EPHEMEROPTERAN

any member of the order Ephemeroptera, comprising the group of insects known as mayflies. Other common names for the winged stages are shadfly, sandfly, dayfly, fishfly, and drake. The aquatic immature stage, called a nymph or naiad, is widely distributed in freshwater; a few species can tolerate the brackish water of marine estuaries. The winged stages attract attention through mass emergences when they may make roads slippery, clog gutters, and taint the air with an odour of decay. Mayfly nymphs are important in the energy transfer cycle that occurs in freshwaters (winged stages cannot feed). Some species are carnivorous, but the majority of nymphs feed on diatoms, algae, higher plants, and organic detritus; nymphs are devoured in turn by many carnivorous animals, especially fishes. a member of the insect order Ephemeroptera, comprising the insects commonly known in their winged stages as mayflies, shadflies, dayflies, fishflies, or drakes. During their larval phase in fresh water or occasionally in brackish water they are called nymphs or naiads. There are approximately 2,000 known species, of which about 600 inhabit the North American continent. They are found on all land areas except Antarctica. While fossil records are incomplete, evidence suggests that ephemeropterans were present during the Upper Carboniferous Period (280,000,000 to 325,000,000 years ago). They range in size in the adult phase from 2.5 to 32 millimetres (0.1 to 1.3 inches). The life cycle encompasses four stages: egg, nymph, subimago, and imago. In the aquatic nymphal stage, ephemeropterans have six legs, each of which ends in a claw. The body is divided into 10 segments, some of which have gills. The body terminates in two or three thin tails. Adaptation in nymphs has produced species with jaws and appendages especially suited for burrowing and species with flattened bodies for mobility through narrow spaces and for minimizing resistance to stream currents. Other species have legs adapted for movement over river bottoms and other surface features and for adhering to aquatic vegetation. Most ephemeropteran species remain in the nymphal stage for about a year. During this period they shed their external layer (molt) up to 50 times. Once they have matured they molt their last nymphal coat and emerge as winged subimagoes or duns. Unlike all other insects, the mayfly molts again after it has acquired fully developed wings. After a period ranging from several minutes to several days, but usually overnight, the imago molts and emerges as the mature adult, known as the imago or spinner. Both imagoes and subimagoes are characterized by wispy tails, two pairs of thickly-veined wings, and short antennae. Both have compound eyes and are equipped with nonfunctioning mouthparts and digestive apparatus. Mating behaviour begins shortly after entrance into the imago phase and ends with the death of both male and female following the extrusion of fertilized eggs (oviposition). The entire life span of the adult mayfly is usually only a few hours, although at least one species lives for up to two days. Mating begins with the dance of a swarm of males over water at nightfall. Females soon respond, and mating is carried out in the air. Fertilized eggs, which are produced in clusters of fewer than 50 to more than 10,000, are usually deposited by the females on the surface of water. Some species descend beneath the water to oviposit on submerged surfaces. Once deposited, eggs adhere to a submerged object or fall to the bottom; they generally hatch within two weeks. Mayflies in all three growth stages are important in the food chains of aquatic ecosystems. As nymphs they consume algae and other aquatic vegetation and the products of organic decay, and they serve as prey for fish and a number of invertebrate carnivores. Predators of subimagoes and imagoes include hornets, dragonflies, bats, birds, beetles, spiders, and mammals; mayflies become food for fish once again as they mate and die at or near the surface of the water. Additional reading L. Berner, A Tabular Summary of the Biology of North American Mayfly Nymphs (Ephemeroptera) (1959); B.D. Burks, The Mayflies, or Ephemeroptera, of Illinois (1953, reprinted 1975), chiefly taxonomic, with good illustrations; A.E. Eaton, A Revisional Monograph of the Ephemeridae, or Mayflies, 6 pt. (188388), taxonomy of world animal life with historical value; G.F. Edmunds, Jr., The Principles Applied in Determining the Hierarchic Level of the Higher Categories of Ephemeroptera, Syst. Zool., 11:2231 (1962), and Biogeography and Evolution of Ephemeroptera, A. Rev. Ent. (1972); J.W. and F.A. Leonard, Mayflies of Michigan Trout Streams (1962), biology and ecology with colour plates; J.G. Needham, J.R. Traver, and Y.C. Hsu, The Biology of Mayflies (1935, reissued 1972), comprehensive handbook. Justin W. Leonard