Looking somewhat like a mosquito on steroids, this robber fly (Diptera: Asilidae) was recently hanging out on the side of my house. These typically large, stocky flies are commonly found perched like this, waiting and watching for prey. They feed on a variety of other insects including wasps, bees, dragonflies, grasshoppers, and other flies, usually capturing them in flight (Triplehorn and Johnson 2005). These bold carnivores often capture and eat insects larger than themselves. Some have even evolved to mimic bees and wasps, presumably as a result of a selection pressure to get close to them without detection.
There are thousands of species of robber flies, and although many resemble the individual shown here they do vary quite a bit. In spite of this, there are a few relatively obvious characteristics which identify members of the family. Each possesses a sunken area between the eyes with three ocelli, a long and tapered abdomen, three-segmented antennae with terminal styles, and a prominent beard (mystax).
Considering their ravenous predatory nature, it’s best to approach robber flies with caution. Although they generally ignore people, many species can inflict painful bites when handled carelessly (Triplehorn and Johnson 2005).
Triplehorn, C.A. and N.F. Johnson. 2005. Borror and DeLong’s Introduction to the Study of Insects. Seventh Edition. Thomson Brooks/Cole, Belmont, CA.