The CDC Thorax Dun is one of our favorite flies to fish during the Blue Winged Olive mayfly hatch! We used olive colored materials in this video to match Blue Winged Olive mayflies, but you can alter the color and size to match other species. This pattern works very well for Pale Morning Duns - adjust the biot color and dubbing color to a pale yellow or PMD yellow. This fly is a staple in our boxes from Spring through Fall!