A number of these small robber flies (size estimated 8-10 mm) were buzzing about a sandy trail on a very hot morning. Perhaps Megaphorus minutus? I caught a couple ovipositing on grass stems--very interesting behavior I have not seen previously in robber flies, and I don't think we have any of oviposition on a stem like this here.

Identification confirmed by Herschel Raney, and he points out that this formation of an egg mass is seen in the related genus Mallophora.