Ephemeroptera is a group of 2,000 insect species commonly known as mayflies. They are considered to be part of the clade Uniramia which includes silverfish and dragonflies, among others. Ephemeroptera and Odonata are the only extant orders of winged insects in the infraclass Paleoptera. All other insects with wings are in the Neoptera, and are characterized by a wing articulation (joint) that allows them to fold their wings back over their abdomens at rest (Carpenter, 1992).
Ephemeroptera are aquatic insects that often go through many nymph stages (living in water) and two flying stages (the subimago and the imago). They are the only insects to have two flying stages, and can be recognized by their three caudal filaments (tails) at the tip of the abdomen, and a single claw on each leg. This differentiates them from the closely related stoneflies which have two tarsal claws. The flying stages are characterized by relatively large forewings, which are usually kept upright, and reduced or nonexistent hind wings.
The first stage of the life of a mayfly is the nymph (larva), which not only looks very different from the adult, but lives in the water. When the nymphs hatch from the eggs, they are less than 1 mm long. They have no gills at first, and their body shape varies according to habitat. For example, those that burrow (such as Ephemera) have more cylindrical bodies, whereas those that slide under rocks (such as Heptagenia) are flatter. Those in the genus Caenis crawl on mossy stones and vegetation, so they have short bodies with squat legs. Ephemeroptera nymphs may grow to 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.
In older nymphs, gills are found in pairs on each segment of the abdomen (see pictures below). The gills extend from the sides of the body and are oval-shaped. These gills beat to control the flow of water through the body, which also controls the amount of oxygen and salt that flows through the body. Nymphs in still waters generally have larger gills, and those in running water have smaller gills; this allows the nymphs of each habitat to get their optimum flow of water. Not only do the gills function in uptake of water, salt, and oxygen, but they also send water off at right angles to the body. This is used to mislead predators. If the water simply flowed out the back of the nymph's body, predators would know that the nymph was sitting at the beginning of the stream. However, since they send water away from their bodies at several points, the nymphs are not as easy to track.
Some signs of sex can be seen in the last few stages of the nymph, even before it becomes an adult. At this stage, male nymphs have the beginnings of clasping organs on the lower portions of their abdomen, with which they hold the female during copulation. In some species, the males have divided eyes that are two colors. The upper portion is for seeing movement, and the lower portion is specialized for seeing details. The females have smaller eyes and oviducts in the lower abdomen.
When it comes time for the last nymph stage to molt into a subimago (the first flying stage), the guts empty out and the mid-gut section fills with air. Often, many nymphs will then simultaneously let go of their hold on their anchor in the water and float up to the top. Once they reach the air, the cuticle splits open on the thorax and the wings come out. This is the time of greatest vulnerability in their lives as they float on the water before they are strong enough to fly. The subimago has short hairs on the wings and on the body; the wings are dull and pigmented. Once it gains some strength, it flies from the water to some form of shelter such as a tree, long grass, or the underside of a bridge and molts again within 24 to 48 hours. Thisadditional molt allows the legs and tails of the insect to grow more. Longer tails give more stability in flight, and longer legs make it easier for the male to grasp the female in mating.
The imago (the final adult stage) has shiny, hairless wings. The longer legs and tails allow for more rapid flight. The corrugation of the wings protects them by making them more flexible and therefore less vulnerable to wind damage. The imago mates and dies within a few hours to a day. (Harker, 1989) This short adult life is what gives the order its name from the Greek ephemeros meaning "lasting but a day."
Mayfly nymphs : At left above, is the nymph of a Baetid mayfly, and at right is a Heptageniid nymph. Notice the Baetid has a slender, cylindrical body and small gills on its abdomen. The small gills indicate that it lives in moving water, and the shape of its body makes it well-suited for swimming against the current. The Heptageniid has a broad flat body, better suited for life clinging to the bottom of the stream to avoid being carried away by the current. Notice also that both nymphs lack the wings of the adult stage, and both have the three caudal filaments (tails) characteristic of mayflies. (Click on either of the pictures above for a larger image).
Mayfly eggs are eaten by snails and by caddisfly larvae. The nymphs may be eaten by fish, frogs, birds, flies, or water beetles. The subimagos are eaten by fish, birds, dragonflies, water beetles, or other predatory insects. For their own nutrition, mayfly nymphs move over stones and weeds to graze off bacteria. They may collect from sediments or feed on detritus. Most mayflies are collectors and scrapers (Edmunds et al, 1976), and most of the consumed bacterial cells pass through their bodies without being used. They are opportunistic generalists, meaning that they eat what they can, when they can. Nymphs have some symbiotic relationships with chironomids that may be commensal. The chironomid larvae attach to the cuticle of the mayfly and eat the unicellular organisms that live there. More harmful to mayflies are nematode parasites, who use nymphs as their hosts by eating their muscles from the inside.
