Mayflies

Mayflies belong to the Phylum Arthropoda, Class Insecta, and Order Ephemeroptera. They are a small, wide-ranging, primitive order of insects that is well known to entomologists, biologists, naturalists, and fly fishermen. There are approximately 3,000 species of mayflies described worldwide; these are grouped into over 400 genera within forty-two families. As of 2019, approximately 676 species within twenty-one families were recognized from North America. The southeastern United States has nearly 400 mayfly species, which is more than half of those known from the United States and almost half of those known from North America alone. No single study summarizing mayflies exists specifically for Arkansas, but an investigation from 2010 noted about 117 species of mayflies reported from the state.

The phylogeny of the Ephemeroptera has been studied; however, there has been no taxonomic stability in ephemeropteran suborders and infraorders. The traditional taxonomic division of the Ephemeroptera places all mayflies into the suborders Schistonota or Pannota; however, this division was deemed erroneous because Pannota is derived from the Schistonota. It was not until 2005 that the phylogeny of the Ephemeroptera was initially studied by molecular analysis. The family Baetidae was recovered as sister to the other clades. In addition, utilizing both morphological and molecular analyses (complete mitochondrial genome) to study mayfly phylogeny, the Asian genus Siphluriscus (the most plesiomorphic mayfly) was found to be sister to all other mayflies.

Mayflies are an ancient group of winged insects. The first recorded mayfly nymphs are from the Late Carboniferous Period (299 million years ago). However, it is difficult to classify early fossils in the correct order. It has been reported that the largest mayfly of all time may have been the gigantic Bojophlebia prokopi from the Upper Carboniferous (359 million years ago) of Moravia (eastern Czech Republic) with a wingspan of 45 cm (18 in.). Similar fossils are known from Oklahoma, and they are very well preserved, with the wing venation clearly visible.

Mayflies are cosmopolitan in geographic distribution and inhabit clean lotic (running water) and lentic (standing water) habitats in fresh water on every continent except Antarctica. Generally, they are absent on oceanic islands or may occasionally be represented by one or two species that have dispersed from a nearby mainland. The greatest generic diversity is found in the Neotropical zoogeographical realm, whereas in the Holarctic realm, there are a smaller number of genera, but a high degree of speciation occurs.

Mayflies are relatively primitive insects and exhibit a number of ancestral traits probably present in the earliest flying insects such as long tails and wings that do not fold flat over their abdomens. Characteristically, mayflies tend to be small to medium-sized insects, 2 to 35 mm in length (0.07 to 1.4 in.), with an elongate, slender, and soft body. The general body plan of a mayfly is typical of any insect. They have a thorax consisting of three segments, with the last two (mesothorax and metathorax) being fused together. Each segment has a pair of legs that terminate in a single claw. Legs are large and often covered in bristles, hairs, or spines. Wing pads develop on most species on the mesothorax; however, in some species, hindwing pads develop on the metathorax. The abdomen consists of ten segments, some of which may be obscured by a large pair of gills, a thoracic shield (expanded part of the prothorax), or the developing wing pads. Up to seven pairs of gills may arise from the top or sides of the abdomen (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 two or three slender, thread-like projections.

Mayflies are unique among insect orders in having a fully winged terrestrial adult stage known as the subimago that molts into a sexually mature adult called the imago. Mayflies have four membranous wings with many veins and cross veins, as well as large and triangular forewings (two to three times the size of the hindwings). Hindwings are small and rounded in shape, although they are sometimes absent. Most mayfly species have two to three long, hair-like filaments (terminal caudal filaments) on the abdomen. Antennae are short (less than the length of the head and thorax combined and hair-like). Mouthparts in adults are absent, and therefore adults do not feed in this stage.

Mayflies exhibit a simple or incomplete metamorphosis (hemimetabolous), which means they lack the pupal stage and the mature nymph molts directly into a winged stage. The life cycle typically consists of four stages: egg, nymph, subadult or subimago, and imago (adult). Mayflies are unique in having two winged stages: the subimago and the imago. Mating usually occurs in the adult stage, but in a few species, females are capable of mating and ovipositing in the subimago stage (e.g., in the genera Tortopus and Ephoron).

Adult male mayflies can be distinguished from females by a number of characteristics. One, the eyes of males are generally of a different shape and larger than those of females; two, the developing genitalia (a pair of forceps or claspers and paired penes) are outgrowths of the ninth sternum and can be seen through the nymphal cuticle, and three, the caudal filaments of the male can be thicker and more tapered, especially at the base, in males versus females.

The first stage of the life of a mayfly is immature mayflies referred to as “naiads” and “nymphs,” and they are usually less than one millimeter long. It is the dominant life history stage of the mayfly. They spend more than 99 percent of their lives in this stage on the river or lake bottom, filling many crucial roles in freshwater ecosystems as they mature. They have no gills initially, and their body shape varies according to habitat. For example, those in the genus Ephemera burrow and have more-cylindrical bodies, whereas those that slide under rocks (such as Heptagenia) are flatter, and those in the genus Caenis crawl on mossy stones and vegetation, so they have short bodies with squat legs. Ephemeroptera nymphs may grow anywhere from 4 mm to 3 cm long. They are generally camouflaged against their background.

