I love it when you are at the right place at the right time with this hatch. Fish don't seem to pay much attention the dun. I think it's because the fly sheds it's exoskeleton underwater and emerges fully formed. Fish do go crazy over the returning females when they return to lay their eggs. I use an imitation called a Dette Coffin Fly with good success then. It's fun to be in the middle of the stream about dust and hear what sounds like someone throwing rocks the size of your head in
Here is a photo of my version of the Dette Coffin Fly.
Here is also the link by Loren Williams with the recipe and step by step instruction. I used golden olive thread and golden badger hackle otherwise it's bascially the same.