Introduction to the Robber Flies (Asilidae)
of British Columbia

(photo)

by
Robert A. Cannings
Royal British Columbia Museum

The robber fly family (Diptera: Asilidae) contains over 7000 described species worldwide (Geller-Grimm 2012). Robber flies are predators 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 (Whitfield 1925, 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. 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 behaviour and preferences for perching sites.

In British Columbia, the general appearance of robber flies ranges from the stout, densely hairy bumble bee mimics in the genus Laphria to the strikingly slender and almost naked Leptogaster species. The largest asilids in the province are also our largest Diptera – species of Proctacanthus, which can be 40 mm long; the smallest is Stichopogon fragilis at 3mm long.

There is usually little obvious difference between the sexes, except for the genitalia, although females tend to be larger than males and often have broader abdomens. Colour patterns sometimes differ between males and females. In Lasiopogon these differences are minor; often the tomentum on the abdomen is more extensive (but less dense) in the female. In some genera, however, differences are marked. In some species of Cyrtopogon, for example, the males have prominent, dark marks on the wings. Other secondary sexual characteristics occur in males, such as the expanded silver abdominal apex in Nicocles, the striking white abdomens of Efferia, and the tarsal ornamentation of some Cyrtopogon species.

Records of prey taken by Asilidae indicate that they are mostly 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. Lasiopogon is known to attack several orders of insects, but is most commonly found with Diptera as prey (Melin 1923, Poulton 1906, Hobby 1931, Lavigne and Holland 1969, Lavigne 1972, Weinberg 1978).

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 (Fab.) was at least two. 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 were treated by Hull (1962) and the North American genera were keyed by Wood (1981). Engel (1930) is long out of date but is the only publication that has dealt with the entire family at the species level in the Palaearctic, although Lehr has produced a significant body of systematic and ecological publications for various groups of the Palaearctic fauna (e.g. Lehr 1962, 1984a&b, 1996). Majer (1997) keyed the European genera. The Palaearctic species are listed in Lehr (1988). Various publications examined parts of the Holarctic fauna from a regional geographic perspective, e.g., Hine (1909), Bromley (1934, 1946), James (1941), Adisoemarto (1967), Cole (1969), Oldroyd (1969b, 1970a), Baker and Fischer (1975), Nelson (1987), Weinberg and Bächli (1995) or a taxonomic one, e.g., Wilcox and Martin (1936), Cole and Wilcox (1938), Wilcox (1966), Adisoemarto and Wood (1975), Martin (1975), Fisher (1977), Lehr (1984a), Cannings (2002). Wood (1981) gave a summary of the morphology, biology and classification of the North American genera. Martin and Wilcox (1965) listed the North American species known at that time; a more recent list is Fisher and Wilcox (1997). Fisher (2009) documented the Central American fauna. Oldroyd (1970b, 1974) and Londt (e.g. 1985, 1994) have treated parts of the Afrotropical fauna. The latest phylogeny of the family, based on both molecular and morphological characters, was published by Dikow (2009), although the resulting classification is not comprehensive. Lavigne (1999) and Lavigne et al. (1978) have produced bibliographies for asilid literature subsequent to Hull's (1962) review and Geller-Grimm (2012) has compiled a comprehensive bibliography on the internet.

Biogeography of BC Asilidae

The majority of the 116 species of Asilidae known from British Columbia are restricted to North America; three (Laphria gilva, Lasiopogon hinei  and Rhadiurgus variabilis) also occur in Eurasia. Species may be grouped with others that share similar distributions to form what can be termed faunal elements:

One species, Lasiopogon prima, is East Beringian. It is confined to the extreme northwest in the New World, presumably having lived in the Beringian glacial refugium during the Pleistocene ice ages; it does not occur west of the Bering Strait. Lasiopogon hinei is Palaearctic-East Beringian, with an East Beringian distribution in in North America (Alaska, Yukon and adjacent areas), but with a widespread range across Eurasia.

Ten species are Boreal; they occur in the northern spruce forests, across the boreal zone from treeline to the southern margin. In general, these species range from the Atlantic Provinces across the northern New England states, Quebec, northern Ontario, parts of the northern tier of mid-western states, the Prairie Provinces north of the Great Plains, and northern British Columbia, often ranging considerably southward in the higher mountains and plateaus of the western Cordillera. Seven of these species are in the genus Laphria.

More than half (60) of the BC species are Cordilleran. They live in the mountains and plateaus of western North America, mainly in coniferous forests. Dominant genera in this group are Cyrtopogon (17 species), Laphria (14 species), Eucyrtopogon (7 species). Three additional species have similar habitat preferences but are restricted to the Coast Mountains.

Intermontane species are largely confined to grasslands, shrub-steppes and adjacent open woodlands in the valleys and plateaus of the Cordillera. Twenty-two species are represented, six of them in Efferia. Another grassland species, Lasiopogon quadrivittatus, is the sole Great Plains species in the province, restricted to the northeastern corner east of the Rockies.

