Per the comment's under your more recent post below:

 

...the projecting, somewhat flattened terminalia indicate a female here.

Moved from Acrocera.

I carefully went through the key in Sabrosky (1948), supported by both Sabrosky (1944) and Cole (1919)...as well as descriptive info on pg. 223 of Cole & Schlinger(1)(1965). I also cross-referenced with the synonymy and geographic range info in the catalog of Stone(2)(1965)...and after much scrutiny (and sleeping on it) I'm fairly well convinced that this is a female of A. melanderi.

It keys to A. steyskali in Sabrosky (1948) via the following characters:
    1) 2nd longitudinal vein (i.e. R2+3) present and complete...as clearly seen in your 1st image;
    2) Mesonotum without stripes, entirely black except for whitish postalar calli;
    3) Coxae yellow, not entirely black (see front leg of 1st image); female cerci more or less acute but not narrowly acuminate (see 3rd image);
    4) Dorsum of abdomen predominantly black (as in Figs. 1 & 1a of Sabrosky (1944)), not (very!) predominantly yellow (as in Fig. 34 of Cole (1919)).

The discussion in Sabrosky of this species is compatible with this individual, and Stone(2) indicates A. steyskali was put under synonymy with A. melanderi (which Sabrosky 1948 anticipated) and it's range includes California.

Finally, this individual agrees well with the other BG post of A. melanderi below, ID'd by Dennis Haines:



The color pattern of the female dorsum for A. steyskali (= melanderi) shown in Fig 1a on pg. 413 of Sabrosky (1944) has smaller amounts of yellow area on T2 and T3 than seen here, but Sabrosky stated that "there is considerable variation in the extent of the color" there. He also stated: "One consistent feature is that the venter of the abdomen is the predominantly dark brown to blackish type (fig. 2a of Sabrosky, 1944)." This appears consistent with the 1st & 2nd images here.

I have to say I am pestered by a slight doubt here, in that this might possibly go to A. arizonensis in Sabrosky's treatment, due to my possibly misinterpreting whether the cerci are acuminate...and also whether the coxae are yellow (although they certainly don't look "shining black" to me, which is the alternative given for arizonensis). Also, A. arizonensis is like A. bakeri in being very predominantly yellow on the abdomen, and it is not recorded outside AZ (and Sonora, Mexico) in the literature. So I think it's quite unlikely this is A. arizonensis. But I hope Dennis and/or another acrocerid expert will check my work here, just in case.

At any rate, this is a(nother) great find, Alice. Good to have excellent field images of this species and a coastal dune California record!!

Moved from Small-headed Flies.

This is definitely Acrocera, from the nearly imperceptible mouthparts; threadlike antennae positioned near the top of the head; and bare eyes.