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.
This specimen resembled several others of around the same size and perhaps the same species, which were pretty common in my February sample from the upper Yakima. Unfortunately, I misplaced the specimen before I could get it under a microscope for a definitive ID.
GONZO on Oct 10, 2006October 10th, 2006, 2:08 pm EDT
I've been staring at this one for quite some time. My first impression was lycorias (Boreal Stone, Giant Brown Stonefly). I'm pretty familiar with this species, but I don't know of a good key to Acroneuria at the species level.
The lack of a row of spinules along the occiput of the head and presence of postocular and pronotal fringes all say Acroneuria for sure.