We don’t normally think of flies as being particularly badass. Annoying and filthy maybe, but nobody’s ever run screaming out of a room at the sight of one flying in through the window. Well maybe if it was a tsetse fly. Or looked vaguely like Jeff Goldblum.

Anyways, there actually is one kind of fly that’s just as voracious of a predator as any savage jungle cat or surfer-mangling shark. They’re called robber flies. Or, if you want to be more accurate about them, you can use their other name: assassin flies (Source #1).

robber flies

Photo: Vengolis

The most noticeable thing that sets the flies in the Asilidae family apart from their peers is that many of them sport these crazy-looking hillbilly beards. It’s called a “mystax” (Source #2), and it serves a very functional purpose aside from looking fashionable with skinny jeans, horn rims and an ironic t-shirt. It’s actually designed to prevent thrashing prey from clobbering the robber fly’s head.
Another thing that makes them visually distinct is the way their eyes are set so widely apart (Source #3). In addition to the inherent creepiness, this gives them the superior depth perception required for effective hunting. Their vision is further enhanced by 3 simple eyes set in the depression between the two larger compound ones.
As “true flies”, in the order Diptera, they have only 2 wings instead of 4. Counterintuitively, this actually makes them more agile in flight, as the 2 missing wings have evolved into small, balancing appendages called “halteres” that act just like tiny gyroscopes (Source #4). Put that all together and you have a fly that’s able to make some impressively spectacular aerobatic maneuvers, to include snatching hapless flying prey right out of the air (Source #5).

But enough with all that technical stuff, let’s get to the murder. With all their predatory adaptations, robber flies are fully capable of taking on some rather unexpected prey (Source #6), such as…

Wasps and bees:


Dragonflies much bigger than themselves:

Large beetles:

And even spiders in their own freaking webs:


They can be sneaky too. Some of them even mimic wasps and bees to facilitate the constant entomolicide:

robber flies bee mimic

Photo: Tom Murray

And just like any proper serial killer, robber flies stab their victims to death. After seizing their prey, they shove their short, sharp proboscises into their victims’ flesh like an oil can spout, inject a paralyzing toxin that liquefies the insides, and then suck out the goop like a straw (Source #7). They’ll gleefully and painfully stab your ass as well, if you annoy them enough.

Just in case you were having trouble picturing the aforementioned puncturing and goop sucking, here you go:

Finally, as if subjecting their prey to the humiliation of being completely obliterated by a lowly fly wasn’t bad enough, they appear to be cocky little bastards about it as well. Just Google “robber fly dangling” (after enabling the proper filters, of course), and you’ll come up with dozens of images of them hanging nonchalantly from a perch by just one or two legs as they devour some poor dupe of a bug. It looks pretty precarious, but for some species, like the “hanging thieves” in the genus Diogmites below, it’s just the way they roll (Source #8). Smug little killers, aren’t they?

Photo: Hemolymph

—- Sources —-

Source #1: Few insects will mess with a nest of hornets. That is, few except the notorious robber — aka assassin — fly.

Source #2: The mystax helps protect the head and face when the fly encounters prey bent on defense.

Source #3: All robber flies have 3 simple eyes (ocelli) in a characteristic depression between their two large compound eyes. The antennae are short, 3-segmented, sometimes with a bristle-like structure called an arista. Their eyes are set wide apart for good depth perception, with even a depression between the eyes to allow full sweeping vision.

Source #4: That bright yellow structur under the main wing is a haltere. Most insects have two sets of wings and we can tell quite a lot about where a given species fits in the insect scheme based on how it uses those two. Dragonflies and damselflies use both for flying, in beetles the forewings are “sclerotised” into a rigid case that protects the flight wings and the “true flies” ( order Diptera) have turned thier hind-wings into halteres – greatly reduced wings which act as gyroscopes stabilising the flies’ flight and allowing them to perform aerobatic tricks.

Source #5: Larval and adult robber flies are predators of other insects. The adults are famous for their ability to capture prey in the air. In fact, robber flies will often catch insects that are larger than themselves, such as bumble bees.

Source #6: The robber flies are an abundant and diverse family (Asilidae) known for their predatory behavior. As their common name implies, robber flies have voracious appetites and feed on a vast array of other arthropods, which may help to maintain a healthy balance between insect populations in various habitats (Joern and Rudd 1982, Shurovnekov 1962). Asilidae adults attack wasps, bees, dragonflies, grasshoppers, other flies, and some spiders.

Source #7: They inject a neurotoxin with their proboscis that renders the quarry helpless, while its insides liquefy.

Source #8: Explanation of Names
Diogmites Loew 1866
from Greek διωγμοσ ‘chase, pursuit’
The common name was suggested by Herschel Raney after Dogmites’s habit of hanging by its forelegs while consuming prey.