Fly puparia found in multiple dead stems of leaf-cup, Polymnia sp. After tearing into stems and finding puparia, I took a closer look at two of the stems I'd torn into - including the one shown here - and noticed a conspicuous hole in the stem right by where each puparium was located. Though I can't say for sure, I think the hole may be associated with the puparium somehow, e.g., made by the larva before pupariating as an exit hole for the adult when it emerges, or made by the larva as it entered the stem. In this first photo the hole in the stem is just to the left of the puparium.

Rearing in progress