Saw these two flying about slowly in our yard in the mid afternoon & tried to photograph them in the air, but couldn't get them in focus. They finally landed on a Ceanothus bush. I think they are Empidids of some kind. According to (1): "Some species of Hilara & Empis that occur in the Northwest construct balloons, which they carry about as a means of attracting females. These balloons can be made of silk (spun from the basal segment of the front tarsi) or of a frothy material from the anus, & they usually contain prey." They are smaller & the proboscis doesn't appear to be as long as the other individuals I have seen in our yard. (live oak/chaparral habitat).