Very nice :)

So, something I'm wondering about here. When I look at the pristine exuvium from the adult you reared, I see an obvious head:



And, just south of (okay, posterior from) the head, there's that strong "double ridge" feature to the exuviae on the dorsal side, with a deep furrow in between the ridges. In the same place, but ventrally, there's the legs-to-be all folded up.

(pupa before eclosure)

But when I look at your soil-bound exuvium example in the current series, and the one in mholland's photo, I don't see a clear "head" part on either of them. In the exuvium poking out of the soil at Red Oak, I see the double ridge feature...but little or nothing anterior to it. In mholland's photo, something similar: above the legs-to-be, there's not much exuvium intact.

It's as if, in both examples, the head part of the exuvium just fell off or partly disintegrated. This would seem to make sense given that the adult fly emerges from this part of the exuvium.

Note that I'm not doubting the family-level identity of any of these exuviae, just interested in what looks to me like partial disintegration of two of them. If this disintegration does indeed happen regularly, then knowing about it would seem to be useful for future efforts to ID xylophagid exuviae.

The head part of the exuvium splits open in Orthorrhapha, but often falls off and is lost, as it only remains weakly attached to the rest of the body. There doesn't seem to be a taxonomic connection in whether or not it is retained; I've seen other exuviae of Coenomyia that retain the entire head.

An observation only:
I have seen these

here in the moist, wooded valley. This was found in the open and sunny prairie area.