Summary ³

Mayflies or shadflies are insects belonging to the order Ephemeroptera (from the Greek εφημερος, ephemeros = "short-lived" (literally "lasting a day" "daily" or "day-long"), πτερον, pteron = "wing", referring to the brief lifespan of adults). They have been placed into an ancient group of insects termed the Palaeoptera, which also contains dragonflies and damselflies. They are aquatic insects whose immature stage (called "naiad" or, colloquially, "nymph") usually lasts one year in fresh water.
Most species feed on algae or diatoms, but a few species are predatory. The naiad stage may last from several months to several years, with a number of moults along the way. Most mayfly naiads are distinctive in having seven pairs of gills on the dorsum of the abdomen. In addition, most possess three long cerci or tails at the end of their bodies. Some species, notably in the genus Epeorus, have only two tails. In the last aquatic stage, dark wingpads are visible. Developmentally, these insects are considered hemimetabolous. A more casual and familiar term is "incomplete metamorphosis". Mayflies are unique among insects in that they moult one more time after acquiring functional wings (this is also known as the alate stage); this last-but-one winged instar usually lives a very short time, often a matter of hours, and is known as a subimago or to fly fishermen as a dun. Mayflies in this stage are a favourite food of many fish, and many fishing flies are modelled to resemble them.
Often, all the mayflies in a population mature at once (a hatch), and for a day or two in the spring or fall, mayflies will be everywhere, dancing around each other in large groups, or resting on every available surface.

Sources and Credits

1. (c) Mick Talbot, some rights reserved (CC BY), http://www.flickr.com/photos/25258702@N04/3627392628
2. (c) nataliemarisa, all rights reserved
3. Adapted by nataliemarisa from a work by (c) Wikipedia, some rights reserved (CC BY-SA), http://en.wikipedia.org/wiki/Ephemeroptera

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