Here is a nice specimen of Pararchytas decisa (male) – not the darkened wing bases and vein rm. I have yet to track down Norm Woodley’s revision* of this genus so the determination is tentative until I can see a description of all 3 species.
EDIT (9/8/2011): I now have Norm Woodley’s key and this is confirmed as Pararchytas decisa.
* Woodley, N.E. 1998. A revision of the genus Pararchytas Brauer and Bergenstamm (Diptera: Tachinidae). Proceedings of the Entomological Society of Washington 100: 409–420.