Mahogany Dun Thorax. An excellent smooth water Mayfly performer, but should not be discounted for slower steam riffles. Mahogony Dun Thorax dry flies are designed to sit in the surface film the lower hackles are trimmed so that its body rides flush with the water surface, this results and the perfect silhouette that will tempt even the most spooky trout and is almost identical from below to the form of the natural Mayfly dun.