On many trout streams across the world, the mayfly is the most abundant food source for trout. Mayflies are typically available to the fish all year in either their nymphal form or as a winged adult often, both are present. The mayfly is most probably the most important aquatic insect to the trout fly fisherman.

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Mayflies are any of an order (Ephemeroptera) of insects with an aquatic nymph and a short-lived, fragile adult lacking mouthparts and having membranous, heavily veined wings and two or three long, threadlike tails. NOTE: the wings of the mayfly are held vertically above the body when closed. 

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Over 3,000 species of mayfly are known worldwide, grouped into over 400 genera in 42 families. The mayfly has three separate stages in its life. They are Nymph, Subimago and Imago. In all three stages mayflies are available to fish. In fly fisherman "talk", the subimago is known as a dun and the imago is known as a spinner.

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Nymphs are Immature mayflies. There are four basic kinds of mayfly nymphs. They are swimmers, clingers, crawlers and burrowers. These terms refer to how they live in the underwater environment. 

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Special Note: I you are a fly tier and/or a fly fisher, I would be remiss if I did not point out what, in my opinion, are some of the best and most informative videos I have found in the many years of fly fishing and fly tying. Rog and Jane Klettke have a channel on YouTube that contains many excellent videos on a number of fly fishing related topics. I am especially impressed with their series on entomology and fly tying. Here are two that relate specifically to Mayflies:

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Mayflies I: Simple Entomology for the Fly Tier and Fly Fisherman Part 11 by Rog and Jane Klettke

Mayflies II, Simple Entomology for the Fly Tier and Fly Fisherman part 12 by Rog and Jane Klettke

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More about mayflies...

​Bugs of the Underworld: Mayfly - by CMU LMDC

A short video on mayfly emergence and life cycle - National Geographic

Mayflies by Wikipedia

Mayflies by Encyclopedia Britannica

Mayfly – The Life Cycle & How to Fish Mayfly Patterns - by FrostyFly

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If you are interested in aquatic insects or you like to tie mayfly nymphs, here is a site for you... lifeinfreshwater.net. There are incredible images of virtually all of the nymph or underwater forms of aquatic insects.

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Should you be interested in capturing and saving aquatic insects from the river/stream, to use when tying nymphs, here is a short video that explains the process very well...see it here...

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