The number of molts a nymph goes through on its way to becoming an adult does not depend on its nutrition, but the increase in size that comes with each molt does. Once mature, nymphs may be distinguished by the presence of dark or swollen, well-developed wing pads. Nymphs are elongate, typically flattened insects with medium to long antennae. Most species have three caudal filaments, a terminal filament, and two cerci. Coloration of nymphs is variable and ranges from nearly uniform to a bold pattern. These forms have an elongated, cylindrical, or somewhat flattened body that passes through a number of stages called instars. Each molt results in an increase in size. 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.). The head has a tough outer cuticular covering of sclerotin, often with various hard ridges and projections; it points either forward or downward, with the mouth at the front. There are two large compound eyes, three ocelli (simple eyes), and a pair of antennae set between or in front of the eyes. Mouthparts are adapted for chewing and consist of a flap-like labrum, a pair of strong mandibles, a pair of maxillae, a membranous hypopharynx, and a labium.

Mayfly nymphs are strictly aquatic, and respiration is by gills. They live in lentic (standing water) situations as well as lotic (running water) habitats and tend to be habitat specialists. Various examples of nymph habitat include spring-fed pools, spring brooks, small streams, small to large rivers, as well as side pools and ponds along stream margins, backwater areas of streams and rivers, bogs, swamps, roadside ditches, temporary pools, permanent ponds, lakes, and reservoirs. General habitat preferences of the main mayfly families are as follows: Baetidae favor warmer water whereas the Heptageniidae live under stones and prefer fast-flowing water, and members of the relatively large Ephemeridae make burrows in sandy lakes or river beds.

The subimago (subadult) resembles the adult mayfly and is the first flying stage; however, it is usually sexually immature, its appearance tends to be duller, it has one or two pairs of opaque wings fringed with minute hairs, and the legs and caudal filaments are shorter. Typically, the winged subimago seeks a sheltered place to rest before molting into the imago stage. Interestingly, this transformation takes as little as a few minutes in some species but up to two days in others. Subimagos are typically poor fliers and lack the color patterns used to attract mates. After a period of one or two days, or in some cases a few minutes, the subimago molts to the full adult form, making mayflies the only insects for which a winged form undergoes a further molt.

Adult mayflies, or imagos, are relatively primitive in structure, exhibiting traits that were probably present in the first flying insects, including long tails and wings that do not fold flat over the abdomen. Mayflies are delicate-looking insects with one or two pairs of membranous, triangular wings, which are extensively covered with veins. When at rest, the wings are held upright, as in a butterfly. The hind wings 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 upward in addition to the lateral eyes. They are capable of detecting ultraviolet light and are thought to be used during courtship to detect females flying above them. 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).

Mayflies are considered aquatic “indicator species” because finding the “naiads” or “nymphs” typically indicates a relatively clean, unpolluted aquatic environment. Mayflies are quite intolerant of aquatic pollution.

In Arkansas, the life histories and ecology of most state mayfly species are poorly known. Generally, mayflies tend to have a one-year life cycle with one or more generations, although some species may have a two-year life cycle. A life history classification of mayflies has been established for four groups and eleven types. Most North American (including Arkansas) mayfly species tend to be type A1, in which the eggs hatch in autumn, nymphs continue to grow during the winter, and adults emerge in spring or summer. Type A2 species possess eggs in diapause (delayed development) until spring or summer, and growth of nymphs is short and quite rapid. Diapause in mayfly eggs has been observed in a number of species.

Female mayflies are capable of laying up to 9,000 eggs. Females may lay their eggs in several ways. They may (1) drop their eggs into the water while in flight, (2) deposit them by touching the surface of the water with the abdomen while in flight, (3) land on the surface of the water and deposit the eggs, or (4) actually enter the water and deposit the eggs on or under the substrate. The eggs stick to the substrate after they are laid and tend to hatch fairly soon after deposition. Although little is known of the number of instars or molts required to reach the subimago stage, researchers have reported from nine to forty-five. Nymphal life can be as little as sixteen days or up to a year or more depending on the species. Most mayflies “hatch” (emerge as adults) from spring through autumn months, but not necessarily in May as their name might imply. These hatches are often massive and noted especially by fly fishermen who choose artificial fishing flies that resemble the species hatching.

Mayflies are hemimetabolous (meaning that they have incomplete metamorphosis). They are unique among insects in that they molt one more time after acquiring functional wings; this last-but-one-winged (alate) instar usually lives a very short time and is known as a subimago, or to fly fishermen as a dun. Mayflies at the subimago stage are a favorite food of many fish, and many fishing flies are modeled to resemble them. The subimago stage usually does not survive more than twenty-four hours; in some species, it may last only for a few minutes.

