Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
I've tentatively called this fragile, highly damaged dun a Serratella species. I had much better pictures of the species but lost them. It's about size 22, but good-sized trout were rising to this hatch; I suspect it has to do with the solid profile shown by the dark, almost-black wings that have given this hatch the name "Darth Vaders" among some Wisconsin anglers.
This mayfly was collected from unknown in Wisconsin on July 14th, 2004 and added to Troutnut.com by Troutnut on January 25th, 2006.