There were a number of these Snipe flies (apparently genus Rhagio) about today. They constantly paused to wave their forelegs in the air, which have prominent white marks. This is clearly some sort of display. It could be to advertise for a mate, but it might be defensive--this reminds me of the antennae pattern of many wasps, and the raised legs do resemble antennae. This reminds me very much of the displays of Stilt-legged flies, Micropezidae. The function of the displays in that group is not known.