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ROBBER FLY
Megaphorus willistoni
Family Asilidae - True Flies
Order Diptera - Flies
Risk Status
Official status
The Robber Fly is currently listed as G5
S1 status by CDC.
Image Credits: Robber Fly sketch by Hannah
Nadel in 'Rare Invertebrates of the South Okanagan' brochure. Photo by
Okanagan Universtiy College.
Distinguishing features
Order Diptera, Flies, are easily distinguished from other insects because
they have only one pair of normal wings. The second pair, just behind
the first, is represented by two knobbed organs, the halteres,
used to stabilize the body during flight. Most flies have large compound
eyes and mouthparts that are modified for piercing, lapping, or sucking
fluids.
Robber flies are common, swiftly flying predators, 5-50 mm long.
Many have stout, spiny legs, a dense "beard" of bristles
on the face, and a depression in the forehead between the compound
eyes. Many species are important predators in grassland habitats.
Megaphorus willistoni is a squat little fly bristling with
grey and yellow hairs, which give it an uncanny resemblance to leaf-cutter
bees as they buzz from plant to plant, hovering here and there in
search of prey.
Distribution
Map
Red dots indicate specimen records or confirmed breeding sites.
British Columbia
Only one specimen of this small robber fly has been recorded in Canada,
just a few hundred metres from the International Boundary in the southern
Similkameen Valley.
North America
Habitat
They live in the hot, dry grasslands and shrublands that remain and
seem to prefer areas with many flowers, since this is where their favourite
prey, small bees and wasps, abound.
Why is it endangered?
Rare invertebrates of the south Okanagan and Similkameen valleys such
as this species are threatened not by direct exploitation, but by loss
or degradation of their habitats. They are at risk because their ecosystems
are at risk.
The grasslands of the southern interior of the province are a valuable
agricultural resource, and their rich soils have been ploughed and irrigated
to produce tree fruits, grapes, and vegetables. Pesticide use has probably
had a great impact on native insects living in around agricultural areas.
As well, heavy grazing has altered the plant composition of grasslands,
changing the invertebrate communities. The Robber Fly is effected by compaction
caused by trampling by cattle.
The massive diversity of invertebrate species in British Columbia makes
it very difficult for entomologists to do a literature or collection survey
to determine which species are endangered or threatened. Specialized,
detailed surveys will be required for almost every species that is suspected
of being endangered. Despite a general ignorance about invertebrate distribution,
information is known about a number of species that are confined to threatened
habitats of very limited extent in the Thompson-Okanagan valleys.
Biology
Eggs are laid in a case-like mass on a dead plant stem; the larvae develop
in the soil where they apparently prey on other insect larvae.
Like all robber flies, they capture their prey in their bristly legs
and kill it with toxic saliva injected through their short proboscis.
The dissolved tissues are then sucked back up through the proboscis.
Flies exhibit complete metamorphosis.
The larvae of many species are soft, legless, and headless. They are called
maggots and live in soil, decaying material, or as parasites of vertebrates,
snails, or other insects.
Sources for more information
Related On-line Sites to Visit
Publications
Rare Invertebrates of the South Okanagan, brochure, MOE, March 1995
Biodiversity in BC, Cannings , Ch.4, 1994, p. 49
The Audubon Society Field Guide to North American Insects and Spiders,
1980, p. 661
Museum Specimens
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Industry Canada
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