Mayflies

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Mayflies

Mayflies, or shadflies, are aquatic insects in the order Ephemeroptera. There are several thousand species of mayflies, distributed among 20 families. Mayflies have a relatively protracted nymphal stage, which occurs in freshwater habitats. The adults are a short-lived stage during which dense aggregations of animals engage in a frenzied procreation, needed to perennate the species in its local habitats.

Biology of mayflies

Mayflies have a simple metamorphosis , with three life-history stages: egg, nymph (or naiad), and adult. The nymphal stages are numerous and relatively protracted, generally lasting for at least one year. Mayfly nymphs occur in aquatic habitats, and account for most of the lifespan of mayflies. Some species have as many as 28 nymphal molts. The last, pre-adult stage occurs when a nymph rises to the water surface, molts, and develops into a winged form called a subimago, which flies a short distance, and usually rests on vegetation. Within a day or so, the subimago molts into the terrestrial, sexually mature adult stage, which is generally found in close proximity to the aquatic nymphal habitat . Mayflies are the only insects that have an additional molt after they have developed functional wings.

Mayflies have soft, elongate bodies, with two or (most commonly) three, distinctive, thread-like appendages projecting from the end of their abdomen. Adult mayflies have short antennae and many-veined, roughly triangular, membranous wings. The wings are held erect and together over the body when the animal is at rest, and cannot be folded up as in most other orders of insects. The aquatic mayfly nymphs have distinctive, leaf-shaped appendages on the sides of their abdomen that serve as gills for the exchange of respiratory gases.

Larval mayflies are aquatic in fresh waters, and have mouthparts adapted for feeding on algae and other relatively soft, organic materials. Most mayflies are herbivores or detritivores, but a few species are carnivores of other aquatic invertebrates . Because they do not feed, adult mayflies have only vestigial mouthparts. Mayflies have large, compound eyes, and in many species the males are larger than the females, and have forelegs adapted for grasping the female during the nuptial (mating) flight. In most species the forewings are relatively large, while the hindwings may be absent, or are reduced in size compared with the forewings.

Adult mayflies do not feed and are short-lived, only living for a few hours or several days. This fact is reflected in the Latin root of the scientific name of this group of insects, Ephemeroptera, which refers to the highly ephemeral nature of the adult stage. The sole purpose of adult mayflies is procreation. To achieve this goal, adults of particular species emerge synchronously within a brief period of time . Adult mayflies sometimes occur in spectacularly large aggregations, in which the animals mate and deposit their eggs to water in frenzied swarms, and then die soon afterwards. Most of the mayflies in the swarm are males. The females fly briefly into the swarm, find a mate, and they couple then leave to copulate and lay their eggs.

Ecological and economic importance of mayflies

Sometimes, the mass emergences of adult mayflies can involve extremely large numbers of animals. During the brief times when mayflies are very abundant, some people may view them as a nuisance, because the insects seem to be flying everywhere and can be bothersome because of this, or because their bodies accumulate abundantly on beaches, streets, window screens, and in other places. Rarely, the accumulated biomass of mayfly bodies can represent a traffic hazard, by making roads rather slippery. However, this is quite a rare event, and in general mayflies should not be thought of as a nuisance. Mayflies are rarely abundant enough to be a bother. Almost always, mayflies are a harmless part of the natural world.

Mayfly nymphs are an important component of many freshwater ecosystems. Grazing by mayflies is important in preventing the build-up of a large biomass of aquatic algae and detritus, and in nutrient cycling. Because mayflies can be quite abundant in many habitats, they are an important food for many species of predators.

Mayfly species are rather particular in their choice of habitat, and in their tolerance of environmental conditions, such as the temperature and chemistry of water. Because of their specific habitat requirements, mayfly species are often studied by aquatic ecologists as indicators of water quality, for example, in studies of pollution . A famous study involving mayflies was conducted in Lake Erie during the 1950s and early 1960s. Lake Erie was badly polluted at that time, especially by organic debris associated with sewage and algal growths, the decomposition of which consumed most of the oxygen in the waters of deeper parts of the lake. The development of anoxic conditions resulted in mass die-offs of nymphs of the mayflies Hexagenia rigida and H. limbata, which were previously extremely abundant. The virtual collapse of the populations of mayflies in Lake Erie was widely reported in the popular press, which interpreted the phenomenon as an indication that the great lake was "dead," and had been rendered as such by pollution caused by humans. Today, the waters of Lake Erie are much cleaner, and its populations of mayflies have recovered somewhat.

Both nymphal and adult mayflies are a very important food for economically important sportfish, such as trout and salmon . Many sport fishers are highly skilled at tying mayfly "flies" as lures for use in fishing. The more realistically the lure portrays the species of mayfly that the trout or salmon are interested in feeding upon in a particular stream at a particular time, the greater is the fisher's success in catching fish .