What does it look like?
Mayflies are dainty insects with lacy wings and three long hair-like “tails” which trail behind in flight. They are often called dayflies because of their short lives. The length of adult Mayflies varies from ¼ inch to a little over 1 inch. Their soft bodies may be yellow, green, or brown. Some are black and white. The long, slender Brown Stream Mayfly is golden-brown in color. The Golden Mayfly has a yellow body as well as yellow antennae, legs, and tails. It has brown markings on its head and abdomen.
Like all insects, they have 6 legs, 3 body parts, and 2 antennae. They have two short, bristly antennae and their front legs are longer than the back ones. Mayflies are not true flies because they have four wings instead of two. The front wings are held upright when at rest. The hind wings are much smaller than the fore wings. A few of the smaller species lack hind wings altogether.
Their transparent wings and wispy tails give Mayflies an uncommonly delicate and graceful appearance. The wings have various markings depending on the species. The Brown Drake Mayfly Ephemera simulans has spotted wings. The wings of the Golden Mayfly Hexagenia are clear and lack markings. Young Mayflies have wings that are cloudy in appearance.
Immature Mayflies are called naiads. Naiads have two forms—flattened and cylindrical. They have long legs and plate-like gills on the sides of the abdomen. They usually have three long thin tail projections and short antennae. Their body color may be green or brown depending on the food they eat.
Names of Mayfly families often refer to the structure of the larvae. For example, Flat-headed Mayflies have flattened larvae. Their shape is an adaptation to living under rocks or logs in flowing streams. Minnow Mayflies have slender minnow-like larvae.
Where does it live?
Mayflies are found in freshwater areas throughout the world. Central Hungary is home to one of the largest Mayfly populations. Experts believe there are about three thousand different Mayfly species. About 600 species are found in North America.
There are about twelve species of common burrowing Mayflies in North America. The larvae are inconspicuous because they burrow in sediment at the bottom of lakes and streams.
Mayfly naiads can be found at almost any time of the year in clear running water. The flattened forms are attached to rocks or other substrates. The cylindrical forms swim well and prefer flowing or highly oxygenated water.
Mayfly larvae are sensitive to changes in water conditions. Recently, pollution in rivers and streams has caused some species to be endangered or become extinct.
What does it eat?
None of the Mayfly species feed during their short adult lives because they have only vestigial mouthparts.
The aquatic immature Mayflies feed by scavenging small pieces of plants or algae that live on rocks under the water.
How does it defend itself?
Although Mayflies do not seem to have any defenses, they have survived mostly unchanged for 350 million years. Scientists believe they were around even before the dinosaurs.
What stages of metamorphosis does it go through?
Mayflies have incomplete metamorphosis. Eggs are placed on the surface or underwater by adult females. Immature stages molt (shed exoskeletons) and develop through several stages (instars) before leaving the water. The number of molts depends on the species and is affected by temperature and water conditions. A typical life cycle lasts one year.
When a naiad hatches from an egg, it may live underwater for several years before surfacing to molt into a subimago. This is the in-between stage after the naiad and before the adult. The subimago usually molts into a full-grown adult in a few hours. It has wings like the adult, but is unable to mate.
Upon reaching the surface of a river or lake, the male subimago pulls itself free from its larval skin which is left floating on the surface. A pair of long filaments trails from the back tip of the abdomen. In this first winged stage the subimago flutters toward the river bank landing in tree branches where it molts again. The Mayfly is the only insect that molts after reaching the winged stage. The fully adult stage is called the imago. Adults live for a day or two, just long enough to mate and lay eggs.
The males swarm in the air to find females and to mate. They have bifocal eyes to help them in their search. The upper part of each eye is enlarged for spotting females in the air above. The males use their long front legs to grab and hold a female. After mating, the female flies away and the male searches for another mate. The males continue to compete in finding females, swooping and fluttering over the water.
Within a few hours, the females deposit several hundred eggs while flying low over the water, or by dipping the abdomen on the water surface. Some submerge themselves to lay eggs underwater. Females often die on the water surface as soon as their eggs are laid.
What special behavior does it exhibit?
In a few species, the males assemble in aerial columns several meters high into which the females fly to be mated. Within the hour, successful or not, they are all dead, and their bodies accumulate in piles on the water’s surface.
Some Mayfly species do not even lay their eggs. The whole abdomen with the eggs that it contains simply breaks off and falls into the water.
Mayflies belong to the order Ephemerida. Their scientific name refers to the very short (ephemeral) lives of adults.
How does this bug affect people?
Immature Mayflies are an important food source for fish. Many lures and artificial fishing flies are patterned after them. Anglers call the subimagos “duns” and the imagos “spinners.”
In general, Mayflies are unnoticed and not considered pests. They do not cause harm to people by transmitting disease or biting. Some problems occur when Mayflies swarm around electric lights at night. Dead Mayflies sometimes cause a foul smell in rivers or lakes.
Emergences of millions of adult Mayflies from the Great Lakes formerly left huge drifts of dead insects on sidewalks in Toledo, Chicago, and Green Bay. These huge hatches disappeared in the 1960s because the lakes had become polluted. Recently Mayfly swarms have reappeared around Lake Erie—a welcome sign that the lake has been cleaned up to healthy levels again.
AgriLIFE Extension Texas A & M System. 
Attenborough, D. (2005). Life in the undergrowth. Princeton, NJ: Princeton University Press.
Eaton, E. K. (2007). Field guide to insects of North America. New York, NY: Houghton-Mifflin.
Leahy, C. (1987). Peterson first guide to insects of North America. Boston, MA: Houghton Mifflin Company.
University of Kentucky Entomology. 
Waldbauer, G. (1996). Insects through the seasons. Cambridge, MA: Harvard University Press.
Wilsdon, C. (1998). National Audubon Society first field guide insects. New York, NY: Scholastic Inc.