Robber Flies (Family Asilidae) are medium to large flies that prey on other flies and insects. These predatory flies are usually seen either waiting on low perches in open, sunny areas or buzzing around as they look for prey. I observed this female Robber Fly below embracing a captured Eristalis Fly (Eristalis sp.) in Arizona's Santa Catalina Mountains. Eristalis Flies are bee-sized, so one of these plump, flower-loving flies would make a good-sized meal for a hungry Robber Fly.
As this photo shows, Robber Flies are quite capable of handling large prey, and they will even take on large, dangerous prey like bees and wasps. However, unlike bees and wasps, Robber Flies do not sting, and the "stinger" on this Robber Fly is actually a harmless ovipositor that she uses for laying eggs. Although they do not sting, Robber Flies do bite and their saliva contains neurotoxins to quickly immobilize prey and proteolytic enzymes to dissolve its internal organs so that they can later be easily sucked out. The proboscis of a Robber Fly is specialized for stabbing and sucking, and after one of these flies captures something, it will return to a perch in its territory to suck out its liquefied meal.
Robber Flies are highly beneficial insects because they prey on a wide variety of pest flies, so I rescue any that get trapped in my house or swimming pool. Although they don't attack or bother humans, Robber Flies can bite quite painfully, so do not capture or pick up these flies with your bare hands.
Although some species of Robber Fly appear bee-like, most species of Robber Fly have a characteristic elongated shape, almost like that of a short, fat dragonfly. Robber Flies have a single pair of powerful wings, spiny legs that aid in grasping prey, a heavily bearded face, and large compound eyes that give them excellent vision for spotting prey.