Female March Brown (Rhithrogena germanica) by Richard Bartz
Mayflies are insects which belong to the Order Ephemeroptera (from the Greek ephemeros = "short-lived", pteron = "wing", referring to the short life span 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 freshwater. The adults are short-lived, from a few minutes to a few days depending on the species. About 2,500 species are known worldwide, including about 630 species in North America. Common names for mayflies include "dayfly", "shadfly", "Green Bay Flies", "lake fly", "fishfly," "midgee", "June bug", and "Canadian Soldier." The mayfly belongs to group 1 taxa, or pollution-sensitive animals. This means if mayflies are in or around the water, the water should be of a good quality.
Eggs are laid on the surface of lakes or streams, and sink to the bottom. Naiads moult 20 to 30 times over a period of a few months up to year, depending on the species. The naiads live primarily in streams under rocks, decaying vegetation, or in the sediment. 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 along the shores of Lake Erie.
Most species feed on algae or diatoms, but there are a few predatory species. The naiad stage may last from several months to as long as several years, with a number of moults along the way. Mayfly naiads are distinctive in that most have 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 insects. A more casual and familiar term is incomplete metamorphosis. Mayflies are unique among the winged insects in that they moult one more time after acquiring functional wings (this is also known as the alate stage); this second-to-last winged instar is usually very short, often a matter of hours, and is known as a subimago or to fly fishermen as a dun. This stage is a favourite food of many fish, and many fishing flies are modeled to resemble them.
The lifespan of an adult mayfly can vary from just 30 minutes to one day depending on the species. The primary function of the adult is reproduction; the mouthparts are vestigial, and the digestive system is filled with air.
The wings are membranous, with extensive venation, and are held upright like those of a butterfly. The hindwings are much smaller than the forewings, and may be vestigial, or entirely absent. The second segment of the thorax, which bears the forewings, is enlarged, holding the main flight muscles.
Adults have short, flexible antennae, large compound eyes and three ocelli. In most species, the males' eyes are large and the front legs unusually long, for use in locating and grasping females during mid-air mating. In some species, all legs aside from the males' front legs are useless. Uniquely among insects, mayflies possess paired genitalia, with the male having two penises and the female two gonopores. The abdomen is roughly cylindrical, with ten segments and two long cerci at the tip.
Ephemeridae, Baetis sp. larvae
The final moult of the naiad is not to the full adult form, but to a winged subimago that physically resembles the adult, but which is usually sexually immature. The subimagos are generally poor fliers, and typically lack the colouration patterns used to attract mates. The subimago eventually moults to the full adult, making mayflies the only insects where a winged form undergoes moulting.
Like the adult itself, the subimago stage does not last for long, and rarely for more than 24 hours. In some species, it may last for just a few minutes before moulting into the adult, while the mayflies in the family Palingeniidae have sexually mature subimagos, and no true adult form at all.
It often happens that all the mayflies in a population mature at once (the 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. This happens in Buffalo, New York and in mid-June on the Tisza River in Serbia and Hungary; this kind of mayfly is called the tiszavirag (in Hungarian) or "tiski cvet" in Serbian which is translated as "Tisza flower" in both languages, though the most widespread Serbian name is "vodeni cvet" (water flower). This natural phenomenon is called Tisza blooming. In certain regions of New Guinea and Africa, mayflies are eaten when they emerge en masse on a certain day.
Because of its short lifespan, the mayfly is also called one-day fly in some languages - French ephemere, German Eintagsfliege, Dutch eendagsvlieg, Slovenian enodnevnica, Swedish dagslanda, Danish and Norwegian dognflue, Finnish paivankorento, Spanish efimera, Romanian efemeride.
Both immature and adult mayflies are an important part of the food chain, particularly for carnivorous fish such as trout in cold water streams or bass and catfish in warm water streams, or Walleye in cool water lakes. Males generally fly in swarms that undulate in the air 5-15 meters above the ground.
The status of most species of mayflies is unknown because many species are only known from the original collection data. Four North American species are believed to be extinct, two of which are listed below:-
- Pentagenia robusta was originally collected from the Ohio River near Cincinnati, but this species has not been seen since its original collection in the 1800s.
- Ephemera compar was reported from the "foothills of Colorado". Despite intensive surveys of the Colorado mayflies, this species has not been collected in the past 50 years.
The International Union for Conservation of Nature (IUCN) red list of threatened species includes one mayfly: Tasmanophlebi lacus-coerulei, the Large Blue Lake Mayfly, is a native of Australia and is listed as vulnerable.