I have finally seen some more of these flies on a stinkhorn mushroom (Phallus ravenelii). There were a couple of the mushrooms up after a good rain, and a number of the flies. Looking at the images, I can see the difference between male and female, presumably, the males having very large eyes. Size estimated.
(I collected a couple of specimens the next day and will preserve them.)

Previous photos of whole mushroom, with its fly associates: