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Mayflies also known as shadflies or fishflies in Canada, are aquatic insects belonging to the order Ephemeroptera. This order is part of an ancient group of insects termed the Palaeoptera, which also contains dragonflies and damselflies. Over 3,000 species of mayfly are known worldwide, grouped into over 400 genera in 42 families.

Mayflies are relatively primitive insects and exhibit a number of ancestral traits that were probably present in the first flying insects, such as long tails and wings that do not fold flat over the abdomen. They are aquatic insects whose immature stages (called «naiads» or «nymphs») live in fresh water, where their presence indicates a clean, unpolluted environment. They are unique among insect orders in having a fully winged terrestrial adult stage, the subimago, which moults into a sexually mature adult, the imago.

Mayflies «hatch» (emerge as adults) from spring to autumn, not necessarily in May, in enormous numbers. Some hatches attract tourists. Fly fishermen make use of mayfly hatches by choosing artificial fishing flies that resemble the species in question. One of the most famous English mayflies is Rhithrogena germanica, the fisherman’s «March brown mayfly».

The brief lives of mayfly adults have been noted by naturalists and encyclopaedists since Aristotle and Pliny the Elder in classical times. The German engraver Albrecht Dürer included a mayfly in his 1495 engraving The Holy Family with the Mayfly to suggest a link between heaven and earth. The English poet George Crabbe compared the brief life of a newspaper with that of a mayfly, both being called «Ephemera», in 1785.

Description Edit

Nymph Edit

Immature mayflies are aquatic, and are known as nymphs or naiads. They have an elongated, cylindrical or somewhat flattened body that passes through a number of instars (stages), moulting and increasing in size each time. When ready to emerge from the water, nymphs vary in length, depending on species, from 3 to 30 mm (0.12 to 1.18 in).[2] The head has a tough outer covering of sclerotin, often with various hard ridges and projections; it points either forwards or downwards, with the mouth at the front. There are two large compound eyes, three ocelli (simple eyes) and a pair of antennae of variable lengths, set between or in front of the eyes. The mouthparts are designed for chewing and consist of a flap-like labrum, a pair of strong mandibles, a pair of maxillae, a membranous hypopharynx and a labium.

The thorax consists of three segments, the hindmost two, the mesothorax and metathorax being fused. Each segment bears a pair of legs which usually terminate in a single claw. The legs are robust and often clad in bristles, hairs or spines. Wing pads develop on the mesothorax, and in some species, hind wing pads develop on the metathorax.[3]

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 wingpads. In most taxa up to seven pairs of gills arise from the top or sides of the abdomen, but in some species they are under the abdomen, and in a very few species the gills are instead located on the coxae of the legs, or the bases of the maxillae. The abdomen terminates in a pair of, or three, slender thread-like projections.

Subimago Edit

The final moult of the nymph is not to the full adult form, but to a winged stage called a subimago that physically resembles the adult, but which is usually sexually immature and duller in colour. The subimago often has partially cloudy wings fringed with minute hairs; its eyes, legs and genitalia are not fully developed. Subimagos are generally poor fliers, and typically lack the colour patterns used to attract mates. After a period, usually lasting one or two days but in some species only a few minutes, the subimago moults to the full adult form, making mayflies the only insects where a winged form undergoes a further moult.

Imago Edit

Adult mayflies, or imagos, are relatively primitive in structure, exhibiting traits that were probably present in the first flying insects. These include long tails and wings that do not fold flat over the abdomen.[4] Mayflies are delicate-looking insects with one or two pairs of membranous, triangular wings, which are extensively covered with veins. At rest, the wings are held upright, like those of a butterfly. The hindwings are much smaller than the forewings, and may be vestigial or absent. The second segment of the thorax, which bears the forewings, is enlarged to hold the main flight muscles. Adults have short, flexible antennae, large compound eyes, three ocelli and non-functional mouthparts. 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. In the males of some families there are two large cylindrical «turban» eyes that face upwards in addition to the lateral eyes.[5] They are capable of detecting ultraviolet light and are thought to be used during courtship to detect females flying above them.[6] In some species, all the legs are functionless, apart from the front pair in males. The abdomen is long and roughly cylindrical, with ten segments and two or three long cerci (tail-like appendages) at the tip. Uniquely among insects, mayflies possess paired genitalia, with the male having two aedeagi (penis-like organs) and the female two gonopores (sexual openings).

