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The main fly visible at center here is a female of Trupanea nigricornis, from the keys in Cavender & Goeden(1983) and Foote, Blanc, & Norrbom(1)(1993). As far as genus confirmation goes, note the 3 pairs of frontal, two pairs of orbital, and one pair of scutellar bristles here...which together are diagnostic for Trupanea. The overall detailed shape of the dark wing infuscation clearly indicates species T. nigricornus...in particular the relatively narrow "stem" of the dark "Y"-shaped marking at the apex of the wing.

These were among a large group of flies that were found within contiguous chambers separated by thin plugs of chewed plant fiber in a hollowed-out stem (of Chrysothamnus = Ericameria?) that appeared to be prey provisioned by wasps (Ectemnius or Crossocerus?) for their larvae. For more details, see comments under the 1st thumbnail of those shown below, which are all from the same stem: