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This stem was found by Dave Wagner in desert scrub habitat in the Owens Valley adjacent to the base of the White Mountains. The stem was weak and broke when Dave reached for it (or brushed past, I'm not sure which), whence he noticed the abundant flies packed within the hollowed out portion of the stem. There were initially more flies present, some had fallen out while folks were handling the disintegrating stem before it was passed to me and these photographs were taken. At least 11 flies are present in the photos (5 in the left half of the stem, 6 in the right half). I'm not sure what the plant was...perhaps an Ericameria (formerly Chrysothamnus).

At first I naively presumed these were all flies of the same species aggregating in the stem for some strange reason. Most could be seen to be tephritids (from the wing patterning) and females (from their conspicuous ovipositors). Upon keying those tephritids in Foote & Blanc(1)(1963) and Foote, Blanc, & Norrbom(2)(1993) in became clear they were either Trupanea nigricornis or Trupanea bisetosa.

Those two species are very closely related and at the publication time of Foote & Blanc(1)(1963) females of the two species were considered diagnostically inseparable...whereas males of T. nigricornis were distinguished by their dark baso-flagellomeres, as compared to the yellow baso-flagellomeres of T. bisetosa. Since the fly just left of center (see last image of this set) had dark baso-flagellomeres, it seemed it might be a male of T. nigricornis...until I noticed its wings were entirely unmarked! ;-) That prompted me to scrutinize its head shape, and notice that the frons doesn't appear as wide & flat as in the other (Trupanea) flies present. At this point I don't know the identity of that fly just left of center in the photo above (nor the fly next to it whose similar wing is visible).

According to the key for Trupanea species in Foote, Blanc, & Norrbom(2)(1993) the main character for separating T. nigricornis and T. bisetosa is the width of the basal "stem" of the dark "Y"-shaped pattern at the apex of the wing: in bisetosa that stem is about twice as wide as the posterior branch of the "Y"; whereas in nigricornis the stem of the "Y" is less than or equal to the width of its posterior branch. This character was first proposed and published in Cavender & Goeden(1983) where figures appear illustrating the distinguishing character. From what's visible of the wings here, that criterion indicates at least one female of T. bisetosa and two females of T. nigricornis among the tephritids in the photo above. The two species can also be separated by host plant preference: nigricornis using a large number of species in a number of genera of Asteraceae, while bisetosa is restricted primarily to members of the genus Helianthus. But host plant species for both Trupanea species are present in the general area here, so that doesn't eliminate either as a candidate here.

After researching a bit, I believe what was found here was the provisioned chambers of a wasp nest of some species of Ectemnius or Crossocerus which are known to provision nests in hollowed-out stems with flies as prey for their larvae [e.g. see Rau(1922), Grissell(2007), and Eiseman, Charney, & Carlson(2010)]. A collage showing nest chambers with tephritid prey appears here...and more images of Penstemon stems with putative nest chambers of Ectemnius appear here. This would explain the observed lack of activity among the flies (due to paralysis from wasp venom) during the many minutes of observation.

According to Aluja & Norrbom(1999) there is a tephritid genus, Strauzia, that are stem-borers...but these are clearly not of that genus (e.g. from wing patterning). Moreover, the lack of movement among the fully-formed non-teneral adults flies here suggests sting-paralyzed, stem-provisioned, wasp prey.

The thumbnails below are from the same stem:

           

I'm not sure what the best node in BugGuide's taxonomic tree would be for placing this post. Most these flies are Trupanea nigricornis, but since one appears to be T. bisetosa and I don't know the ID of the two just left of center, I'm provisionally placing the post under Diptera. If those two flies are ID'd at some point, the post can be moved to the lowest common rank of the flies present...or simply to Trupanea if that's judged to be its best resting place.