Today the mayflies were hatching on the Ipswich River, Mass.  Uniformly dark gray wings and dark body.  Since I saw the animal in focus, I will say it is Leptophlebia cupida.  The pattern is an Iron Blue Dun or Black Quill tied on a number 14 hook.    They began to fly mid-day and there were a few at the Ipswich River Watershed office on April 22 and in Boxford on May 1.

Fly fishermen visit the troutnut for more info: http://www.troutnut.com/hatch/79/Mayfly-Leptophlebia-cupida-Black-Quill