No, it’s not the name of a cool new band you haven’t discovered yet; the Hanging-thieves are a genus (Diogmites) of robber flies that is known for hanging by one or two legs from a perch site while consuming their prey. Robber flies (family Asilidae) are also known as assassin flies, because they catch other bugs while on the wing and eat them with their piercing mouthparts. In general, robber flies are considered to be beneficial, as they eat many pest insects.
I was lucky enough to stumble upon a Hanging-thief (it looks like Diogmites salutans) as it was engrossed in eating a wasp. The robber fly was perched on one of my AED’s (Armadillo Excluder Devices), small wire cages that I set up to protect my fake lizard nests from predation by wily armadillos (that story another time…). The fly had already paralyzed its prey with venom, which also contains enzymes which liquefy the prey’s innards (as in spiders). As I watched, the robber fly carefully manipulated the wasp, turning its body over and around, and inserting its proboscis into likely sites, as if its prey were some sort of delicious juice box. In the video below, you can see the mouthparts of the fly working to slurp up its presumably delicious meal.
My favorite part of seeing this robberfly was observing its moustache, or mystax, which is so tough that it may help protect the fly from injury when it is subduing struggling prey.