This rather small & delicate bee fly was nectaring on a flower head of Chaenactis (fremontii?)...along with an intriguing and much smaller black & white beetle, visible to the left (it's not a beetle, but a thrips!...see comments thread below). The habitat was an open, sandy, flower-filled, canyon bottom; with braided washes nestled within gentle slopes covered in creosote scrub on the (desert-transition) east side of the southern Sierra Nevada.

One can see from the images in this series that:
1) the clypeus reaches to directly below the antennae; 2) the wings have 4 posterior cells; 3) there is a (very small) antennal sulcus bordered by (minute) dorsal and ventral prongs (see full-size image).
The above characters indicate subfamily Phthiriinae, using the key to subfamilies of Bombyliidae in Evenhuis(1). This subfamily is represented by two tribes in the nearctic: Phthiriini (with genera Acreophthiria and Neacreotrichus); and Poecilognathini (with genera Euryphthiria, Poecilognathus, Relictiphthiria, and Tmemophlebia). The key on page 41 of Evenhuis(1) separates the tribes by:

1) Metapleuron with micropubescence (best seen under high magnification); plus lots of genitalia characters......Phthiriini; 1) Metapleuron bare, without micropubescence; plus lots of opposing genitalia characters......Poecilognathini;
I think the metapleuron is visible in the full-size version of the 4th image in the series...it appears as a small, tawny-colored, downward-pointing triangular sclerite, just below, and slightly posterior to, the very base of the wing. To the resolution discernible in the photo, it looks "bare"...which suggests tribe Poecilignathini. Beyond that, the keys in Evenhuis focus mostly on genitalia However, the markings on the head, thorax, and abdomen here look a lot like those of a number of posts currently placed under Neacreotrichus:

     

I believe males of most nearctic taxa of Phthiriinae are black to grayish, but in some genera (e.g. Poecilognathus, Neacreotrichus) the females are constrastingly colorful, with yellow and/or browns. The color here, as well as the well-separated eyes, suggest this is a female. (Although, somewhat unusually, males of some taxa in the subfamily also have eyes well-separated.)