top of page

Don't Shoo This Fly; Mayflies Are Important, Beneficial

This is the time of year when, if you are observant, you may see adult giant mayflies calmly resting on a leaf, stem or even a trail. When we see them, they are near the end of their lives. These lovely insects exist in their adult form for mere days.

A giant mayfly. (Photo by Angela Rafac)

Giant mayflies are the most common mayfly in North America. There is fossil evidence showing that mayflies have existed for 300 million years! Adults are yellowish in color with brown highlights. Their bodies are about 3/4 inch long to 1 1/4 inches long, with females being on the larger side. They have two short antenna, two very long tails and a distinct vein pattern in their wings.

Stunning swarms

Their purpose as adults is to mate. This guarantees that their population continues. Females lay fertilized eggs above the surface of a river, lake or pond. In some locations, giant mayflies and their close relatives are known to swarm.

Thousands of adults emerge at the same time and stay together in a large group. Staying together is a great way to ensure finding a mate. Depending on your perspective, this is either an amazing spectacle and photo opportunity or a temporary annoyance.

Unusual and unknown

All species of insects go through different stages in their life cycle. It can look very different for different species. What almost every single species has in common is that once the wings are developed, that is the final stage. (That is if they have wings.) Mayflies are the only exception to this rule. They have two winged phases, the subimago and the imago.


Words to know

Algae: A nonflowering plant that usually lives in the water.

Annoyance: A nuisance, or something that annoys someone.

Burrow: A hole or tunnel dug by a small animal.

Molt: To shed old skin, shells, hair or feathers.

Silt: Fine sand, clay or other material carried by water and deposited as sediment.

Spectacle: A visually striking display.

Swarm: A large of dense group of insects.


Scientists don’t understand it completely and debate about what it means and why it happens. Fishermen don’t need to understand the details; they know this first winged phase, the subimago, is great for bait.

Bottom dwellers

Before they are flying around the skies, mayflies are living in the silt and sand in the bottoms of rivers, ponds and lakes. They have three stages in their life cycle: egg, nymph and adult. The adult female releases a few eggs at a time into or over the surface of the water. The eggs sink to the bottom. When they hatch, the nymphs are only 1 millimeter in length and do not have gills.

A mayfly larva. (Photo via Shutterstock)

As these tiny nymphs grow, they molt, or shed their skin. They do this again and again and again, up to 30 times! They eventually have gills, a three-pronged tail and a head very similar to the adults'. They may live from four months to two years in the nymph stage. They make little burrows in the silt for protection and head out to feed on algae and plant material.

When they are ready to molt into their first winged form, they crack their outer skin and fill it with oxygen so they float up to the surface of the water. Then they wiggle out ready for flight. Many fish and birds like to eat them.

Informative indicators

In their nymph stage, giant mayflies are incredibly beneficial to humans. The giant mayfly nymph is known as an indicator species. Indicator means that these species can tell us something. Specifically, they tell us about the quality of the water or aquatic ecosystem. They are very sensitive to their environment and cannot survive in a lot of pollution. Finding them indicates or tells us that the water they are living in is high quality.

You can learn about many amazing plants, animals, insects and other species all around us by visiting the Diversity of Life Interpretive Trail Exhibit at Isle a la Cache Museum in Romeoville. The exhibit will be on display through Sunday, Oct. 2.


Follow Willy's Wilderness on Facebook for more kid-friendly nature stories and activities.


Commenting has been turned off.
bottom of page