This rather plump Zelia was hanging very closely about a dead stump (a plum) in my yard. (An advantage of not having a neat yard and neighbors who are not fussy--interesting insects.) Possibly this was a female looking for whatever its host is. Size estimated.

My guess is this is indeed Z. vertebrata, and that this was a female looking for a host: Brimley, p. 367 (1), states that Zelia vertebrata parasitizes the "Betsy Beetle (Passalus cornutus)", which is now called Odontoaenius disjunctus.
The standing, decaying stump this fly was frequenting is a good habitat for the likely host beetle--abundant in my yard.