There is a tendency for all the mayflies in a population to mature at the same time, and for a day or two in the spring or autumn, mayflies are everywhere. The emergence may be synchronized with dawn or dusk, and light intensity seems to be an important cue for emergence. For example, the small eastern blue-winged olive (Baetis intercalaris) usually emerges just after sunset in July and August; however, a large hatch has been observed at midday in June. Subimagos are very attractive to predators. As with other animals, synchronous emergence may be an adaptive strategy that reduces the chance of being consumed. 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. One species, Dolania americana of the coastal plain of the southeastern United States, has the shortest lifespan of any mayfly and perhaps any insect, less than five minutes.

Reproduction is accomplished when adult males swarm a few feet above the water surface and perform a nuptial dance. Each species has a characteristic up-and-down pattern of movement; strong wingbeats propel it upward and forward with the tail sloping down, and when it stops moving its wings, it falls passively with the abdomen tilted upward. Mating occurs when the females fly into these swarms. The male clasps the thorax of a female from below using his front legs and inseminates her. Typically, copulation lasts only a few seconds. Males hide in vegetation during the night, and in the daytime they again perform the nuptial dance.

When the nymph is ready to emerge, several strategies may be used. Transformation of the nymph may occur underwater with the subimago swimming to the surface and launching itself into the air. Or, the nymph may rise to the surface, burst out of its skin, remain for a minute or two resting on the exuviae (cast skin) and then fly upward; in some cases, the nymph actually climbs out of the water before transforming.

Nymphs live primarily in streams under rocks, leaf packets, decaying vegetation, or sediments. Although few species live in lakes, they are often among the most prolific. For example, the emergence of a species of Hexagenia (Emphemeridae) was recorded on Doppler weather radar by the shore of Lake Erie. 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 the European species, Ephemera vulgata burrows, the filamentous gills are used in gaseous exchange and serve as true accessory respiratory organs.

In most species, the nymphs are herbivores or detritivores feeding on algae, diatoms, and detritus, but in a few species, they are predators of chironomid (midge) larvae. Along with caddisfly larvae and gastropod mollusks, the grazing of mayfly nymphs has a significant impact on the primary producers on the bed of streams and rivers, including plants and algae.

Mayfly eggs are eaten by snails and trichopteran (caddisfly) larvae. Mayfly nymphs are eaten by a wide range of predators and are an important part of the aquatic food chain. Fish (particularly trout) are the main predators, consuming nymphs from the substrate or water column, and feeding on adults at the surface of the water. A few caddisfly, stonefly, and dragonfly larvae also feed on mayfly nymphs, as well as aquatic beetles, leeches, crayfishes, and frogs and salamanders. After hatching, mayflies may also be preyed upon by other insects, birds, or bats.

The first attempt to document mayflies of Arkansas was in a 1964 unpublished report of only eighteen species collected from sites in the Ozark Mountains of Benton, Carroll, Madison, and Washington counties. This was followed by a 1966 publication reporting twenty-seven nominal species from light trapping adults in Washington County. Eight additional species from the scattered literature were added prior to a more comprehensive study in 1978, bringing to thirty-five the number of mayflies known from Arkansas. The 1978 study documented seventy species from the mountainous regions of the uplands (Ouachita Mountains and Ozarks) of the state. The most recent investigation, published in 2010, noted about 117 species of mayflies reported from Arkansas, including the following fifteen families, their common name and number of species (in parentheses) as follows: brown duns, Ameletidae (1), blue-winged olives, Baetidae (28), armored mayflies, Baetiscidae (1), angler’s curses, Caenidae (7), Hendrickson’s, Ephemerellidae (20), hexes/big drakes, Ephemeridae (6), March browns, Heptageniidae (24), slate drakes, Isonychiidae (4), tricos, Leptohyphidae (2), blue quills, Leptophlebiidae (10), pseudo-gray drakes, Metretopodidae (1), No common name, Palingeniidae (1), white flies, Polymitarcyidae (4), golden drakes, Potamanthidae (4), and gray drakes, Siphlonuridae (4). Three families—Baetidae, Emphemerellidae, and Heptageniidae—account for over sixty percent of the total number of species known from the state. A previously reported endemic mayfly, the leptophlebiid, Paraleptophlebia calcarica, was originally known only from the type locality at Gutter Rock Creek on Mount Magazine in Logan County. However, it was later reported from Kansas. It is a species of greatest conservation need according to the Arkansas Game and Fish Commission. Another leptophlebiid species, Habrophlebiodes annulata, appears to be restricted to the central highlands of Arkansas and western adjacent areas (Kiamichi River, Oklahoma).

A variety of organisms live on or in mayflies, including a range of bacterial, protozoan, trematode, cestode, and nematode parasites. As well as being final hosts for parasites, mayflies serve a major role as intermediate hosts, most notably for numerous fish parasites.

For additional information:
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Henry W. Robison
Sherwood, Arkansas

Chris T. McAllister
Eastern Oklahoma State College