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The biggest, plumpest fly feeding at the Sugar Maple sap.

Either Calliphoridae or Muscidae, right?

If this much is correct, then how to separate the two? The fly has plumose arista, which could be either, right? The coloration is gray-black like most Muscids but there was just a tiny hint of green on the abdomen so maybe Calliphoridae after all?

I guess the real bad news is that I didn't get a lateral view :(.

Any ID help would be appreciated.

Woodlands, West Virginia USA, 11 November 2006. Length 14.5 mm from the front of the eyes to the rear of the abdomen.