Species Profile - Mayfly


Ephemeroptera or Ephemerida The short-lived adults, found near water, have long tail appendages and large, transparent wings; the larvae are aquatic.
adult mayfly Adult Mayfly


As the sun begins to gently dip below the horizon, you notice an array of glittering wings frantically beating the air above the river. Cast against the golds, reds and pinks of the setting sun the cycle has come full circle. The return of the mayfly!




Mayflies can be identified both in the nymph (larval) or adult stages.

Larvae - Larvae are elongate and cylindrical in form. The head has well-developed eyes, chewing mouth parts and short antennae. Behind the head you will find the small, developing wings trailing behind a ‘wing pad’ . Along the abdomen there will be a series of gill structures that range in shape from paddles to feathers to ribbons. The abdomen ends in 2 or 3 tail filaments, 3 is most typical.

Adults – delicate insects with two or four triangular shaped wings with many veins. You will notice that the wings remain vertical when the insect is at rest. The eyes are well developed, but the mouth parts are not well formed. Adults do not feed in many cases. The tail filaments are transferred to the adult as well.

Mayfly Larva


Mayflies can be found in almost all freshwater systems with adequate oxygen levels. Some species larvae burrow into the sediment, while others lead a more free living life.

Mayfly larvae feed on algae, fungi and decaying plant materials.


Mayflies utilize what is termed an incomplete metamorphosis in their development from egg to adult and will occupy the benthic (bottom) zone until moments before emerging as a winged, terrestrial adult.

Mayflies spend about 99% of their lives as nymphs. The ‘hatch’ as it is referred to by fly anglers, is often synchronized so that many (sometime very many!!) individuals emerge at once. Others will emerge in smaller numbers over a long period of time.

Once emerged, the subimago (a transformational stage preceding the sexually mature adult) rests on nearby vegetation until becoming a mature adult a few days later.

Mating takes place in flight as female flies through a swarm of males. The female then returns to the water to lay eggs and dies.

Adult mayflies rarely live more than a month, most often much less.


Mayflies are extremely important in the aquatic food web. As nymphs they are fed upon by other invertebrates, amphibians, reptiles, fish, birds and mammals.

The adults are often taken by birds and bats while they are alive. Once the mass of swarming, mating adults dies there is a literal smorgasbord on the surface of many lakes and rivers when they often number in the millions.

Natural Connections

One way to demonstrate to students how environmental stressors affect the mayflies in their local stream or river is to use Lesson 3:6 - Macroinvertebrate Mayhem. This interactive game helps students to realize how stressors can change the balance and health of an aquatic ecosystem.

If you or your students are not familiar with the amazing diversity of organisms in your local water bodies, use Lesson 1:4 - Water Habitat Site Study to explore who is living in the water near you. Remember to download the macroinvertebrate identification keys from our website to help you figure out what you’ve found. 

Fun Facts - A History of the Mayfly

It may not be news to all of you, but it wasn’t that long ago that many of our river and streams in Minnesota no longer had mayfly ‘hatches’.

The mayfly, a corner stone of the aquatic food web, is a very sensitive creature. Sensitive to chemical pollutants, increases in suspended solids (erosion) and decreased dissolved oxygen (DO) levels.

The primary cause of the collapse of mayfly populations during the late 1960’s, 1970’s and early 1980’s was sewage. Little or no treatment of sewage was occurring during this time. We were literally flushing our toilets into the river in many parts of the state.

As the population of Minnesota increased, especially in the Twin Cities and Duluth, the amount of sewage naturally increased. There were reports from this period of ‘cakes of fecal matter’ floating on top of the rivers.

While this sounds bad enough, the problem for the mayflies wasn’t the poop itself, but what happened next. As bacteria began to break down this mass of organic sludge they used up tremendous amounts of dissolved oxygen (DO). In fact, they used up so much DO that mayflies, stoneflies, caddisflies and even fish disappeared from the Mississippi River from the Twin Cities all the way down to Lake Pepin. (To understand how bacteria can affect the DO of any natural system, refer to Lesson 2:8 - Fish in Winter)

Today because of modern sewage treatment facilities and regulations on the disposal of toxic chemicals mayflies have returned to most of our waterways.

New challenges for mayflies come in more subtle forms and revolve around chemicals that often come from our homes and yards. Medications and other pharmaceuticals that leave our bodies go right through the sewage plants untreated and are affecting aquatic life that eat and breathe these compounds. (Internet search: feminization of fish)

There are things that we can do to make sure another mayfly collapse doesn’t occur in the future. You students may not realize ‘storm sewers’ or ‘storm drains’ flow untreated directly to the river, stream or lake nearest to them. What we do on the land affects the water. Picking up pet waste, keeping leaves and grass clippings out of the street, using minimal or no lawn fertilizers, following best practices when spreading manure or applying agricultural chemicals, and many others all affect water quality and mayfly abundance

Maintaining healthy population of mayflies and the fish that feed on them depends on the choices that we all make every day. So next time you’re fishing on the shore at sunset, watching the mayfly clouds gather, think about how you can make sure that this cycle will continue each and every year.

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