It ranges through Alaska and the Yukon, reaching the Arctic Ocean, and occurs in the Montane Cordillera only in the eastern foothills of the Rockies in Alberta. L. aldrichii and L. monticola are found mostly along trails in spruce woods across the southern mountains in the Ecozone. L. cinereus and L. trivittatus are found along streams; the former ranges as far north as the Skeena River, northwest of the Ecozone, the latter lives on the eastern side of the Rocky Mountains and spills out onto the prairies. The sole Stichopogon species, S. fragilis, is a tiny silver fly from the sandy Okanagan grasslands at Osoyoos, right on the United States border. Only a single specimen has been collected in Canada. The common Canadian species, S. trifasciatus, widespread across the continent, occurs in southern Alberta grasslands but has not been collected in the Montane Cordillera Ecozone.
SUBFAMILY ASILINAE (19 species, 19%)
In the Montane Cordillera Ecozone the asilines are mostly medium to large, grey, elongate species. Perhaps the most distinctive ones are in the genus Efferia, a common and striking group of flies in grasslands. The males have large club-shaped genitalia; the ovipositors of the females are long and sword-like. These are used to place the eggs in cracks in the soil, in dead plant inflorescences, and so on. The larvae, as in all species in the subfamily, as far as is known, develop in the soil and the ones studied feed on the larvae of scarab beetles (Scarabaeidae). Adult males of all the species have silver-white abdominal segments; in most, only the sixth and seventh segments are white, but in E. benedicti and E. staminea most of the segments are white and clothed with long, white hairs parted along the midline. E. albibarbis is widespread right across the United States, but in Canada it occurs only in sandy habitats in the extreme southern Okanagan and on the beaches of Lake Erie in Ontario. In Canada E. frewingi is mainly a species of the southwestern Great Plains, but in the Montane Cordillera it lives only in the grasslands adjacent to the Columbia and Kootenay Rivers in the Rocky Mountain Trench. E. coulei is a spring species, flying mostly in May and early June; E. benedicti flies mainly from mid-June to the end of July. E. harveyi is active mostly from August through September. All three are among the most common and distinctive grassland asilids and can be present in high densities. Where they occur sympatrically, their staggered flight seasons allow them to fill similar ecological niches. An undescribed species, closely related to E. coulei, occurs in the Okanagan and Thompson valleys.
Other grassland members of the subfamily are Dictropaltum mesae, a small, widespread golden species; Machimus occidentalis; M. vescus; Megaphorus willistoni, a rare little leaf-cutting bee mimic known from only one specimen in Canada (from the southern Similkameen Valley); Neomochtherus willistoni; Proctacanthus milbertii and P. occidentis. Machimus occidentalis and Neomochtherus willistoni are very common species and are also found in open dry forests. However, these two similar species seldom are found together; the former flies mostly in June and is replaced by the latter in late July and August. The two Proctacanthus species are the largest robber flies in the Montane Cordillera, reaching a length of about 40 millimetres.
The remaining species are forest dwellers, mostly Cordilleran species: Machimus callidus, M. erythocnemius, Neoitamus brevicomus and Nevadasilus auriannulatus. Rhadiurgus variabilis is an exception in this list. This species is a member of the Boreal element and is one of only two Holarctic robber flies. It is one of the most northerly dwelling asilids and one of the most common species in the spruce forests of the Ecozone, from the Cascade Mountains to the Rockies, from the Okanagan Highlands to the Omineca Mountains.
