A number of these bombyliids were seen on the flowers of Arctostaphylos viscida ssp. pulchella, practicing secondary nectar-robbing...i.e. taking nectar through pre-existing holes likely cut by bees with strong mandibles, who were the initial primary nectar thieves. I'm often amazed at the large proportion of manzanita flowers I find which show evidence of nectar-robbery (i.e. holes chewed near the bases of the corollas).

This bee fly keys to subfamily Phthiriinae, tribe Phthiriini in the MND(1)...major diagnostic character are: Eyes entire along posterior edge; occiput not swollen/bilobate above; setae present but no strongly developed bristles on thorax and legs; abdomen tapering-cylindrical (not swollen apically); R4+5 branched; first flagellomere stout, blunt, with a minute stylus arising from a small sulcus before apex; Crossvein dm-cu present, closing off discal cell distally; proboscis long, projecting; four posterior cells (i.e. vein M2 present); R2+3 originating proximal to base of cell dm.

In the current circumscription of nearctic Phthiriini there are two genera: Acreophthiria and Neacreotrichus. From Joel's comment here, the black, pilose condition here suggests Neacreothichus. Evenhuis & Greathead(2) list 4 species of Neacreothichus in California: diversus, florale, humilis, and vittiventris.