The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
Like most common names,"Olive Dun" can refer to more than one taxon. They're previewed below, along with 1 specimen. For more detail click through to the scientific names.
An imitation is rarely required, but notes show that when it is needed it is needed badly.