Family Asilidae - Robber Flies - North American Insects & spiders

Robber Flies – Family Asilidae

robber fly Efferia aestuans Efferia aestuans
There are over 7,500 species of robber flies world wide, ~1,000 in North America. Florida alone has more than 100 species. [5]
Species counts are, of course, approximate (and I suspect are wildly inaccurate on the low side).

All robber flies have stout, spiny legs, a dense moustache of bristles on the face (mystax), and 3 simple eyes (ocelli) in a characteristic depression between their two large compound eyes. The mystax helps protect the head and face when the fly encounters prey armed with weapons. e.g., bee stingers. The antennae are short, 3-segmented, sometimes with a bristle-like structure called an arista. The word arista comes from Latin “beard of grain” which, of course is what one looks like. [6]

Female Robber Fly, Laphria grossa Female Robber Fly Laphria grossa

The short, strong proboscis is used to stab and inject victims with saliva containing neurotoxic and proteolytic enzymes which paralyze and digest the insides; the fly then sucks the liquefied meal much like we vacuum up an ice cream soda through a straw. The proboscis is the short, stabby thing protruding beneath this fly’s eyes.

You yourself have probably been personally attacked by an organism with a proboscis a time or two — because that what mosquitoes and deer flies and all sorts of biting insects use to pump blood out your body. Proboscises are used by bugs to suck juices out of plants – you may have experience with that as well if thrips are devouring your roses. Such a dastardly business, interspecies warfare.

Insect Orders That Suck

Order	Example Species	Approx. Species Count	Proboscis Present?	Notes
Lepidoptera	Butterflies, moths	~180,000	✅ Yes	Coiled proboscis for nectar
Diptera	Flies, mosquitoes	~160,000	✅ Many	Mosquitoes, some flies use proboscis
Hemiptera	True bugs (aphids, cicadas)	~80,000	✅ Yes (stylet-type)	Piercing-sucking proboscis
Hymenoptera	Bees, wasps, ants	~150,000	❌ Mostly no	Bees have tongue-like structures but not true proboscis
Coleoptera	Beetles	~400,000	❌ No	Chewing mouthparts
Orthoptera	Grasshoppers, crickets	~25,000	❌ No	Chewing mouthparts
Others	Various minor orders	~5,000+	❌ Rare	Varies, but mostly no proboscis

Giant Robber Fly – Promachus sp.

The Buzz About Robber Flies
Much has been made of the speed and agility of these flies. Many books and sites cite them as fast and agile flyers, taking insects on the wing. Others attribute to them “still-hunting”, that is, perching and attacking prey in mid-air; My National Audubon Society Field Guide to Insects asserts they “pounce on resting insects from above.”

From my experience, I can say not all robber flies are fast, and not all robber flies are agile. The big robbers (Promachus) lumber and buzz in flight – it is very easy to follow them, and I’ve often seen them pounce on honeybees busy at flowers. Then there are the fast, agile ones – good luck seeing them do anything but disappear.

I’ve seen some smaller species (Tolmerus, Laphria canis) perch and attack midair prey – many times missing even slow-flying moths. So much for agility. Whatever the species, robbers are fun to watch. I consider myself lucky to see a capture: most often I find the fly enjoying a meal, or flying off with prey firmly impaled on that terrible beak.

Laphria sericea

Robber flies mating – Machimus sp.

	Dioctria hyalipennis
	Laphria canis
	Cerotainia albipilosa

Bumblebee Mimicry

Laphria with beetle prey
	Laphria thoracica
	Laphria flavicollis

Female Robber Fly, Laphria grossa

Male Brownbelted Bumble Bee - Bombus griseocollis

Male Bumble Bee

Some robber flies are convincing bumble bee mimics. This evolutionary adaptation allows them to reap the benefits of aposematic (warning) coloration without the high metabolic cost of constructing and maintaining a venom-delivery system.

Adult robber flies attack other flies, beetles, butterflies and moths, various bees, dragon and damselflies, ichneumon wasps, grasshoppers, and some spiders. Courtship behavior consists of the male glomming onto the female as if she were prey. Copulation is accomplished tail-to-tail, with the genitalia interlocked. The duo has no trouble flying around in this condition; the male generally towing the female backwards.