Biology Edit

Reproduction and life cycle Edit

Mayflies are hemimetabolous (they have «incomplete metamorphosis»). They are unique among insects in that they moult one more time after acquiring functional wings;[7] this last-but-one winged (alate) instar usually lives a very short time and is known as an imago, or to fly fishermen as a spinner. Mayflies at the imago stage are a favorite food of many fish, and many fishing flies are modeled to resemble them. The imago stage does not survive for long, rarely for more than 24 hours. In some species, it may last for just a few minutes, while the mayflies in the family Palingeniidae have sexually mature subimagos and no true adult form at all.

Often, all the mayflies in a population mature at once (a hatch), and for a day or two in the spring or autumn, mayflies are everywhere, dancing around each other in large groups, or resting on every available surface.[2] In many species the emergence is synchronized with dawn or dusk, and light intensity seems to be an important cue for emergence, but other factors may also be involved. Baetis intercalaris, for example, usually emerges just after sunset in July and August, but in one year, a large hatch was observed at midday in June. The soft-bodied subimagos are very attractive to predators. Synchronous emergence is probably an adaptive strategy that reduces the individual’s risk of being eaten.[8] The lifespan of an adult mayfly is very short, varying with the species. The primary function of the adult is reproduction; adults do not feed, and have only vestigial (unusable) mouthparts, while their digestive systems are filled with air.[7] Dolania Americana has the shortest lifespan of any mayfly: the adult females of the species live for less than five minutes.[9]

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 (courtship) dance. Each insect has a characteristic up-and-down pattern of movement; strong wingbeats propel it upwards and forwards with the tail sloping down; when it stops moving its wings, it falls passively with the abdomen tilted upwards. Females fly into these swarms, and mating takes place in the air. A rising male clasps the thorax of a female from below using his front legs bent upwards, and inseminates her. Copulation may last just a few seconds, but occasionally a pair remains in tandem and flutters to the ground.[10] Males may spend the night in vegetation and return to the nuptial dance the following day. Although they do not feed, some briefly touch the surface to drink a little water before flying off.[10]

Females typically lay between four hundred and three thousand eggs. The eggs are often dropped onto the surface of the water; sometimes the female deposits them by dipping the tip of her abdomen into the water during flight, releasing a small batch of eggs each time, or deposits them in bulk while standing next to the water. In a few species, the female submerges and places the eggs among plants or in crevices underwater, but in general, they sink to the bottom. The incubation time is variable, depending at least in part on temperature, and may be anything from a few days to nearly a year. Eggs can go into a quiet dormant phase or diapause.[11] The larval growth rate is also temperature-dependent, as is the number of moults. At anywhere between ten and fifty, these post-embryonic moults are more numerous in mayflies than in most other insect orders. The nymphal stage of mayflies may last from several months to several years, depending on species and environmental conditions.[3]

Many species breed in moving water, where there is a tendency for the eggs and nymphs to get washed downstream. To counteract this, females may fly upriver before depositing their eggs. For example, the female Tisza mayfly, the largest European species with a length of 10 cm (4 in), flies up to 3 kilometres (1.9 mi) upstream before depositing eggs on the water surface. These sink to the bottom and hatch after 45 days, the nymphs burrowing their way into the sediment where they spend two or three years before hatching into subimagos.[12]

When ready to emerge, several different strategies are used. In some species, the transformation of the nymph occurs underwater and the subimago swims to the surface and launches itself into the air.[2] In other species, the nymph rises to the surface, bursts out of its skin, remains quiescent for a minute or two resting on the exuviae (cast skin) and then flies upwards, and in some, the nymph climbs out of the water before transforming.

