I have read that only the females drop to lie spent on the water. These nymphs are clingers and only swim poorly. The hatch is generally confined to silty, sandy, slow to medium water, while I fish the spinner fall at the glide below the riffles. My observations suggest the egg laying occurs in the riffle, where a spinner imitation is impossible to fish. The fish don t seem to feed on them much in the fast water, preferring to wait on the spinners to collect in the slower back waters and whirlpools where mats of spent flies collect like algae on the surface of a summer pond.
Trout seem to feed on the dun in similar places.
I have never fished this hatch well. I have watched my betters fish it with some success and have returned for lunch to reflect on the error of my ways over a single malt or two. I was never enlightened. Rusty, another single malt, please. The trick seems to be to catch the event on a day with little wind, casting to individual fish and timing the cast to that trouts feeding rhythm.