Isonychia or Mahogany Mayfly
These flies are a big thing in the East and Midwestern US. These mayflies usually show up a week or so before the brown drake in the Midwest. The hatch usually occurs in early June and produces great spinner falls in the late evening at dark. The flies mate pretty high above the water and will fall spent wing shortly after breeding. I just fished a good Isonychia spinner fall last night in North West Michigan and had good success. The patterns I used were the 2 following flies. I used the mahogany parachute leading up to the spinner fall for about 1.5 hour before dark, once the sun set I switched to the spent wing spinner fly pattern shown in the second image below. Both of these great flies produced a lot of fish throughout the afternoon and evening, as they have in many previous fishing trips.
Mahogany Parachute (Isonychia) This very simple pattern is tied with mahogany dubbing, a calf tail parachute, blue dun tail and hackles.
Isonychia Spinner Pattern
Isonychia bicolor spinner pattern.This very standard looking spent wing spinner is very effective during the spinner fall of a variety of mayfly species.