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 keys out to Suwallia, for which I did not find any nymph species keys. However, I'm placing it in Suwallia pallidula because I caught a few adults in the same spot that closely resembled the abundant nymphs and keyed them out to species. Features I noted under the microscope when keying this specimen to genus included apical hairs of cercal segments that were directed at posterior angles, and the longest apical hairs of distal segments were shorter than their following segment.
This stonefly was collected from Mystery Creek #199 in Washington on July 28th, 2019 and added to Troutnut.com by Troutnut on July 30th, 2019.