Ever the river has risen and brought us the flood, the mayfly floating on the water. On the face of the sun its countenance gazes, then all of a sudden nothing is there.The Epic of Gilgamesh
On a recent walk in the forest by the Otonabee River one afternoon, I noticed many adult mayflies (shadflies) clinging to leaves of the riparian shrubs and trees. The shed skin from the mayfly’s dun stage often lay attached nearby. I also noticed several still emerging from their dun stage to their final spinner stage.
When I examined the shoreline of the river, I saw thousands of floating nymphal skins. These were left behind as the dun stage of the mayfly emerged, escaping their water home of two years to land on the shore vegetation for their next stage.
I returned later that evening, during a fine breeze at dusk, to walk the same forest by the river. This time mayflies were everywhere! They covered plants, tree trunks and rocks, lined up on large basswood leaves. They were in the air. Landing on my camera. I realized that I was witnessing the start of the mayfly dance: when they emerge, swarm and mate then lay their eggs and die.
I only had to look up. There—braving the wind and shivering in place or diving low then rising again—was a single mayfly. Then, seeing one, I saw them all. Dozens. Hundreds. Then so many more. All performing a drunken tango between male and female partners as dusk fell into night.
I fetched good friend Merridy to watch the dance and in that short space of time most had taken flight and were truly swarming.
About the Mayfly
Mayflies are small elegant creatures with slender bodies, bulbous eyes and tails as long or longer than their bodies. The adults range from a few milimeters to two centimeters. Mayflies belong to one of the oldest orders of insects, the Ephemeroptera, with an evolutionary history dating back some three hundred million years to the late Carboniferous Period. There are over two thousand species worldwide. Some mayflies identified in the Otonabee watershed belong to the flat-headed Heptageniidae and include species of the genus Stenacronand Stenonema (in Thompson Creek, a tributary of the Otonabee River).
Adult (winged) mayflies have large compound eyes, short, bristle-like antennae, and functionless mouthparts and digestive tracts. Once mayflies enter the winged stages they don’t feed—they don’t need to because they don’t live long enough in this stage. Their membranous wings include a large, triangular front pair and a much smaller, rounded hind pair. The adult mayfly has two or three threadlike tails, usually as long as, or longer than, the body. The ones I observed had three tails that were twice the length of the mayfly’s body. They likely belonged to the Heptageniidae, given the rocky shores of the river.
The life cycle of mayflies occurs in four stages: egg, nymph, subimago (dun), and imago (spinner), altogether spanning up to several years and mostly occurring in the aquatic environment, with egg and nymph stages lasting usually a year and adult stage from hours to a few days. Eggs, which vary widely in size and surface detail, may be oblong, oval, or rounded. Depending on the species, a female may produce fewer than 50 or more than 10,000 eggs.
Mayfly Life Cycle
The aquatic nymph stage of the mayfly varies from a few weeks to several years, depending on the species. The nymph is a voracious feeder, undergoing as many as fifty stages (instars) as it grows from a tiny egg in river or lake silt and detritus and feeds on anything from aquatic plants, plant detritus, algae and diatoms to preying on other creatures. When its growth is complete, the nymphal skin splits down the back and the nymph emerges from the water as a dull-coloured winged sub-imago or dun (with hairy wings to prevent adhesion to the water surface) that seeks shelter in bankside vegetation and trees. This happens usually in May and often en masse. After a period of several hours, the sub-imago (dun) once again sheds its skin, and transforms into the brightly coloured imago (or spinner). It is not clear why mayflies have retained this unique step in their lifecycle; some think that they may not be able to achieve the change from nymph to sexually mature adult in one step. Others suggest that the dun stage is particularly evolved for one function: to escape the water.
Mating starts soon after the final molt. As dusk approaches, males swarm above the water, flying into the breeze and performing a mating flight or dance. Females join the swarm, rising and falling as the dance continues. The male grabs a passing female with its elongated front legs and the pair mate in flight. After copulation, the male releases the female, who then descends to the surface of the water where she lays her eggs. She lays them by dipping them into the water while flying, releasing a few eggs with each dip. The mayfly eggs fall to the bottom of the water where they stick to plants and stones.Spent from mating and egg laying, the females remain on the water surface, wings flat on the surface, where fish pick them off at their leisure. The male mayfly rarely returns to the water; instead he remains on the land nearby, clinging to rocks and vegetation to quietly die.
Of course, the cycle doesn’t end with the fish; the next morning I spotted a great blue heron, catching fish. The small fish were feeding on spent mayflies on the water surface, themselves easy pickings for the heron. In the space of ten minutes, I watched the heron gulp down two fingerlings.
What I witnessed is not unique. Several species of mayfly exhibit great synchronicity in their hatching. The North American species Hexagenia limbata hatches in huge numbers from the Mississippi every year, sometimes in the trillions. Blogger Craig Macadam on Freshwaterblog.net writes that the numbers were “estimated at 18 trillion—more than 3,000 times the number of people on earth.” Craig adds that, “Ironically, what is seen as a nuisance in America is seen as a gift in Africa. Locals around Lake Victoria gather adults of the mayfly Povilla adusta together with Chironomid midges to make a type of patty called ‘Kungu’. This protein rich food stuff is an important part of their diet.”
Mayflies as Indicators of Clean Water
In my work as a limnologist and aquatic ecologist, I studied benthos in streams as indicators of water quality. Mayflies belong to the order Ephemeroptera and are joined by two other orders, Plecoptera (stoneflies) and Tricoptera (caddisflies), all good indicators of oxygenated, cool moving water. Mayflies are a known sensitive invertebrate to habitat disturbance or degradation.
