Mayflies
(order Ephemeroptera)

Mayfly subimago or "dun"

    Mayflies are the best-known and most widely imitated insects in fly fishing.  They are the "traditional" insect that has the longest history and association with the sport.  They are unique among insects in that they are the only ones with a fully-winged terrestrial stage prior to the mature adult stage.  This is called the "subimago" (or, "dun") stage.  And, because they are so widespread and are a preferred food for many freshwater animals, including fish, they form a critical link in the freshwater food chain.

    Some mayfly larva resemble those of stoneflies, and are also sometimes mistaken as damselfly larva.  Mayfly adults, however, are unique in appearance and are not likely to be confused with other winged insects.  The most distinguishing feature is their upright wings, that give them the appearance of "sailboats."  As "upwings", adult mayflies look much different than the "downwings" such as the "tentwing" caddis . . . "flatwing" stoneflies . . . or midges, often referred to as "glasswings."

    Most mayfly species have one or two generations each year.  Development time for the larva ranges from a few weeks to two years.  The process of "hatching" to which the fly fishers' attention is often drawn, refers to the act of transformation between larva and sub-adult, or subimago/dun.  This can occur in a very synchronized and predictable fashion, or it can be drawn out over several weeks.  However, some species hatch, mate, lay eggs and die all within a few hours.  The likely triggering factors to these "hatches" is likely due to temperature and duration of daylight.

    The subimago/dun stage usually lasts for one or two days.  During this time, their characteristic "smoky" or cloudy colored wings can be easily seen as the insect mostly rests awaiting the final molt.  The last stage, when the insect becomes a sexually mature adult, is distinguished by the clearing of the wings (to almost transparent) and the intensification of color and contrast in the body.

    Adult mayflies generally do not feed.  They live for less than a month, solely for the purpose of mating.  The mating ritual often involves the "swarming" of both sexes, most often at dawn and dusk, with large congregations gathering.  The males particularly may be noticed flying up and down in a rhythmical manner.  After mating, females lay eggs in 3 ways: (1) drop eggs while flying low over water (2) lay them on the water by intermittent touching or landing (3) submerge themselves and deposit under the water.  The female then dies, often in a "spent" fashion, with wings spread out to the sides.  This activity is often called a "spinner fall", and is another notable time to fly fish with topwater ("dry fly") mayfly patterns.

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