A CDC-tied mayfly dun in black from RIO. CDC (cul de canard) feathers trap tiny air bubbles for natural buoyancy, giving this pattern a soft, lifelike footprint on the water.
At #18 it suits small dark mayflies and midges. Fish it on a dead drift, and refresh the CDC with desiccant or false casts to keep it floating.
On flat, glassy water, lengthen your leader and downsize your tippet — selective trout notice everything except a perfectly presented mayfly.
