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This small fly was on the rocks at the edge of Rock Creek in Kankakee SP, IL. It was about 3/16"

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A nice example of an ephydrid. Kahis may be able to tell you the genus. I image there are not too many with a wing pattern like this.

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There are a few genera with infuscated wings + clear spots, like Scatella and Notiphila. In this case I'd suggest Scatophila. (scatological names abound) There seems to be more than five spots on each wing, although it's difficult to tell when the wings overlap like in the pic. Here in Europe most Scatophila species have a protuding face and I think this character is visible, although very poorly due to the angle.

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Just noticed the size given, 3/16" - thats about 4.5 mm, too large (I think) for Scatophila which has mostly very small species (<2mm = 0.08" ).

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This appears to be Scatella quadrinotata.