In my personal record of selective fish feeding during hatches of small to medium sized mayflies, the no-hackle pattern with open wings of paired feather sections has been scoring far better the anything else. Particularly on larger fish that was snobbing the most popular CDC and hackles patterns. During some phases of the predation, the fish prefer the opaque and soft wings in slip of feather. On the right side: the natural and the imitation, photographed from under the surface.