Mayflies – the ‘Circle of Life’

Mayflies – the ‘Circle of Life’

The emergence of the mayfly is a spectacular but very short-lived sight to see. Each year from May through Augustadult mayflies emerge from lakes and rivers. In some parts of North America, swarms of emerging mayflies are so large (up to 80 billion) that scientists have been able to track their presence in the skies on weather radar systems that are usually used to track rain and snow (National Geographic, 2020) 

While we may not see swarms of 80 billion mayflies here in the Credit River Watershed, mayflies are very much an important presence.

Adult mayfly in Credit River Watershed. 

The transformation into the adult form is fascinatingThe mayfly experiences two separate winged adult forms during their lifecycle.

Credit: Art Station, Jana Růžičková      

1. First, a mating female called an imago, also more commonly known as a spinner, lays her eggs in the water. These eggs then settle to the bottom of the stream and stick to stones and plants 


 2. The mayfly egg then hatches into a nymph at the bottom of stream, river, lake or wetland where it spends most of its life. 


3. The first winged-stage, known as the dull-coloured sub-imago or more commonly known as a dun, lasts from 30 minutes to a few hours. During this stage, the nymphs make their way to the water surface and shed their skin. They rest momentarily to dry off their newly exposed wings. They may also look for shelter among streamside plants or trees. During this adult stage, mayflies play an important role in the ecosystem as they serve as food for a wide variety of animals, including fish and birds.

Adult Mayfly emerging from cast skin. Credit: Frost Fly 

1. If the sub-imago survives, they will enter the second winged-stage, known as the brightly coloured imago. This is when mating and reproduction takes place. Once the females have dispersed their eggs, they will fall to the water surface tired out where they become food for fish. The male will typically go to nearby land where they also return to the food chain.

Through our Integrated Watershed Monitoring Program (IWMP), we study the community of mayfly nymphs and other aquatic invertebrates that live on the bottom of the stream. Aquatic invertebrates serve as bio-indicators. That means their presence, numbers and species diversity can tell us a lot about the health of a waterbody and the surrounding environment. Abundant, healthy and diverse communities of aquatic invertebrates typically indicate healthy water and a thriving local environment.  

Typically, we spend about two months each year collecting samples from over 50 stream sites throughout the Credit River Watershed. 

CVC staff collecting aquatic invertebrates at a West Credit River Tributary.

Much of the fall and winter are spent in the lab putting all the pieces together. We work on correctly identifying the organisms using a microscope.

A view through the microscope at CVC. Left: Caddisfly larva. Right: Mayfly nymph.  

This year, we’ve had to postpone our IWMP field work due to COVID-19 in order to ensure the safety of our staff and the communitySo, while we may not be outside right now our work continues. We have a tremendous amount of data collected from 2019’s field season that we have been working to analyze and summarize so that we can accurately report on conditions of the Credit River Watershed’s ecosystems. We also continue to learn more about aquatic invertebrates and sharpen our identification skills.  

If you are interested in learning how to identify aquatic invertebrates, a great resource we have been using is The Atlas of Common Freshwater Macroinvertebrates of Eastern North America. Here you can learn about the different families of aquatic invertebrates that you may discover across our watershed, including the many different types of mayflies. 

Happy learning! 


By CVC’s Chantalle Jacob-Okorn, Technician, Watershed Monitoring

Feature photo by Michigan Radio, 2019.

4 Comment
  • Bob Marshall says:

    I was wondering how the population of green drake mayflies is doing in both the main Credit and the west Credit above the Forks? I know that there has been a severe reduction in the main Credit, but understood that there was still a viable population in the West Credit between the Forks and Belfountain.

  • ed mckelvey says:

    Very interesting. A good time to learn in this environment.

  • Kelly Mulholland says:

    Nice article. A little sad but very beautiful too. I wont ever look at Mayflys the same way.

  • John Watts says:

    I am interested and I know that fishermen use flies that imitate May flies. When I was young in England I would take frog spawn home and watch the evolution of frogs. It was also possible to catch newts and they would produce eggs. My father in law would take frog spawn into ponds where the frogs had been eliminated
    to maintain their existence.

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