Nature News: The Ancient Life Cycle of the Mayfly

published 05/10/2021 in The York Weekly, Portsmouth Herald and other Seacoast Media Group publications

Life aquatic-what lives under the mirror-like surface of a local stream?

I don’t know why I was worried, but I was.  While preparing to lead a stream exploration I was worried that it was too early in the season to find anything interesting in the stream.  Worried despite the evidence all around me-while kayaking last weekend my son and I had encountered flights of mayflies, the aquatic juveniles metamorphosing into flying adults all around us, ethereal sprites fluttering up from the surface of the water into the sun.  Despite the black flies that are already plaguing us in the garden. Despite the warm spring days.  So, I went down to the little river/big stream that meanders through the valley behind my house and checked to see who was out and about in the stream bed.  I found tiny water boatmen, diving beetles and tons of juvenile mayflies (aka nymphs) and so realized I could stop worrying about a lack of aquatic life.  

What lives under the glassy surface of a stream? sue pike photo

A large number of insects spend a large part of their life cycle in an aquatic environment-take mosquitoes, their aquatic juvenile stages love growing up in stagnant water.  Black flies, on the other hand, prefer to lay their eggs in faster moving streams which, because of all that movement, are better oxygenated.  Damsel and dragonflies, caddisflies, stoneflies and mayflies are also aquatic up until they molt into the adult stages of their life cycles. 

It’s a great time to look for mayflies, the water is getting warm enough to wade in and swish a net around in search of interesting critters.  And over the next few weeks (and long into the summer–the name mayfly is misleading, different species in this group emerge at different times), if you are lucky, you might encounter mayfly nymphs molting into adults and swarming over the surface of a local stream.  

Mayflies are among the most ancient orders of insects on Earth!

Mayflies are an ancient order of insect, the Ephemeroptera, a taxonomic order that was here about 100 million years before the dinosaurs.  The term “Ephemeroptera” comes from the word “ephemeral” meaning short-lived and “optera” meaning “winged” referring to the short lifespan (a couple hours to a couple days) of the winged adults whose only purpose is to mate. The aquatic juveniles, called nymphs (or naiads) pass through numerous instars (growth stages) becoming gradually more adultlike with each molt.  This is called incomplete metamorphosis.  Unlike the complete metamorphosis of something like a butterfly that has a pupal stage in which everything about the body gets drastically rearranged, the mayflies gradually change with each molt (in which they shed the exoskeleton they’ve outgrown) until they become adults.  Mayflies are the only insects to have 2 adult stages-the first winged damselfly to emerge from the water is sexually immature, this stage quickly molts into a fully reproductive adult-the final stage in its life cycle that mates and dies.  

Life aquatic: two mayflies and a diving beetle. sue pike photo

Mayfly anatomy follows the basic rules of all insects.

If you want to look for mayfly nymphs you need to know a little bit about their anatomy in order to identify them.  Mayflies have 3 body regions: a head with relatively big compound eyes, a thorax to which the 6 legs are attached and an elongated abdomen which has beautiful leaf-like (or feathery, depending upon the species) gills extending from the sides and 3 cerci (thread or antenna-like appendages) that extend from the tip. The gills can be different sizes depending upon whether the mayfly lives in still water or running water.  In still water the gills are larger than in running water because there is less oxygen in still versus turbulent water. The gills also help protect the mayflies from predators by sending water off and away from their bodies at different angles-making it hard for predators to track them. If you want to explore mayfly identification in more detail, check out the UNH Center for Freshwater Biology ( or the Stroud Center ( for really great identification keys.

The mayfly life cycle is a beautiful, ancient cycle happening in a stream near you.

The mayfly life cycle ends (or begins-depends upon how you look at it) with reproduction.  Their life cycles are timed so that entire populations of nymphs molt and leave the water in synchrony, with the flying adults forming huge swarms over the water. The males grab passing females with elongated front legs and they mate in flight.  The female then dips down to the surface of the water to lay her eggs, when done she often falls to the water’s surface to die, oftentimes feeding fish in the process. The males don’t stick around but rather go off to the nearby land to die.  It is a beautiful, ancient cycle, not to be missed, and could be happening at a stream near you.

You can find more of my posts about backyard nature on Instagram @pikeshikes

One Reply to “Nature News: The Ancient Life Cycle of the Mayfly”

  1. Beautifully done and hyper-interesting, as are all of your short pieces. Too bad I detest flies and bugs of all sort!

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