The Slate Dun is the Epeorus longimanus, a western species of the Heptageniidae family. It’s also called the Slate Brown Dun. This mayfly prefers the fast moving water of mountain streams. The mayfly emerges anytime from June to early October depending on the location. Spinners can fall from late afternoon to dark. In some cases they fall in the early evenings. The hatches are generally not huge but they usually hatch when there are few other hatches occurring. That can make them very important.