In the beginning, I did think these were crane flies, but I wasn't certain. Thanks for clearing things up. Is this a pretty common species? I ask because this is the first I've seen of it around here. I discovered an oak tree that was infested with them.

their abundance status in your area. They are not as common as the other look alike species Gnophomyia trisitissima which can be separated by their yellow halteres. The larvae of both species feed on dead wood, and the adults usually gether in fair number for mating purpose. That maybe the reason you saw many of them at one time. They don't infest live plants. - Chen