Some species of bee fly appear similar to small bumble bees, being very hairy and quite heavy-bodied. Of these, our local species range in color from a medium brown to silver-brown, sometimes with whitish or blackish abdominal markings. (Bumble bees are black with yellow-white markings.) A bee fly in this group typically has rather long legs and a very long slender proboscis. Other species appear more like house flies overall. Of these, our local species appear covered with a dense velvet pile, often dark brown to black in color. Some of these have narrow whitish segments on their abdomens and are referred to as progressive bee flies in the Audubon Society Field Guide to the Insects and Spiders. There are many other species that do not fit into either of these categories. Bee flies range in size from about 1 to 22 mm.
The wings of many species have extensive black or brown markings. Because of this, people from the north-east might easily mistake some species for blood-thirsty deer flies.
The larvae (juveniles) of some species feed on grasshopper eggs. A number of other species are parasites on different kinds of moth caterpillars.
I often see bee flies hovering around or feeding at small flowers. While they appear to land, many do not stop beating their wings in their brief feeding stops, so they are accompanied by an audible hum. Bee flies often rest on the bare ground of the trails where they favor sunlit patches. If disturbed they may move on or hover just above the ground. The humming sound of the beating wings and the prominent proboscis of some of the larger bee flies give one the impression of a very mean-looking mosquito. While some entomologists, who have attempted to catch these in their bare hands, can attest to their ability to pierce flesh, the bee fly is just waiting for the disturbance to pass. Watch for them.
Ron Lyons (volunteer 1990-1999)