If this is Hemipepsis then I think they have the size wrong on the info page because this beast was very large - way over 20 mm. I noticed another photographer said their subject was from 1 1/2 inches to 2 inches. I photograph wasps all the time but this wasp was one of the largest I've ever seen here. Just wondering what you think, Aaron.

You're right, Carol. The "12-20mm" size currently appearing on the Hemipepsis info page is wrong. (It was correctly quoted from the Audobon Field Guide(1)...but that field guide is in error there!)

The treatment in Townes(2)(1957) indicates that in Hemipepsis the forewing alone can range from 12-27mm. Add to that the head, antennae, and long, dangling leg lengths...and one can understand how the overall size impression for a Hemipepsis can seem much greater than 20mm. For instance, the Hemipepsis in the post below was assigned an approximate size of 30mm:

 

Now...how about ascertaining the genus here?

To start, this is certainty not Pepsis, as that genus can be distinguished among all pepsine genera by:

  1) the marginal cell is apically rounded and slightly separated from the costal edge of the wing, and
  2) the 1st recurrent vein (or "1m-cu") meets the posterior edge of the 2nd submarginal cell (or "S2") within its basal third.

This is illustrated in the posts thumbnailed below:

 

So if this isn't Pepsis, what is it? A commonly held notion is that all the "large" pepsines are either in Pepsis or Hemipepsis, but that's not entirely correct. Entypus can have fore wing lengths up to to 26 mm according to Townes(2)(1957). However, all the other pepsine genera have wing lengths at most 18mm (most significantly less).

So, given your size estimate, this is either Hemipepsis or Entypus. These two genera can be distinguished by the following somewhat subtle wing venation characters (which are illustrated in the thumbnails that follow):

In Hemipepsis the 1st recurrent vein (or "1m-cu") meets the posterior edge of the 2nd submarginal cell ("S2") at its distal end...or at most within a 1/10 th the posterior edge length from that point.
Whereas in Entypus the 1st recurrent vein meets the posterior edge of the 2nd submarginal cell about 1/4 th its length before its distal end.

 

The first time around with your post, I didn't check these details very carefully enough. But I just now measured the relevant distances from your photo using Photoshop, and found the position where the 1st recurrent vein meets the 2nd marginal cell is very close to 1/4 the length of the posterior edge of the cell short of its distal end. So, it appears I erred in previously referring your post to Hemipepsis, and that it's actually Entypus.

Not only does that conclusion agree well with the post below:

 

...but in studying Townes, the only Entypus species that seems to fit here is E. atratus. Furthermore, and the range map for that species on pg. 52 of Townes indicates records around the Great Salt Lake. If we were to stick with Hemipepsis, the best species match there would have been H. ustulata, with the nearest records to your locale being around the Zion/Cedar Breaks area (see the range map on pg. 46 for H. ustulata).

Thus, I think we have a good case for E. atratus here.

Thanks for taking the time to check that out, Aaron.

I wish I had gotten two good shots off but instead I got these. I'm pretty sure it hadn't eaten yet because it was a little cranky. Next time I'm tossing it a tarantula.