They are unique among insect orders in having a fully winged terrestrial preadult stage, the subimago, which moults into a sexually mature adult, the imago.

[5] The abdomen consists of ten segments, some of which may be obscured by a large pair of operculate gills, a thoracic shield (expanded part of the prothorax) or the developing wing pads.

The subimago, or dun,[6] often has partially cloudy wings fringed with minute hairs known as microtrichia; its eyes, legs and genitalia are not fully developed.

In males of Ephoron leukon, the subimagos have forelegs that are short and compressed, with accordion like folds, and expands to more than double its length after moulting.

In most species, the males' eyes are large and the front legs unusually long, for use in locating and grasping females during the mid-air mating.

[11][12] Uniquely among insects, mayflies possess paired genitalia, with the male having two aedeagi (penis-like organs) and the female two gonopores (sexual openings).

[1] Often, all the individuals in a population mature at once (a hatch), and for a day or two in the spring or autumn, mayflies are extremely abundant, dancing around each other in large groups, or resting on every available surface.

[15] Male adults may patrol individually, but most congregate in swarms a few metres above water with clear open sky above it, and perform a nuptial or courtship dance.

For example, the female Tisza mayfly, the largest European species with a length of 12 cm (4.7 in), flies up to 3 kilometres (2 mi) upstream before depositing eggs on the water surface.

However, in low-oxygen environments such as the mud at the bottom of ponds in which Ephemera vulgata burrows, the filamentous gills act as true accessory respiratory organs and are used in gaseous exchange.

They process a great quantity of organic matter as nymphs and transfer a lot of phosphates and nitrates to terrestrial environments when they emerge from the water, thus helping to remove pollutants from aqueous systems.

[5] Along with caddisfly larvae and gastropod molluscs, the grazing of mayfly nymphs has a significant impact on the primary producers, the plants and algae, on the bed of streams and rivers.

Carnivorous stonefly, caddisfly, alderfly and dragonfly larvae feed on bottom-dwelling mayfly nymphs, as do aquatic beetles, leeches, crayfish and amphibians.

[27] Besides the direct mortality caused by these predators, the behaviour of their potential prey is also affected, with the nymphs' growth rate being slowed by the need to hide rather than feed.

[28][29] Other nematodes turn adult male mayflies into quasi-females which haunt the edges of streams, enabling the parasites to break their way out into the aqueous environment they need to complete their life cycles.

A study in laboratory simulated streams revealed that the mayfly genus Centroptilum increased the export of periphyton,[32] thus indirectly affecting primary production positively, which is an essential process for ecosystems.

The adverse effects on the insects of pollution may be either lethal or sub-lethal, in the latter case resulting in altered enzyme function, poor growth, changed behaviour or lack of reproductive success.

[55][56] From the Permian, numerous stem group representatives of mayflies are known, which are often lumped into a separate taxon Permoplectoptera (e.g. including Protereisma permianum in the Protereismatidae,[53] and Misthodotidae).

However, from the same locality the strange larvae and adults of the extinct family Mickoleitiidae (order Coxoplectoptera) have been described,[57] which represents the fossil sister group of modern mayflies, even though they had very peculiar adaptations such as raptorial forelegs.

In the much younger Baltic amber numerous inclusions of several modern families of mayflies have been found (Ephemeridae, Potamanthidae, Leptophlebiidae, Ametropodidae, Siphlonuridae, Isonychiidae, Heptageniidae, and Ephemerellidae).

After[18] Siphluriscidae Baetidae Baetiscidae Prosopistomatidae Coloburiscidae Leptophlebiidae Chromarcyidae Oligoneuriidae Vietnamellidae Austremerellidae Teloganodidae Teloganellidae Melanemerellidae Ephemerythidae Machadorythidae Tricorythidae Dicercomyzidae Coryphoridae Leptohyphidae Ephemerellidae Neoephemeridae Caenidae Potamanthidae Polymitarcyidae Behningiidae Euthyplociidae Ichthybotidae Palingeniidae Ephemeridae Acanthametropodidae Ametropodidae Metropodidae Isonychiidae Heptageniidae Rallidentidae Siphlaenigmatidae Dipteromimidae Oniscigastridae Ameletopsidae Nesameletidae Ameletidae Siphlonuridae The Dutch Golden Age author Augerius Clutius (Outgert Cluyt) illustrated some mayflies in his 1634 De Hemerobio ("On the Mayfly"), the earliest book written on the group.

[67] The critics Larry Silver and Pamela H. Smith argue that the image provides "an explicit link between heaven and earth ... to suggest a cosmic resonance between sacred and profane, celestial and terrestrial, macrocosm and microcosm.

[72]In his 1789 book The Natural History and Antiquities of Selborne, Gilbert White described in the entry for "June 10th, 1771" how Myriads of May-flies appear for the first time on the Alresford stream.

Large trouts sucked them in as they lay struggling on the surface of the stream, unable to rise till their wings were dried ... Their motions are very peculiar, up and down for so many yards almost in a perpendicular line.

[74] The English poet George Crabbe, known to have been interested in insects,[75] compared the brief life of a newspaper with that of mayflies, both being known as "Ephemera",[76] things that live for a day:[77] In shoals the hours their constant numbers bring Like insects waking to th' advancing spring; Which take their rise from grubs obscene that lie In shallow pools, or thence ascend the sky: Such are these base ephemeras, so born To die before the next revolving morn.

[78] The American Poet Laureate Richard Wilbur's 2005 poem "Mayflies" includes the lines "I saw from unseen pools a mist of flies, In their quadrillions rise, And animate a ragged patch of glow, With sudden glittering".

[81] The American playwright David Ives wrote a short comedic play, Time Flies, in 2001, as to what two mayflies might discuss during their one day of existence.

A large number of these species have common names among fly fishermen, who need to develop a substantial knowledge of mayfly "habitat, distribution, seasonality, morphology and behavior" in order to match precisely the look and movements of the insects that the local trout are expecting.

[92] The 2014 hatch of the large black-brown mayfly Hexagenia bilineata on the Mississippi River in the US was imaged on weather radar; the swarm flew up to 760 m (2,500 feet) above the ground near La Crosse, Wisconsin, creating a radar signature that resembled a "significant rain storm", and the mass of dead insects covering roads, cars and buildings caused a "slimy mess".

[96] Two vessels of the Royal Navy were named HMS Mayfly: a torpedo boat launched in January 1907,[97] and a Fly-class river gunboat constructed in sections at Yarrow in 1915.