Ten species from a wide variety of genera, are Pacific Coastal, inhabiting forests, meadows and beaches of the Pacific coastal lowlands. Species confined to west of the 100th meridian but otherwise ranging widely in North America are termed Western; four of these occur. Another four, the Austral species, range across the continent mostly south of the boreal forests, and with most of their range in the United States.

References

Adisoemarto, S. 1967. The Asilidae (Diptera) of Alberta. Quaestiones entomologicae 3: 3-90.

Adisoemarto, S. and D.M. Wood. 1975. The Nearctic species of Dioctria and six related genera (Diptera: Asilidae). Quaestiones entomologicae 11: 505-576.

Baker, N.T. and R.L. Fischer. 1975. A taxonomic and ecologic study of the Asilidae of Michigan. Great Lakes Entomologist 8: 31-91.

Bromley, S.W. 1934. The robber flies of Texas (Diptera: Asilidae). Annals of the Entomological Society of America 27: 74-113.

Bromley, S.W. 1946. Asilidae. The Diptera or True Flies of Connecticut, Part VI. Guide to the insects of Connecticut. Connecticut State Geological and Natural History Survey Bulletin No. 69. Hartford, Conn. 47 pp.

Cannings, R.A. 2002. The systematics of Lasiopogon (Diptera: Asilidae). Royal British Columbia Museum, Victoria, BC. 354 pp.

Cole, F.R. 1969. The flies of western North America. University of California Press, Berkeley. 693 pp.

Cole, F.R. and J. Wilcox, 1938. The genera Lasiopogon Loew and Alexiopogon Curran in North America (Diptera-Asilidae). Entomologica Americana 43: 1-90.

Dikow, T. 2009. A phylogenetic hypothesis for Asilidae based on a total evidence analysis of morphological and DNA sequence data (Insecta: Diptera: Asiloidea). Organisms, Diversity and Evolution 9: 165-188.

Engel, E.O. 1930. Asilidae (Part 24), in E. Lindner (ed.) Die Fliegen der palaearktischen Region, vol. 4. Schweizerbart'sche, Stuttgart. 491 pp.

Fisher, E.M. 1977. A review of the North American genera of Laphystiini with a revision of the genus Zabrops Hull (Insecta: Diptera: Asilidae). Proceedings of the California Academy of Sciences, Series 4, 41(5): 183-213.

Fisher, E.M. 2009. Asilidae (robber flies, assassin flies, moscas cazadoras, moscas ladronas). Pp. 585-632 in B.V. Brown et al. (eds.) Manual of Central American Diptera: Volume 1. NRC Research Press, Ottawa, ON. 714 pp.

Fisher, E.M. and J. Wilcox. 1997. Catalogue of the Robber flies (Diptera: Asilidae) of the Nearctic Region. Unpublished draft. 48 pp.

Geller-Grimm, F. 2012. Robber Flies (Asilidae). Internet site at http://www.geller-grimm.de/asilidae.htm

Hine, J.S. 1909. Robberflies of the genus Asilus. Annals of the Entomological Society of America 2: 136-172.

Hobby, B.M. 1931. The British species of Asilidae (Diptera) and their prey. Transactions of the Entomomolgical Society of the South of England 6 (1930): 1-42.

Hull, F.M. 1962. Robberflies of the world: the genera ofthe family Asilidae. Smithsonian Institution Bulletin 224 (Parts 1 & 2): 1-907.

James, M.T. 1941. The robber flies of Colorado (Diptera: Asilidae). Journal of the Kansas Entomological Society 14: 27-53.

Knutson, L.V. 1972. Pupa of Neomochtherus angustipennis (Hine), with notes on feeding habits of robber flies and a review of publications on morphology of immature stages. (Diptera: Asilidae). Proceedings of the Biological Society of Washington 85: 163-178.

Lavigne, R.J. 1972. Asilidae of the Pawnee National Grasslands in northeastern Colorado. University of Wyoming Agricultural Experiment Station, Science Monograph 25: 1-35.

Lavigne, R.J. 1999. Bibliography update for the Asilidae (Insecta: Diptera). University of Wyoming Agricultural Experiment Station, Science Monograph 55: 1-91.

Lavigne, R.J. and F. Holland. 1969. Comparative behavior of eleven species of Wyoming robber flies (Diptera, Asilidae). University of Wyoming Agriculture Experiment Station, Science Monograph 18: 1-61.

Lavigne, R.J., D.S. Dennis and J.A. Gowen. 1978. Asilid literature update 1956-1976 including a brief review of robber fly biology. University of Wyoming Agricultural Experiment Station, Science Monograph 36: 1-134.

Lehr, P.A. 1962. Some aspects of the evolution of robber flies. Trudy nauchno-issled. Inst. Zasch. Rast. KazASKhN, Alma-Ata 7: 347-382.

