I've seen these in my back yard and at other nearby sites for the last couple of weeks. As they fly, the fringed legs are held out to the side, giving an odd effect. (This might be mimicry of lycid beetles--the outstretched legs resemble the spread elytra of a beetle, perhaps.) I finally got one to hold still and got photos. This one was perched quite still on some violets in the understory of the woods in my back yard. The time was 6:54 p.m. EST, approaching dusk. (Reference below indicates females swarm near dusk.) You can see the air sacks on the abdomen as well in this photo--see explanation below. Length was measured by photographing the leaf it was perched on next to a scale. Image updated 2/22/2020!
Identification based on other photos in the guide, for instance:
That photo is identified as a likely Rhamphomyia longicauda
. See that photo for references on the interesting mating patterns of some members of the Empididae. In some species, this one apparently included, females form swarms and males chose desirable females. Females eat only the nuptial gifts provided by males. The fringe on the female legs, plus the inflated air sacks (visible in this photo) serve to mimic an abdomen filled with eggs. See, for instance: FEMALEMATING SWARMS INCREASE PREDATION RISK...
have caught a male of the same species at my porch light about ten days earlier: