I've never come across this species in the angling literature, but I found several mature nymphs in a kicknet sample in a small stream around 3,000 feet in the Washington Cascades in late July. Their coloration is striking.
Pictures of 2 Mayfly Specimens in the Species Attenella delantala:
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Nymphs: The juvenile, underwater stages of mayflies, stoneflies, dragonflies, and damselflies and other aquatic insects whose juvenile stages are covered by hard exoskeletons. The word can also refer to the fishing flies which imitate these creatures, in which case it is used as a blanket term for flies imitating any underwater stage of an invertebrate (except for crayfish and leeches).