Insects in your yard: mayflies

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Mayflies are found all around the world, except the Arctic and Antarctic. There are about 630 species in North America and more than 2,000 species in the world. If you live near water, especially a stream, they may see them in your yard, too.

Mayflies have existed since the Carboniferous Period, 354 to 298 million years ago, which places them among the oldest winged insects ever. Their scientific order, Ephemeroptera, is from the Greek ephemera for “lasting a day.” Mayflies spend almost all of their lives in water, in an immature stage. They leave water as adults only to mate and deposit eggs. Within a day or two they die.

Mayflies, dragonflies and damselflies are grouped together in a scientific Division named Palaeoptera. These insects differ from all other insects in that they’re unable to fold their wings down flat over their abdomen.

Read all about dragonflies and damselflies

Mayflies are among the most important bottom-dwelling animals. First and foremost, as nymphs, they’re food for fish and other aquatic animals. On land, they’re a food source for birds and other animals. They’re important to humans, too, because mayflies are freshwater species that are sensitive to pollution and their presence and diversity are indicators of water health. 

Physical Characteristics
Adult mayflies are easy to recognize. Like other insects they have six legs, but their forelegs are distinctively long (extremely so with males) and slender. The abdomen is long and slender, too, and usually held noticeably curved upward, while the wings are held together vertically. The wings are large, triangle-shaped and transparent, with many veins and sometimes markings or mottling. The hind wings are tiny and rounded, or sometimes absent, depending on the species.

Mayflies are not harmful. (Jason Means / Flickr; cc by-nc-nd 2.0)

Mayflies are harmless. (Jason Means / Flickr; cc by-nc-nd 2.0)

Mayflies are often seen with their forelegs extended forward and sometimes also angled upward. At the tip of the abdomen are, depending on the species, two or three threadlike tails, called caudal filaments. The tails are fragile and easily broken off, so sometimes a two-tail mayfly is really a three-tail that has met with an accident.

Adults range in size from 0.1 inch to 1.25 inches (2.5 to 32 mm) long, excluding the tails. The head is triangular and the antennae usually are short and thin. The eyes are large and compound. In females they’re separated. The male’s eyes meet and are particularly interesting because they’re split into a large upper lobe and a smaller lower lobe. Mayflies also have three very tiny simple eyes, called ocelli (oh-CELL-ee), which are used for detecting light.

The body is soft and quite delicate. Some are nearly transparent, with others being yellowish, orange, green or dark. Wings may be clear or tinted and some have spots, mottling or dark veining. Some mayflies have markings or dark bands on their body.

Nymphs
Mayflies spend most of their life in a series of immature stages in water. Called nymphs or naiads, their body is elongated and either flattened or cylindrical. Coloration is usually a drab green or brown. They have a distinct head with compound eyes, chewing mouthparts and antennae. They molt several times as their body grows larger. Their wing pads can be seen on top of their thorax. Three pair of legs extend from the thorax and the abdomen is usually tipped with tails.

Mayfly nymph (Rhithrogena sp.). Notice the feathery gills along each side of the abdomen.  (Dave Huth / Flickr; cc by 2.0)

Mayfly nymph (Stenacron interpunctatum / proximum). Notice that this species has feathery gills. (Dave Huth / Flickr; cc by 2.0)

Nymphs breathe through a set of several gills located along each side of their abdomen. The gills are highly variable, being leaf-shaped, feathery, plate-like, sometimes even forked. They swim by wiggling their bodies and, with some, feathery tails help move them along. Some species live in silt and mud on the bottom and others cling to rocks, plants or logs. Most feed on microscopic plant matter, but a few species are predators. Mayflies do all their eating as nymphs. As adults, they lack functioning mouthparts.

Mayflies are the only insects that have fully developed wings prior to reaching the adult, or imago (em-AH-go), stage. After many, many molts the nymph becomes nearly ready for flight. Now a pre-adult called a subimago, it swims to the surface and crawls out of the water. Breathing now through tiny openings in its exoskeleton, it finds a suitable spot and waits for its wings to dry. The wings are opaque and dull because there’s still one last layer to shed. It can fly, but weakly. Within a day it will molt for the final time, revealing transparent wings that glisten in the sun. Soon the adult will lift itself into the air and begin its search for a mate.

Although called “May” flies, one species or another can be seen throughout the summer and fall. Most prefer a habitat with flowing water, such as a stream or spillway, but some species live in rivers and ponds, or even still water. Look for adults and subimagoes on vegetation along water’s edge. In the water, look for nymphs in the places they feed.

If one comes into your house, handle it carefully. You can slowly coax one onto your finger or a piece of paper and set it down outdoors. Mayflies are harmless, they don’t bite and they don’t have stingers.

Life cycle
Mayflies go through incomplete metamorphosis: They progress from an egg to a nymph to an adult.

Mayfly courtship usually begins with huge swarms of adult males dancing in the air above water. Females fly into the swarm, where males grab them at the base of their forewings (the reason for the males’ extra-long forelegs). Each twosome leaves the party, with many mating in flight while others alight (some do both: they start out in mid-air, but end up falling to the ground or water still coupled.) In some areas, parts of Lake Erie, for example, swarming includes so many individuals some years that they become a nuisance and a hazard by flying into people, making roads and sidewalks slippery and splattering on windshields.

Females usually lay their eggs in low-light conditions, like at dusk or on an overcast day, on the surface of water either one-by-one, in small batches, or all at once. Some species go into the water to lay their eggs. A few species lay unfertilized eggs that hatch into more females, a process known as parthenogenesis. Eggs sink to the bottom and hatch there. Some nymphs will feed at that level and others will cling to the underside of rocks or on plants.

Subimago molting. (Andy Reago and Chrissy McClarren / Flickr; cc by 2.0)

Subimago molting. (Andy Reago and Chrissy McClarren / Flickr; cc by 2.0)

As with all other insects, a nymph’s “skin” is actually its skeleton (called an exoskeleton), worn on the outside of its body instead of within. As it eats and grows increasingly larger, the nymph must shed (molt) the inflexible exoskeleton each time it begins to outgrow it. Molting occurs 20 to 30 or more times over a period of three weeks up to three years, depending on the species, with one year being typical.

Most mayfly species have one or two generations a year. Adult mayflies live only one or two days.

Predators
Fish are the main predators of mayfly nymphs. Other aquatic life, such as dragonfly nymphs, leeches and crayfish, also feed on mayflies. Adult mayflies are prey for carnivorous insects, spiders, birds and some mammals.

Top photo: Mick Talbot / Flickr; cc by 2.0

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