These are rather small as robber flies go: 10mm (about 3/8"). Adult robber flies attack other flies, beetles, butterflies and moths, various bees, dragon and damselflies, ichneumon wasps, grasshoppers, and some spiders. This orange-abdomened female has managed to capture an ichneumon wasp, and is busy sucking out the liquified guts.

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. Take a close look at any insect that looks like a bumble bee if it's sitting on a leaf - chances are, it's a robber fly. (Bumble bees as a rule do not sit in one spot for more than a few seconds).