People passionate about nature

Robber Flies

Ruthless killers of the insect world

Photos and article by Deanna Dodgson

Found worldwide, robber flies belong to a large order of predatory flies known as Asilidae.  According to one source, their proficiency at hunting insects is what prompted beekeepers in Germany two centuries ago to dub the bee-like flies that predated their colonies as thieves, or robbers.

Above: Neoitamus-orphne basking

More than 200 species of robber flies, or asilids, are found in Canada.  It is estimated that 40-50 species live in Manitoba.  In North America, asilid diversity is highest in the arid and semiarid regions of the southwest. 

Robber flies come in a variety of sizes (5-40 millimeters) and shapes (slender to stout).   Base colours range from grey to black or sometimes reddish.  The extent, location and colour of hairs and bristles on the body varies, but all robber flies are bearded and have a a row of hairs and/or bristles (mystax) above the mouthparts.   There are black and yellow bee mimics and others are striped on the abdomen, resembling sand wasps.  Some have patterned wings.  The sexes generally look alike but can easily be told apart by genitalia.  Females are often larger than males.  Their legs are normally stout and sometimes patterned.  A proboscis is used for feeding. 

Above: Male Laphria cinerea, a denizen of eastern pine forests

Genrally preferring open, sandy areas, robber flies may be found in grasslands, pastures, dry coniferous forests and meadows.  A small number of asilids perfer shady woods.  The bulk of their daily activity occurs during the warmest parts of the day.  Individuals of different species may perch anywhere from ground level up to fifteen feet above ground level on grasses, leaves or tree trunks. 

Most robber flies hunt flying insects, others pounce on their prey.  Asilids use their long, bristled legs to capture and carry the victim to a nearby perch.  The prey is impaled by the robber fly's proboscis, and special enzymes in their saliva quickly immobilize the victim, turning the contents of the insect into an ingestable liquid.  The liquid is then sucked up through the proboscis.  Finishing a large meal such as a Bombus bee might take upwards of 2 ½ hours.

Above: Female Laphria janus with prey

Usually targeting arthropods smaller than themselves, prey includes leafhoppers, beetles, antlions, grasshoppers, cicadas, wasps, bees, moths, butterflies, and dragonflies (even large ones), and occasionally spiders.  Other robber flies are also on the menu – these may also include their own species.   Some asilids feed preferentially on individuals from  certain insect orders but most are generalists.

Courtship among robber flies is fairly rare; most males simply locate and overpower a female with which to mate.  Promachus males, however, will noisily hover near the female for up to one half minute or more before attempting to mate.  Courtship behaviour in robber flies is thought to have evolved as a means for males to avoid being eaten by the larger females who would otherwise see them as prey! 

When looking for suitable locations to lay eggs, females use the tip of the ovipositor to test the substrate.    Various species will lay eggs in soil or loose sand, in or on live or dead vegetation, or in crevices within decaying wood.  The larvae pass through 4 to 7 instars (stages) before becoming adults, and are also predatory, hunting eggs, larvae and pupae of other insects.   It takes 1 to 3 years to complete the life cycle. 

Above:  Female Promachus probing rush stem

The asilid fauna of Manitoba is imperfectly known.  With effort, changes in distrubtion for known species will likely be recorded and discovery of new species may be documented.

If you are interested in learning more about robber flies, here are some resources to get you started:

• Canadian grassland species are discussed in “The Robber Flies (Diptera: Asilidae) of Western Canadian Grasslandsands” by Dr. Rob Cannings.   PDF available here.

BugGuide, an insect identification/informational website, has some general information, as well as species photos and links to other resources.