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 several mayflies (shadflies) clinging to leaves of shrubs and trees. The carcass from a fresh molt often lay attached nearby. I also noticed several mayflies coupled together, as if mating.
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 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 then saw them all. Dozens. Hundreds. All performing a drunken tango between male and female partners as dusk fell into night.
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.
The mayfly’s life cycle is as fascinating as it is apparently fleeting. It reads a little like a fabled princess tragedy, complete with dramatic changes in appearance from childhood, two versions of adulthood, a mating dance in mid-flight, followed by sudden death shortly after. However, as with salmon who have long juvenile lives then die once they spawn, the mayfly’s single role as an adult is to reproduce; and it does this with style and aplomb, not to mention doing it all in the air.
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 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:
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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.