This image shows a relatively small clump of Ephydra hians along the shore of Mono Lake.

These keyed to genus Ephydra subgenus Hydropyrus in the MND(1) via couplets: 1b; 64a; 65a; 66b; 67a; 68. They went to E. hians in Aldrich (1912), and moreover, E. hians is the only species of Ephydra occurring at Mono Lake according to the 1988 paper "Comparative population ecology of Ephydra hians Say (Diptera: Ephydridae) at Mono Lake (California) and Abert Lake (Oregon)" by D. B. Herbst (PDF here).

This is an interesting (even famous?) fly...Mono Lake, the Mono Paiute (or Kucadikadi), and Mono County, CA are all named after it. The pupae were collected in large numbers as a food source in historic times. The natural history of this species has been well-researched and is quite fascinating. See the info page for links to more detailed references.