Ecology Edit

Nymphs live primarily in streams under rocks, in decaying vegetation, or in sediments. Few species live in lakes, but they are among the most prolific. For example, the emergence of one species of Hexagenia was recorded on Doppler weather radar by the shoreline of Lake Erie in 2003.[14] In the nymphs of most mayfly species, the paddle-like gills do not function as respiratory surfaces because sufficient oxygen is absorbed through the integument, instead serving to create a respiratory current. 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.[15]

In most species, the nymphs are herbivores or detritivores, feeding on algae, diatoms or detritus, but in a few species, they are predators of chironomid and other small insect larvae and nymphs.[16][17] Nymphs of Povilla burrow into submerged wood and can be problem for boat owners in Asia.[18] Some are able to shift from one feeding group to another as they grow, thus enabling them to utilize a variety of food resources. 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.[3] Along with caddisfly larvae and gastropod mollusks, the grazing of mayfly nymphs has a significant impact on the primary producers, the plants and algae, on the bed of streams and rivers.[19]

The nymphs are eaten by a wide range of predators and form an important part of the aquatic food chain. Fish are among the main predators, picking nymphs off the bottom or ingesting them in the water column, and feeding on emerging nymphs and adults on the water surface. Carnivorous stonefly, caddisfly, alderfly and dragonfly larvae feed on bottom-dwelling mayfly nymphs, as do aquatic beetles, leeches, crayfish and amphibians.[20] Besides the direct mortality caused by these predators, the behavior of their potential prey is also affected, with the nymphs’ growth rate being slowed by the need to hide rather than feed.[19] The nymphs are highly susceptible to pollution and can be useful in the biomonitoring of water bodies.[2] Once they have emerged, large numbers are preyed on by birds, bats and by other insects.[3]

Mayfly nymphs may serve as hosts for parasites such as nematodes and trematodes. Some of these affect the nymphs’ behavior in such a way that they become more likely to be predated.[21][22] 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.[23] The nymphs can also serve as intermediate hosts for the horsehair worm Paragordius varius, which causes its definitive host, a grasshopper, to jump into water and drown.

Effects on ecosystem functioning Edit

Mayflies are involved in both primary production and bioturbation. A study in laboratory simulated streams revealed that the Centroptilum genus of the mayfly increased the export of periphyton,[25] thus indirectly affecting primary production positively, which is essential process for ecosystems. The mayfly can also reallocate and alter the nutrient availability in aquatic habitats through the process of bioturbation. By burrowing in the bottom of lakes and redistributing nutrients, mayflies indirectly regulate phytoplankton and epibenthic primary production.[26] Once burrowing to the bottom of the lake, mayfly nymphs begin to billow their respiratory gills. This motion creates current that carries food particles through the burrow and allows the nymph to filter feed. Other mayfly nymphs possess elaborate filter feeding mechanisms like that of the genus Isonychia. The nymph have forelegs that contain long bristle-like structures that have two rows of hairs. Interlocking hairs form the filter by which the insect traps food particles. The action of filter feeding has a small impact on water purification but an even larger impact on the convergence of small particulate matter into matter of a more complex form that goes on to benefit consumers later in the food chain.

Distribution Edit

Mayflies are distributed all over the world in clean freshwater habitats,[28] though absent from Antarctica.[29] They tend to be absent from oceanic islands or represented by one or two species that have dispersed from nearby mainland. Female mayflies may be dispersed by wind, and eggs may be transferred by adhesion to the legs of waterbirds.[30] The greatest generic diversity is found in the Neotropic ecozone, while the Holarctic has a smaller number of genera but a high degree of speciation. Some thirteen families are restricted to a single bioregion.[31] The main families have some general habitat preferences: the Baetidae favour warm water; the Heptageniidae live under stones and prefer fast-flowing water; and the relatively large Ephemeridae make burrows in sandy lake or river beds.


Mayfly Facts

Green, Brown, Tan, Grey, Black, Yellow Skin Type: Forest and woodland close to water Average Litter Size: Algae, Larvae, Aquatic Plants Predators: Birds, Rodents, Reptiles Special Features: Long bodies and large wings

Mayfly Location


Mayflies are aquatic insects that get their name from the fact that the adult appears in May. Mayflies hatch out in large numbers in the spring but continue hatching until fall. Since the adult mayfly’s purpose is to reproduce, it has a short lifespan. Mayflies are beloved and celebrated creatures. There have been poems and books written about mayflies, and even festivals are named after this insect with a brief lifespan.

Mayfly Facts

There are as many as 3000 species of mayflies with 700 species living in North America. Except for the Arctic and Antarctica, mayflies exist throughout the world. When mayflies swarm near freshwater habitats, the gathering can be so dense that it can be hard to see when driving.

