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Dun: Mayflies have two adult stages. They first emerge from the water as duns (scientifically known as the subimago stage). They then molt into the spinner (imago) stage, in which they mate and die. Sometimes the word "dun" is confusingly used to refer to a brownish gray color in fly tying materials.
Natural: A natural is a real insect (or similar creature) a trout might eat. The term is used to specify the real thing as opposed to its artificial imitation.