[Note: Click this link for the full-size version of the above collage; the inset Figs. 183-185 are from Wasbauer & Kimsey(1); Figs. 244-245 are from Plate XIII of Evans(1951)(2).]

The remarks here concern the ID of this wasp. Happily, Frank Kurczewski emailed me out of the blue with suggestions on the ID...and that inspired me to put an effort into using the keys, descriptions, discussions, and figures in Evans(1951)(2), Evans(1966), and Wasbauer & Kimsey(1)(1985) to try to get this to genus and species.

From studying the above-mentioned references, it's clear this is a female (12 antennal articles and presence of protarsal rake) in the tribe Pompilini and genus Aporinellus. The presence of the conical tooth on the postero-lateral corner of the propodeum (indicated by the green arrows in the image and inset above) is a particularly distinctive character of the genus Aporinellus.

As for species, a number of the characters needed to key this are not decisively discernible in my images. If only I'd gotten a clear shot of the 2nd submarginal cell ("sm2" in the Figs.), then I could have verified whether or not this is A. taeniatus...which I suspect it is. It appears there are only 7 currently recognized U.S. species in the genus (scroll down at this link for a current listing)...and after studying the keys and descriptions in the literature for those species (as well as searching out and reading a number of original descriptions of current synonyms, e.g. see here)...I believe the most likely candidate here is A. taeniatus (or possibly A. borregoensis...more on that below).

The overall "castaneous" + "rufous" coloration of the thorax here is rather unusual...and it struck both Frank K. and I as remarkable. However, regarding A. taeniatus, Wasbauer & Kimsey(1)(1985) state that it is:

"...a highly variable species, in both the color and extent of the patterns of appressed pubescence and also in integumental color. Females vary from entirely black to extensively banded with whitish or silvery pubescence, and the integumental color may be entirely black or partly to entirely red."
And Evans(1951)(2) adds that: "...in some specimens the pubescence is almost entirely brownish." All this makes it seem like, color-wise, my post here could fit within A. taeniatus.

None of the other species descriptions I've read would seen to accomodate the abundant patches of beige (= "castaneous") coloring on the thorax here...which I'm thinking is mostly the result of a fine pubescence or pollinose dusting, rather than the actual integument...which looks reddish-brown (="rufous").

The only species I haven't been able to read a detailed description of is A. borregoensis, which is described in:

  Evans, H. E. (1957). Three new California spider wasps. Pan-Pac. Entomol, 33:181-186

Two aspects of that species mentioned in Wasbauer & Kimsey(1)(1985) make it intriguing:

1) it's a desert species, and the Colorado River locale here is nicely tucked into the middle of the southwestern portion dipicted in its range map; and...
2) it's the only species mentioned to have records of females visiting flowers of Pluchea sericea...which was a near monoculture nearby to where this wasp was photographed.