Looking carefully at the area between the hind coxae here (i.e. at the very bases of the hind legs, see full-size image here), I had thought that area looked to be somewhat thinly "membranous" (as opposed to "chitinously scleroterized"). That would indicate Sphiximorpha rather than Polybiomyia.

But apparently I misinterpreted that character. (Indeed, it now seems to me that the view provided here doesn't even allow one to see the relevant portion of the sternum to check for the character...see images here for comparison.)

For more details on this post, see remarks under 1st image of the series.