Lehr, P.A. 1984a. Assassin Flies of the tribe Stichopogonini (Diptera: Asilidae) from the fauna of the U.S.S.R. 1. Zoologicheskii Zhurnal 63: 696-706 (In Russian).

Lehr, P.A. 1984b. Assassin Flies of the tribe Stichopogonini (Diptera: Asilidae) from the fauna of the U.S.S.R. 2. Zoologicheskii Zhurnal 63: 859-864 (In Russian).

Lehr, P.A. 1988. Family Asilidae, pp. 197-326 in A. Soos and L. Papp (eds.) Catalogue of Palaearctic Diptera, Vol. 5, Athericidae-Asilidae. Elsevier, Amsterdam. 446 pp.

Lehr, P.A. 1996. Robber Flies of Subfamily Asilinae (Diptera, Asilidae) of Palaearctic: ecological and morphological analysis, taxonomy and evolution. Dalnauka, Vladivostok. 184 pp. (In Russian).

Londt, J.G.H. 1985. Afrotropical Asilidae (Diptera) 10. The genus Hypenetes Loew. Annals of the Natal Museum 26(2): 377-405.

Londt, J.G.H. 1994. Afrotropical Asilidae (Diptera) 25. A key to the genera of the subfamily Stenopogoninae with new synonomy and descriptions of six new genera. Annals of the Natal Museum 35: 71-96.

Majer, J.M. 1997. European Asilidae. Pp. 549-567 in L. Papp and B. Darvas (Eds.). Contributions to a Manual of Palaearctic Diptera. Vol 2. Nematocera and Lower Brachycera. Science Herald, Budapest.

Martin, C.H. 1975. The generic and specific characters of four old and six new Asilini genera in the western United States, Mexico and Central America (Diptera: Asilidae). Occasional Papers of the California Academy of Sciences 119:1-107.

Martin, C.H. and J. Wilcox. 1965. Family Asilidae, pp. 360-401 in A. Stone, C.W. Sabrosky, W.W. Wirth, R.H. Foote and J.R. Coulson (Eds.), A catalogue of the Diptera of America north of Mexico. U.S. Department of Agriculture, Washington, D.C. 1696 pp.

Melin, D. 1923. Contributions to the knowledge of the biology, metamorphosis and distribution of the Swedish asilids. Zoologiska Bidrag Fron Uppsala 8: 1-317.

Nelson, C.R. 1987. Robber flies of Utah (Diptera: Asilidae). Great Basin Naturalist 47: 38-90.

Oldroyd, H. 1969b. Diptera Brachycera, Section A -- Tabanoidea and Asiloidea. Handbooks for the identification of British insects. Vol. 9, Part 4. Royal Entomological Society of London. 136 pp.

Oldroyd, H. 1970a. Diptera, I -- Introduction and key to families. Handbooks for the identification of British insects. Vol. 9, Part 1. Royal Entomological Society of London. 104 pp.

Oldroyd, H. 1970b. Studies of African Asilidae (Diptera). I. Asilidae of the Congo Basin. Bulletin of the British Museum (Natural History), Entomology 24: 207-334.

Oldroyd, H. 1974. An introduction to the robber flies (Diptera: Asilidae) of South Africa. Annals of the Natal Museum 22: 1-172.

Poulton, E.B. 1906. Predaceous insects and their prey. Part 1: Predaceous Diptera, Neuroptera, Hemiptera, Orthoptera and Coleoptera. Transactions of the Entomological Society of London 1906: 323-409.

Theodor, O. 1980. Diptera: Asilidae. Fauna Palestina, Insecta II. The Israel Academy of Sciences and Humanities, Jerusalem. 448 pp.

Weinberg, M. 1978. Contribution to the knowledge of the morphology and biology of the species Lasiopogon montanus Schin. (Diptera, Asilidae) from the southern Carpathian mountains. Traveaux du Museum d'Histoire Naturelle "Grigore Antipa" 19: 293-296.

Weinberg, M and G. Bächli. 1995. Diptera: Asilidae. Vol. 11, Insecta Helvetica (Fauna). Schweizerischen Entomologischen Gesellschaft, Geneva. 124 pp.

Whitfield, F.G.S. 1925. The relationship between the feeding habits and structure of the mouthparts in the Asilidae. Proceedings of the Zoological Society of London 1925: 599-638.

Wilcox, J. and C.H. Martin. 1936. A review of the genus Cyrtopogon Loew in North America (Diptera- Asilidae). Entomologica Americana 16 (New series): 1-95.

Wilcox, J. 1966. Efferia Coquillett in America north of Mexico (Diptera: Asilidae). Proceedings of the California Academy of Sciences 34: 85-234.

Wood, G.C. 1981. Asilidae. Chapter 42. Pp. 549-573 in J.F. McAlpine, B.V. Peterson, G.E. Shewell, H.J. Teskey, J.R. Vockeroth, and D.M. Wood (Eds.). Manual of Nearctic Diptera. Volume 1. Agriculture Canada Monograph 27: vi+1-674.