Introduction to the Robber Flies (Asilidae)
of British Columbia
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.
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,
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
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.
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
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.
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.
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:
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
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,
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Brown et al. (eds.) Manual of Central American Diptera: Volume 1. NRC Research
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Fisher, E.M. and J. Wilcox. 1997. Catalogue
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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.
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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
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James, M.T. 1941. The robber flies of
Colorado (Diptera: Asilidae). Journal of the Kansas Entomological Society 14:
Knutson, L.V. 1972. Pupa of Neomochtherus
angustipennis (Hine), with notes on feeding habits of robber flies and a
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Lavigne, R.J. 1972. Asilidae of the Pawnee
National Grasslands in northeastern Colorado. University of Wyoming
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Lavigne, R.J. and F. Holland. 1969.
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