Ephemeroptera is derived from the Greek ephemeros = "short-lived", pteron = "wing", referring to the brief lifespan of adults. Sometimes they are grouped with the ancient Palaeoptera, which also includes dragonflies and damselflies. All are aquatic with immature stages naiads or nymphs. Development is usually one year in freshwater. The adults do not live very long, ranging from only a few minutes to a few days. About 2,510 species hve been identified, including about 632 species in North America. Ther common nmes also include "dayfly", "shadfly", "Green Bay flies", "lake fly", "fishfly."
The eggs are deposited onto the surface of lakes or streams, from which they sink to the benthos. Naiads may molt 20-32 times over a few months up to year. The naiads live primarily in streams under rocks, decaying vegetation, or in bottom sediment. The few species that live in lakes are very prolific, numbering in the millions
The primary food is algae or diatoms, but there are also some predatory species. The naiad stage may take from several months up to several years. Naiads are distinctive in that most have seven pairs of gills on the dorsum of the abdomen. They also frequently possess three long cerci or tails at the end of their abdomen. Some species, such as the genus Epeorus, have only two tails. They are hemimetabolous insects. Mayflies are unique among the winged insects in that they molt one more time after acquiring functional wings stage. The duration of the second-to-last winged instar, or subimago, only lasts a few minutes. This stage is a primary food source for fish.
The lifespan of an adult mayfly may vary from 30 minutes to one day. The primary function of the adult is reproduction; the mouthparts are vestigial, and the digestive system is filled with air. The wings are membranous (similar to a house fly's wings but with many more veins) and are held upright like those of a butterfly. The forewings are much larger than the hind wings. In most species, the males' eyes are usually 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.
The wings are membranous, and have many veins. They are held upright like those of a butterfly. The hindwings are smaller than the forewings, and may be vestigial, or absent. The second segment of the thorax, which bears the forewings, is enlarged, holding the main flight muscles.
Adults also have short, flexible antennae, large compound eyes and three ocelli. The male's eyes are large and the front legs unusually long, In some species, all other legs are not used. Mayflies possess paired genitalia, with the male having two penises and the female two gonopores. The abdomen is mostly cylindrical, with 10 segments and two cerci.
Mayflies in a population often mature at one time and for a day or two in the spring or autumn, they are very abundant. This usually occurs in early summer.
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