A fly-fishing DUN is the stage of the mayfly between a nymph and adult mayfly. The technical term is the SUBIMAGIO stage. This is a vulnerable phase for the mayfly since the wings are formed, but have a dull opaque color. Often this stage results in trout actively feeding from the water surface.
Brown Drakes are burrowing Mayfly and thrive in areas with loose sand and gravel bottoms.The Brown Drake hatch usually starts about dark or just after dark and when the weather is mild. Since it is warm, their wings dry fast and they are not on the water long.
Brown Drakes are the most widespread of all the big mayflies, making them nationally important on many of the country's best trout streams.