Mayfly Scientific Name

The scientific name for the Mayfly is Ephemeroptera, which comes from the Greek language and means “short-lived.” Mayflies emerge in large groups but have short lifespans. Other names for the mayfly include the dayfly, drake, fishfly, sandfly, and shadfly. Fishfly is a popular name for the mayfly.

Mayfly Appearance and Behavior

Adult mayflies have large eyes and short antennae. A mayfly’s slender body makes the eyes seem more pronounced, which is why they are described as “bug eyes.” Mayflies have pairs of large clear triangular wings with vertical and horizontal veins that give them a delicate netlike appearance.

The mayfly’s wings are similar to butterfly wings in how they are attached to the insect’s thorax. The mayfly’s larger wings are at the front of the body and with small round wings behind. The tiny hind wings on some species can be challenging to see, and some appear to have no hind wings. The mayfly has two or three tails that look like threads. The tails can be longer than the insect’s body. Mayflies vary in color and size, but they tend to blend in with their background.

The different colors and sizes of mayflies seen in an area are the result of different species inhabiting a single water source. However, the mayfly is one of the easiest aquatic insects to identify because of its large eyes, slender body, and threadlike tails. The mayfly can range from one-tenth of an inch to just over an inch or three centimeters long—about the size of a quarter.

Mayflies sometimes emerge in such large numbers that they cover light posts, trees, and tall grasses, making them nuisances around homes and businesses. Swarms of mayflies can be so large that they appear on Doppler weather radars.

Mayfly Habitat

Most mayfly nymphs or naiads live in streams with clear, shallow water, but some reside in still waters and around the edges of lakes. As naiads age, they start to develop gills. Naiads that live in still waters have larger gills, and those living in moving streams have smaller gills. Naiad’s gills control water flow, salt, and oxygen intake. The gills also deflect water at angles, which can mislead predators because it makes the naiads harder to track.

Nymphs can live for several months and then emerge from the water as adults. Seeing mayflies around streams can be a sign of good water quality since a naiad’s gills are vulnerable to polluted waters. When large numbers of mayflies hatch near bodies of water, that can be reassuring because it indicates that the habitat is environmentally sound. Efforts by communities to keep rivers and streams clean ensure the existence of mayflies.

Mayfly Diet

Mayfly naiads feed on algae, microscopic sea organisms, organic matter consisting of leaves and decaying animals, and plants. Once a mayfly gets its wings, it can no longer feed. Also, adult mayflies have no mouths, so they are unable to eat. Like any creature that needs food to live, mayflies could not live long without eating. Food is not an issue for the adult since it dies within hours or days after emerging.

Mayfly Predators and Threats

Trout and other fish consume mayfly naiads as food. Mayfly naiads are also the food choice of birds, flies, frogs, parasitic roundworms, and water beetles. Caddisfly larvae and snails may eat the eggs of mayflies. Birds, dragonflies, fish, and water beetles eat mayflies that are in the early adult stage. When mayflies swarm, they tend to cause fish to swarm, which is helpful to fishermen looking for places to cast their lines. Fishermen sometimes use lures crafted to look like mayflies.

Mayflies reproduction, babies, and lifespan

The purpose of the adult mayfly is to reproduce, and once they reproduce, they die. During swarming, adult mayflies mate. Once the male mates with a female, he protects her to prevent other males from mating with her. Females can produce as few as 50 to several thousand eggs. After mating, a female mayfly deposits her eggs into the water by dipping. A female mayfly dips several times to release eggs into the water. Some mayflies drop their eggs on the water’s surface. The eggs sink into the water, resting among debris and aquatic plants. However, when mayflies deposit eggs in this manner, the eggs can be eaten by fish before sinking.

Mayfly larvae are known as naiads or nymphs, which emerge a short time after the female lays the eggs. The new naiads are very tiny with no gills. The developmental stages of naiads are known as instars. Depending on the species of the nymph, the number of instars can range from 12 to 45. The location where naiads live and the water temperature determine how long a species remains in the naiad stage.
Eventually, the nymph molts or sheds its outer layer. Mayflies are unique, being the only insect with two adult molts. Molting is the process of shedding the outer shell or outer skin.

