Cluster Fly - Pollenia sp.
Family Calliphoridae (Blow Flies)
genus Pollenia (Cluster flies)
Live cluster flies photographed in the wild at northern Illinois and central Pennsylvania
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Flies in the genus Pollenia are commonly called "cluster" flies after their habit of  gathering in conglomerations. They superficially resemble house flies, but are slightly larger. Cluster flies at rest overlap their wings, whereas house flies do not. The thorax is without distinct stripes, and is covered with short, yellow-gold hairs. The abdomen is dark grey, with patches of darker gray, resembling the coloring of some flesh flies in the family Sarcophagidae.


Male cluster flies have eyes that nearly meet at the top of the head.
Females' eyes are more wide-set (far right and below). These early spring flies are feeding on tree sap.

Here's a better look at that "checkerboard" pattern on the abdomen
References
  1. Bugguide.net Cluster Flies
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Flies of North America - Order Diptera. Flies are prevalent in virtually all habitats, with over 16,000 species in North America. Flies can be distinguished from all other insects in that they only have one pair of normal wings. Most flies have compound eyes and mouthparts adapted for piercing, lapping or sucking fluids.
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