Looks like it might be Geum.

Apparently the larvae of some flies live inside geum leaves (leaf-miners). This article is about geums in Britain, but I assume there could be similar leaf-mining by fly larvae in the U.S.:
http://www.ukflymines.co.uk/Keys/GEUM.php

And in both the U.S. and Britain, some geums are pollinated by flies. For example, flies are the primary pollinators of Geum peckii in NH:
http://www.centerforplantconservation.org/collection/cpc_viewprofile.asp?CPCNum=2022

Wow! That certainly explains all the flies.

Thank you both!

This one's dead body looks similar to the Geum, but the leaves are very different. Could it be another type of Geum?

It does look more like a Geum than a Sanicula (one of the black snakeroots), but I'm not good enough at identifying to make a distinction from the photo.