Mayflies are the most important group of insects to most fisherman, since the history of fly fishing relates primarily to them. There are over 700 species, but only a few are of importance to the fly fisherman.

 Mayflies like midges and caddis flies have four stages of metamorphosis. Egg, larvae, pupa, and adult. How ever the fisherman must be prepared with 6 basic patterns to successfully imitate the stage being eaten by the trout at the time. Mature mayfly nymph, emerging nymph, floating hatching nymph, stillborn Dun, Dun, and spinner.

Writing in his classic 1969 edition of The New Streamside Guide, Art Flick wrote, "to date I have never met a fisherman who had fished any stream where trout could not be taken on this fly".