Pyrgota undata (Waved Light Fly).
Size: approx. 15 mm. Photo: 06/07/15, 11 pm.

Now this is crazy ... the females of this species land on May beetles causing them to take flight. Then the fly lays an egg into the beetle's back while soft parts are exposed during flight. Flies usually attack female beetles only and may pursue them under lights. The larvae are about 1 cm long and kill the host beetle after about two weeks to finally consume the entire interior. The fly pupates inside the host remains and emerges following spring.  
bug guide (this photo):