At the time E. tristis was descibed it included several undescribed species. Sabrosky notes that E. tristis has a large ocellar tubercle on top of the head, and yours does not. So its something else in Eulonchus.

Eliminating tristis here on clear grounds is progress. Patient accumulation of observations and insights may eventually lead to a species ID...I like BugGuide!

Q: Does "Sabrosky" here refer to this reference?

Regarding the absence of a distinct ocellar tubercle on this particular specimen, that appears to be a distinctive character compared to the other 5 species of Eulonchus currently posted on BugGuide...they all seem to have a more or less noticeable bump on the top of the head. Hard to see on some, but you can have a look here:

E. tristis; E. marginatus; E. sapphirinus; the possible E. sierraensis; and the unknown species from WA.

Meanwhile, this Cal Acad of Sciences image of E. halli has what appear to be two "little horns" on the vertex (but anterior to the ocelli?). Interesting...thanks for mentioning the tubercle character :)

Yes, that's the one. Its old (1948), but it still has some good observations on the family. The "Manual of Nearctic Diptera, Vol 1, 1981" has somemore good general information. Then you can access Frank Cole's 1919 overview of Acroceridae (Cyrtidae at that time) at http://chla.library.cornell.edu/cgi/t/text/text-idx?c=chla;cc=chla;sid=d41cc3aada5b6cc17024c5c80c324252;rgn=main;view=text;idno=5077659_204_001

With California's diverse habitats and climatic conditions, and rapidly changing geology there are many opportunities for speciation. Also with the difficulty in locating Acroceridae on a regular basis (other than those that visit flowers, ie. Eulonchus) there are still new species to be discovered. Last season I got a new Eulonchus in San Benito County. The females are metallic green and the males are dull metallic red. They reminded me of Christmas ornaments in July. Unfortunately the colors didn't hold after drying and now the females are blue-green and the males bronzy-green. There's also several undescribed species of Ocnaea that are represented my a single specimen. So there's still fun work to be done in the west.

to the genus page, and will add the Manual of Nearctic Diptera you mentioned.
There are new pages for spp. as well.

Good to know...thanks Eric. I recently found an old photo from Mt Pinos area (Kern Co. , CA) with what also appears to be a (metallic-blue) Eulonchus on a Monardella flower.

I'm relatively new to identifying insects...coming from a botanical background where we're fortunate enough to have a reasonably complete and accessible reference literature, at least in CA. I've seen mention in various entomology texts of a monograph on Diptera by McAlpine. Maybe I could find that at a local university library? I may need to start collecting specimens to make headway, though I prefer to photograph and observe without killing. (And of course, I may not get a conclusive "name" anyway, if many are undescribed as you mention. But I'd learn a whole lot more about Dipterans if I tried :-)

Thanks Ron. I have lots more photos, just need more time to post them :-)