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I am posting two photos of two different individual flies, but I think they are the same species. One is very swollen?a gravid female at this time of the year? Or just a fly that has been drinking too much sap?

Ephydridae?

I measured 3.2 mm to the rear of the abodmen in the one photo where you can see the abdomen. In the other photo I measured 3.6 mm to the rear of the wings.

West Virginia USA, November 2006, at Sugar Maple sap, woodlands.

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Swollen, with Eggs or Sap?

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http://www.diptera.info/photogallery.php?photo_id=338

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Thank-you Nikita! It is been a little while since I photographed a new family (new to me that is). I see Black also photographed his flies at sap. Very nice, thank-you for the ID help!

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So, to save everyone from going to the Gallery: that is Aulacigaster leucopeza. ;)

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Anyone know how come the abdomen is so swollen this late time of the year? Eggs?

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Quoting Professor Donald Chandler who is summarizing the recent literature: "There was only one species (the Holarctic A. leucopeza) known from North America until the revision by Wayne Mathis and Amnon Friedberg in 1994. A. leucopeza was restricted to the Palearctic region, and what had been called "leucopeza" in North America became three new species. The species do look a lot alike, and if you are unaware of this paper, and familiar with earlier records of leucopeza from North America, leucopeza would be a logical identification. However, it is no longer the correct name, and I don't know which of the two Northeastern species your photo shows."

Don was commenting on someone else's photo on BugGuide.

Nomina Nearctica has this listing for the genus in North America:

Aulacigaster mcalpinei Mathis and Freidberg 1994 (Aulacigaster)
Aulacigaster neoleucopeza Mathis and Freidberg 1994 (Aulacigaster)
Aulacigaster sabroskyi Mathis and Freidberg 1994 (Aulacigaster)