Ephemeroptera nymphs are usually microhabitat specialists. Each species survives best on a specific substrate at a certain depth under water with a certain amount of wave action. For example, Rithrogena generally live in medium to large trout streams. Ephemeridae burrow into soft areas where flow is slower, or in areas of lakes and rivers where deposits occur; the particular substrate and burrow depends on the genus. The primitive habitat of schistonate mayflies is still water even though most extant mayflies live in running water (McCafferty, 1990). In some areas, succession occurs by different species. For example, in Utah Epeorus longimanus is followed by E. deceptivus. Some species dominate in the spring while others dominate in autumn (Edmunds et al, 1976). Some mayfly nymphs are quite sensitive to pollution and are used to evaluate water pollution and stream health.
Mating occurs in a swarm, and at these times there may be such dense clouds of mayflies in the air near streams that driving becomes impossible. Because their wings are so fragile, the imagos need calm weather in order to mate. Males usually swarm very near the water, though swarm formations vary from species to species. Within the swarm, the insects are always changing positions. When a female enters the swarm, males try to mate with her. Some float to the ground while mating, and others continue flying. Once a male has successfully mated, he will guard the female to make sure that no other male mates with her. The female then flies to water to lay her eggs. She dips into the water while flying and releases a few eggs each time. The eggs sink to the bottom and their surface changes. Some become covered with a sticky substance and some have adhesive disks. Some species are parthenogenic, meaning that they do not need sperm to produce fertile eggs (Harker, 1989). The time it takes for emergence into the subimago form varies depending on temperature -- the milder the temperature, the earlier the emergence (Edmunds et al, 1976). Many species have synchronized emergence of subimagos. In these cases, the emergence occurs at a specific time of day under certain weather conditions (Harker, 1989).
Mayfly Fossil Record
The first recorded mayfly nymphs are from the Late Carboniferous (Fenton, 1989). In most places, mayflies are represented primarily by larval fossils (Sinitshenkova, 1990). Presumably, this is because of the very short lifespan of the adult stage. The siphlonurid form, which is shrimp-like with short caudal filaments that have lateral hair fringes, is the primitive form (Riek, 1970). However, it is difficult to classify early fossils in the correct order. The fossil Triplosoba pulchella is the only insect from the Carboniferous that is consistently placed in the order Ephemeroptera (McCafferty, 1990).
In recent years, certain fossils found in Moravia (eastern Czech Republic) and Oklahoma (central U.S.) previously placed in the order Archodonata have been re-classified as Ephemeroptera. The Oklahoma fossils are very well preserved with the wing venation clearly shown. Hubbard and Kukalova-Peck argue that the presence of three caudal filaments, which is a plesiomorphy, and a well developed costal brace, which is a uniquely derived character of Ephemeroptera, make it impossible to place these fossils in any order other than Ephemeroptera. It has been argued that the presence of segmented tarsi double tarsal claws, traits which are not found in modern mayfly nymphs, means that the fossils were not Ephemeroptera. However, this difference merely means that changes have resulted through evolution since the origin of the group. The simple tarsi and single tarsal claw must be apomorphies, because the double claws and segmented tarsi can still be found in the closely related orders Odonata (dragonflies and Damselflies) and Plecoptera (stoneflies) (Hubbard & Kukalova-Peck,1980).
Early mayfly adults differ from the Permian also differ from their living descendants. Fossil imagos of Protereisma from Kansas have functional mouthparts and fore and hind wings of similar size and shape (Carpenter, 1992). Modern mayflies do not feed as adults, and have smaller hindwings, or no hindwings at all in some species. The highest diversity of Ephemeroptera appears to have been during the Jurassic. Fossils of nine families have been found during this period.
Currently, the Ephemeroptera are classified in several different ways, depending on who does the cladistic analysis. According to Riek, this order has six superfamilies with 14 families. There are also two extinct superfamilies, Proterismatoidea and Mesephemeroidea, which are the Permian insects that appear to be mayflies or their precursors. During the lower Cretaceous, there was extinction and emigration of many mayflies in Brazil -- very few of these have survived to present day (McCafferty, 1990). The evolution from living in still-water to living in running water occurred before the Cenozoic Era (McCafferty, 1990).