The images in this post are of the adult and spent pupa in this first photo of dried, curled Milkweed leaf with dead aphids, 1 adult Gall fly, 2 spent Gall Fly pupal cases.

Another good happenstance story.
September 21, 2016: collected leaves of Butterfly Milkweed, Asclepias tuberosa, with aphids. Found fly larvae which I assumed (wrong) were Syrphids.
Had to leave town for 9 days and couldn't take all the rearings so some suffered, including these, or so I thought.

October 21, 2016: before tossing the desiccated and dead contents of rearing jars, I take a closer look and lo!, in this jar found a dead Gall Fly, then another, then several more, plus spent pupal cases. I checked with A. Williams who sent a list of the several Milkweed associated Gall Flies. I checked Gagné's Red Book and found 2 possible species.

I was perplexed by the spent pupal cases which seemed random; their abdominal ends were not wedged into any sort of gall. Perhaps I could find spent pupal cases wedged properly if I looked on leaves of my plants. So I went outside and collected about 50 Milkweed leaves for a closer look. I did find some leaf imperfections that might be galls, but no spent pupae. Perhaps they had all blown away.

Checking BG further I happened onto Aphidoletes aphidimyza. The antennae seemed to fit the ONE Gall Fly whose antennae weren't horridly desiccated. However, it may not fit with all the adult Gall Flies. BG also says larvae are predaceous which might explain the random spent cases. However, the halteres of my flies, dried as they are, look a different shape.

Reviewing my photos I saw that what I thought were Syrphids probably were not. They also seem to fit this species as per

Batch rearing. I think there may possibly be more than one species of Gall Fly.
Another adult, antennae
Another spent pupal case
and larvae