This fly is a mimic of the Vespid wasps, most notably the potter wasps. It waves its front legs in front of its face to simulate the jointed antennae of wasps. Also, the darkened leading edge of the wings resemble the longitudinal folding of wasp wings. You can see side by side pictures of this fly and the wasp it mimics at http://www.cirrusimage.com/fly_syrphid_Temnostoma.htm