Mayflies live most of their lives underwater. After several months of living underwater, mayfly naiads float to the top and molt into the stage known as subimago or the sub-adult state. At this stage, just before it can fly, the young mayfly is vulnerable to predators. As a sub-adult, the mayfly is not able to mate or reproduce. However, within hours the mayfly molts again into the imago state, becoming an adult winged insect with the ability to reproduce, but not the ability to eat or drink. This version of the mayfly lives for hours or, at most, a few days.

Mayfly Population

The mayfly population in a specific area can depend on the quality of the habitat. Cleaner streams attract more mayflies. Since mayflies can deposit from 50 to several thousand eggs, the number of mayflies in an area depends on the number of eggs adult females deposit in the water. Mayflies have made a comeback in some areas recently because of improved water quality.

Mayfly Frequently Asked Questions

Are Mayflies Dangerous?

When mayflies fly in large groups, they are attracted to lights and typically congregate on the sides of buildings, on posts, and in trees. In some areas, officials may order streetlights turned out to avoid attracting mayflies. When mayflies die, they can leave behind an unpleasant aroma. Since they die quickly in large numbers, piles of dead mayflies can trigger allergies in some people. Mayflies in large quantities can end up on roadways causing the surfaces to become slick and dangerous for drivers. They can get into electrical substations and knock out the power near the waters where they originate. However, they are more of a nuisance than a danger to humans.

What are mayflies good for?

Mayflies add value to the areas where they live. Mayfly habitats tend to create excellent fishing opportunities. Large numbers of mayflies attract fish, and they can boost tourism in areas with fishing businesses. The insects tend to live in clean water, so the presence of many mayflies near streams is a sign of favorable environmental quality. As a result, some people are willing to tolerate mayflies since they are a sign of a healthy ecosystem.

Do mayflies sting or bite

Mayflies do not have mouths to eat, and with such a short life cycle, mayflies do not need food. Although they may appear to be a stinging insect with their long tails, they neither bite nor sting. The mayfly’s primary purpose is to reproduce, and shortly after that, they die.

The mayfly’s lifecycle: a fascinating, fleeting story

© John Slader Mayfly. Commended Riverfly Partnership / National Insect Week 2010

The mayfly’s lifecycle is one of the most fascinating and fleeting stories in the natural world. One of the many characteristics that makes mayflies the unique insects they are is the potential for two different winged adult forms in their life cycle. The nymph emerges from the water as a dull-coloured sub-imago (or dun) that seeks shelter in bankside vegetation and trees. After a period of a couple of hours or more, the sub-imago once again sheds its skin to transform into the brightly coloured imago (or spinner). It is not clear why mayflies have retained this unique step in their lifecycle, however it is thought that they may not be able to achieve the change from nymph to sexually mature adult in one step.

A mayfly’s life cycle starts with the males forming a swarm above the water and the females flying into the swarm to mate. The male grabs a passing female with its elongated front legs and the pair mate in flight. After copulation, the male releases the female, which then descends to the surface of the water where she lays her eggs. Once mated she will fall, spent, onto the water surface to lie motionless, with her wings flat on the surface, where fish pick them off at their leisure. The male fly rarely returns to the water but instead he goes off to die on the nearby land.

The eggs fall to the bottom of the water where they stick to plants and stones. Flies of the Mayfly family Baetidae pull themselves under the water to attach their eggs directly to the bed before being drowned by the current. The nymphs take anything between a few days to a number of weeks to hatch depending on water conditions and the species, and the resultant nymphs will spend various lengths of time, up to two years, foraging on the bottom before emerging as an adult fly.

When it is time to emerge, the nymphs make their way to the surface where they pull themselves free of their nymphal shuck and emerge as a sub-imago. While they rest here to dry their newly exposed wings, they are at their most vulnerable to attack from fish.

Some species exhibit great synchronicity in their hatching. The North American species Hexagenia limbata hatches in huge numbers from the Mississippi every year. The total number of mayflies in this hatch are estimated to be around 18 trillion – more than 3,000 times the number of people on earth. The newly emerged insects are attracted to lights in riverside towns and villages and the local authorities deploy snow clearing vehicle to remove their rotting corpses. Ironically, what is seen as a nuisance in America is seen as a gift in Africa. Locals around Lake Victoria gather adults of the mayfly Povilla adusta together with Chironomid midges to make a type of patty called ‘Kungu’. This protein rich food stuff is an important part of their diet.

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