dipteran mating display?

[Ken Kinman]

Dear All,
      Especially to entomologists on the list.  I know next to nothing about mating displays in flies, but a gnat (ca. 4 mm. long) crawling on my door window caught my eye.  It seemed to be displaying in a way that is reminiscent of male jumping spiders trying to attract a female.  However, the appendages being raised by this gnat seem to be the third (posterior) pair of legs thrust upwards and arching somewhat forward towards the head.  After putting the gnat in a vial, it continued doing this, so I thought perhaps it might be a threat posture as well.

      Anyway, I'm just wondering if this is some kind of posturing that certain male gnats usually do, and if so, to what family of gnats might it belong?
           ----- Thanks,
                          Ken