This specimen shows one of two distinct color types of Rhithrogena impersonata nymphs. This type is very reddish, especially in the gills, and it was formerly in its own species called Rhithrogena sanguinea.
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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).