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.
On the way home from deer hunting somewhere in Idaho, I had just a couple hours to check out the South Fork Boise River. I drove up and down the canyon for a bit, looking at the water, trying to decide where to stop to fish. Eventually I found some fish rising to some little Baetid duns, and I waded into a bouldery tailout to try for them. They stopped rising pretty quickly, so I switched to my Euro nymphing rig. I hooked two nice rainbows in quick succession, landing a snaky, geriatric-looking 19.5" rainbow and losing one that was a bit shorter but a lot stronger.