Since 1909, when Kolkwitz and Marssondemonstrated that benthic invertebrates demonstrated specific tolerances to organic enrichment and other sources of pollution, scientists have used these communities to study various impacts to stream health, including chemical pollution, flow disruption and habitat destruction. Stenonema and Stenacron, mayflies found in the Otonabee watershed, are known for being sensitive to aquatic pollutants and often used as water quality indicators. Many Ephemeroptera (mayflies), as well as Plecoptera (stoneflies) and Tricoptera (caddisflies) tend to disappear in areas of poor water quality, organic enrichment, low oxygen, and high metal levels. Because of this the EPT Richness Index was developed, based on these three groups being generally pollution-intolerant. Given that mayflies are generally intolerant of oxygen-depriving pollution and toxins, this is good news for the Otonabee River. However, the river is still overly rich in organics (I have seen several algal blooms along the river’s shores and bays).
The Otonabee Region Conservation Authority, which releases an annual watershed report card on the river, has given the Otonabee River a B (good) to C (fair) grade for water quality. “Urbanization appears to have impacted surface water quality as the most developed subwatersheds received a C grade,” the report said. Impacts to river health include road stormwater runoff (chloride, heavy metals, PCBs, PAHs), agricultural and yard/garden runoff (organic enrichment), overuse of water and use of pesticides, herbicides, and fertilizers.
The conservation authority suggested several things residents can do to protect and enhance the health of the watershed environment. They include using phosphate free products, planting trees, conserving water with low flow taps or showerheads, and getting involved in community cleanups.
Here is a short video of the life story of a mayfly in the UK:
In response to my post, Twitter friend Designer / Artist Luminita Serbanescu created a beautiful painting and said: “My painting called ‘Wings for a Day’ was dedicated to the Mayfly. It was an ode to the joy of that one day of flying, freedom and joy after 2 years of waiting.” Below is Luminita’s painting:
Luminita Serbanescu: “With a degree in architecture and relocation to the free world many years ago, I found myself absorbed by a passion for art and environment. Our beautiful blue planet is under attack! Human race was born in the middle of lush forests, powerful rivers and vast oceans and under clean blue skies. Now we are endangering it! I feel that art can and must fight to awaken our long dormant conscience and save the planet! My art shows that desire!”
Brittain JE. 1982. “The biology of mayflies.” Annu Rev Entomol.27:119–47.
Brown, Brandeis L., Neil H. Ringler, Kimberly L. Schulz. 2015. “Sediment and water quality limit mayfly survivorship in an urban lake undergoing remediation.”Lake and Reservoir Management Vol. 31 (2): 145-156.
Brusca, R. C, and Brusca, G. J., 2003. “Invertebrates” 2nd ed. Sinauer Associates, Inc., Massachusetts. Page 595-596.
Canada’s Aquatic Environments (2002). Ephemeroptera. Available here.
Kolkwitz, R. and M. Marsson. 1909. “Okolgie der tierischen Saprobien.” Int. Rev. Hydrobiol.2: 126–152.
Metcalf-Smith, J.L. 1991. “Biological Water Quality Assessment of Rivers Based on Macroinvertebrate Communities.” Section 3.3: NWRI Contribution No. 91–71 in “Rivers Handbook.” Vol. 2, Chapter 3 (“Monitoring Programmes”). Nat. Water Res. Inst., Burlington, ON.
Munteanu, Nina. 2016. “Water Is… The Meaning of Water” Pixl Press, Vancouver. 586pp.
Richmond, Sonya & David C. Lasenby. 2006. “The behavioural response of mayfly nymphs (Stenonemasp.) to chemical cues from cryfish (Orconectes rusticus).”Hydrobiologia560: 335-343.
Rowe, Locke and Michael Berrill. 1989. “The Life Cycles of Five Closely Related Mayfly Species (Ephemeroptera: Heptageniidae) Coexisting in a Small Southern Ontario Stream Pool.” Aquatic Insects, Vol. II (1989), No. 2, pp. 73-80.
Slàdecek, V. 1973. “System of Water Quality from the Biological Point of View.” Arch. Hydrobiol. Beih. 7: 1–218.
Winner, R.W., B.W. Boesel, and M.P. Farrell. 1980. “Insect Community Structure as an Index of Heavy Metal Pollution in Lotic Ecosystems.” Can. J. Fish. Aquat. Sci. 37: 647–655.
This article was updated with information, photos and images from May 28, 2022.
Nina Munteanu is a Canadian ecologist / limnologist and novelist. She is co-editor of Europa SF and currently teaches writing courses at George Brown College and the University of Toronto. Visit www.ninamunteanu.ca for the latest on her books. Nina’s bilingual “La natura dell’acqua / The Way of Water” was published by Mincione Edizioni in Rome. Her non-fiction book “Water Is…” by Pixl Press(Vancouver) was selected by Margaret Atwood in the New York Times‘Year in Reading’ and was chosen as the 2017 Summer Read by Water Canada. Her novel “A Diary in the Age of Water” was released by Inanna Publications (Toronto) in June 2020.
3 thoughts on “The Mayfly Dance”
Like butterflies, mayflies demonstrate metamorphosis. Instead of caterpillars, they have long-lived larva that have evolved to cling to rocks underwater in a river or stream. Unlike butterflies, the adult lives for only a few days.
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That is true of adult males, but not for adult females, who live for hours at best. But in that fleeting moment of adulthood, what an incredible feat they perform: an exquisite dance of life to create more life, their beauty ever more intense in their brief existential purpose to breed. Once their eggs are fertilized by the male, the female returns to the water and, still in flight, stitches her eggs into the water and dies of exhaustion, food for fish. Life goes on…