Note that this Polybiomyia has conspicuous yellow spots on the "pleura" (= lateral faces of the thorax). Technically, those spots are visible on the "mesopleuron" and the "pleurotergite" here. And, though blocked from view by the wing, if this is indeed well-behaved P. townsendi...there should also be a yellow spot on the "sternopleuron", but not on the "pteropleuron"! ;-). What do all those quoted terms mean?! See the labelled image below to learn the specifics of where those "pleural plates" (or "sclerites") are located:

   

The point here is that the specific details of where the yellow pleural spots occur is a critical part of the circumscription of a number of Polybiomyia species, as can be seen in the key from Shannon(1925) appearing as an inset in the image here.

For instance, Shannon's key indicates that while P. townsendi has yellow spots on the mesopleuron, sternopleuron, and pleurotergite...P. bellardii & P. pedicellata have no yellow markings at all on their pleura (at least as currently described and circumscribed, and up to exceptions due to potential intraspecies variation).

So a nettlesome ID question arises...how can you distinguish P. townsendi from P. bellardii when all that's available is a dorsal view photo, where the pleura aren't visible? For instance in images like those below:

     

Note that P. townsendi comes in both largely black and largely reddish color forms...so overall color alone is not sufficient to separate them in dorsal view. Also, though P. townsendi usually has supra-alar stripes, it doesn't necessarily always have them (according to Curran's description).

Looking carefully at BugGuide posts and studying the descriptions of the two species carefully, I think I have a good answer to this "dorsal discriminant" question. Namely, consider the scutellum! In P. townsendi the scutellum is basically entirely yellow (perhaps with an almost imperceptible black anterior rim). However, in P. bellardii the scutellum...though also mainly yellow...is described by Curran as having a dark transverse stripe as well.

It's also helpful to know that P. bellardii is found in Texas...but P. townsendi can range into Texas as well (and also west to CA and south into Mexico).

Moved from Ceriana.

It is, and it belongs to the genus Ceriana!
According to Nomina diptera, in the Nearctic there are 5 species of Ceriana. I know Ceriana abbreviata, and it´s different from this one, so that leaves us with four species:
C. mime, C. pictula, C. snowi and C. tridens.
Greetings,

and some interesting comments!

This is NOT a wasp (nor anything else in Hymenoptera). This is in the order Diptera ! Its a fly, by the one pair of wings and the clearly visible halteres. It is, however, a wasp mimic, even praying on wasps - or bumble bees -(female briefly pounces on a host, oviposits, & larvae develop as internal parasitoids, that is they kill the host): family Conopidae , probably either Physocephala , or Physoconops ; I think the former, because of the way the hind femur is thickened. Too bad, the wing venation isn't clearly showing, that would make it easier. Anyway this is a start, and Ron had the right direction in going to flies.

Moved from Mischocyttarus mexicanus cubicola.

I think you have a wasp mimic here (and in one other just-posted photo #156937). Check the shape of the antennae - sort of spaded here and typically round cross-section on a wasp - thicker upper abdomen here and non-overlapping pattern on the lower abdomen. Also, aren't those halteres behind the wings?