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
Royal British Columbia Museum
P.O. Box 9815, Stn. Prov. Govt.
Victoria, B.C.
V8W 9W2

INTRODUCTION

The Asilidae contains about 5000 described species worldwide, and about 200 in Canada (McAlpine 1979). Our knowledge of the Canadian fauna as a whole is only fair, and for this reason it is difficult to put the fauna of the Montane Cordillera Ecozone into perspective. Although in North America the Asilidae is predominantly a southern family, especially diverse in arid and semiarid environments, over the years specimens have been rather frequently collected in the Ecozone. With its dissected physiography and extremes of elevation, the Montane Cordillera contains a wide variety of robber fly habitats, from many in grasslands to those in subalpine forests. The diversity of asilids present reflects this richness of habitats, and the Ecozone supports more species than any other in Canada. The species total for the Montane Cordillera now stands at 101, but it is certain that more remain to be recorded. Thus, the Montane Cordillera supports about half the Canadian species of Asilidae.

Vaseux Lake

Nevertheless, little has been published on the robber flies of the Montane Cordillera. Revisions of large genera such as Cyrtopogon (Wilcox and Martin 1936), Efferia (Wilcox 1966), Lasiopogon (Cole and Wilcox 1938) and Dioctria and related genera (Adisoemarto and Wood 1975) included references to species of the Ecozone. The various taxonomic works of Curran, for example, the designation and summary of the genus Eucyrtopogon (Curran 1923) also are relevant. Foxlee's intensive collecting around Robson in the Columbia Valley of the West Kootenays resulted in specimens (Foxlee 1942) that are still the main source of our knowledge for that region. Adisoemarto (1967), in his overview of the Asilidae of Alberta, included species known from the narrow, mountainous part of the Ecozone in that province. Cannings (1994) updated the species list for the region and published (Cannings 1989) an account of the species found in a grassland typical of mesic sites at low elevations in the southern Okanagan Valley. He analysed the geographic variation in Rhadiurgus variabilis over its holarctic range and documented its distribution, including its range in the ecozone. The fauna is now moderately well-known for the Okanagan Valley, where considerable collecting in the grasslands has occurred, but for the rest of the Ecozone, especially the northern areas, the family has been little studied.

Asilids are predatory flies that as adults pursue other insects (usually flying ones), seize them, and kill them with paralyzing saliva injected through the hypopharynx (tongue). The liquified contents of the prey are then sucked up through the proboscis (Wood 1981). The morphology of the adult fly (especially the prominent eyes, the mouthparts, and the raptorial legs) reflects this mode of prey capture and feeding.

predatory flies

In temperate climates, robber flies usually hunt in open areas where there is plenty of light, and are most active in the warmest parts of the day. Overcast skies greatly curtail their activity. Different genera, and often different species within a genus, have different hunting behaviours and preferences for perching sites.

There is usually little obvious difference between the sexes, except for the genitalia (the morphology of which is sometimes striking, as in Efferia), although females tend to be larger than males and often have broader abdomens. Colour patterns sometimes differ between males and females; in Montane Cordillera species this is particularly evident in Cyrtopogon bimacula (Walker), C. dasyllis Williston, and C. dasylloides Williston in which the males have prominent, dark marks on the wings. Other secondary sexual characteristics occur in males; some that are displayed in species found in the Montane Cordillera include the expanded silver abdominal apex in Nicocles, the striking white abdomens of Efferia, the tufted golden abdominal hairs of Cyrtopogon auratus and the tarsal ornamentation of some other Cyrtopogon species. Males of these latter species signal with their decorated legs during mating displays.

Records of prey taken by Asilidae indicate that they are often opportunistic predators, feeding upon any insect that they can subdue and kill. However, some species show a strong preference for prey from one or two insect orders (Wood 1981). In many instances this may simply reflect the availability of prey in the habitat where the particular robber fly lives.

Detailed life-history studies of robber flies are rare. Melin (1923), studying Asilidae in Sweden showed that in northern species, at least, the larva is the overwintering stage and the pupal stage lasts two to six weeks. He estimated that the life cycle of Laphria species was at least three years and that of Lasiopogon cinctus (Fabricius) was at least two. Both these genera are common in the Montane Cordillera Ecozone. It is likely that larval growth is faster in warmer regions and many species probably live only one year (Theodor 1980).

Larvae are predators of the eggs, larvae and pupae of other insects in the soil or in rotting wood, although in a few species studied the immature larvae, especially, are ectoparasitic on their hosts (Wood 1981). Knutson (1972) has reviewed the literature on this subject.

The world genera of Asilidae are examined by Hull (1962) and the North American genera are keyed by Wood (1981), although these treatments are out-of-date. Wood (1981) gives a summary of the morphology, biology and classification of the North American fauna. The higher classification of the family is still in some turmoil, and a lack of phylogenetic studies has hindered understanding of the relationships of taxa at all levels. The tentative scheme of Artigas and Papavero (1988) for the New World fauna has been adopted by Fisher and Wilcox (1997) for the new Nearctic catalogue of the family, and is also used here.

 

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