The plot thickens 5/31/08 - Diplazon laetatorius -
Churchville Nature Center, Bucks County, Pennsylvania, USA May 31, 2008 Size: 7-8 mm.
Parasitic wasp and the Syrphid fly cocoon from which it emerged.
You can see the emergence hole in this one; it wasn't there on 5/29/08.
Related images here