There are over 7,000 species of robber flies world wide; nearly 1,000 in North America.  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 bent on defense. The antennae are short, 3-segmented, sometimes with a bristle-like structure called an arista.

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. Many species have long, tapering abdomens, sometimes with a sword-like ovipositor. Others are fat-bodied bumble bee mimics; the effect is quite convincing.

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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 in mid-air; My National Audubon Society Field Guide to Insects asserts they "pounce on resting insects from above."  Well, 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.

I've seen some smaller species (Tolmerus, Laphria canis) perch and attack - many times missing even slow-flying moths. So much for agility. Then there are the fast, agile ones - good luck seeing them do anything but disappear. 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.