Thanks, Brad! The one I posted at Diptera.info is a different individual (posted on Bugguide here). I'll definitely keep an eye out for more.

Specimen now a photo-voucher, thanks John.

This falls within a group of species treated by Steyskal & Ahlmark (1995). There was a suggestion on a picture you posted on Diptera.info that this may be E. atlantica. However, I'm not sure that it is that species. Characters for altantica include "mostly yellow legs" and having a distinct yellow pattern on the abdomen. You can see in the lateral photo the legs are mostly dark and the abdomen is neither colored nor shaped as described for E. atlantica.

This leaves two described species, one present in California and the other from Mexico. The specimen matches very well with what is known about the Mexican species, E. nigricans. Wing pattern matches almost perfectly. However, it is only known (per Steyskal & Ahlmark 1995) from the unique male type collected in Guerrero, Mexico.

I'd say this is either E. nigricans or a closely related undescribed species. Please collect additional specimens if you see them.

Your other Euxesta may be a different species (some have quite a bit of yellow on the abdomen).