This is the picture that shows why this is Stenonema instead of Maccaffertium. The main gills are rounded rather than truncate (square-ish) and the last, thread-like pair of gills is visibly tracheated (that little stripe through the center). Those characteristics distinguish Stenonema femoratum.
Stenonema femoratum (Cream Cahill) Mayfly Nymph from Mongaup Creek in New York
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