Shucks and spinners, imagos and duns, clingers and burrowers ... You would need a dictionary to decode the language of the mayfly and the many terms describing its life stages. These insects spend years as juveniles on the beds of streams, rivers and lakes – some pressed flat beneath rocks – before transforming into winged adults, swarming above the water with a single, urgent goal: to mate.
One evening last week, after attending the launch of Take Me to the River at the Solstice Arts Centre in Meath – a collaboration of artists, scientists and local communities working to restore the river Boyne and her many tributaries, now rapidly dying – I took to the banks of the river as she meanders through Navan and walked the banks. The evening sun bathed the fields and water with a golden light, revealing the insects darting through the air. They looked like specks of dust flitting back and forth, occasionally scattered apart by swallows which darted through them at speed.
As I walked along the mown path beside the river, the adult mayfly stood out, pulsing through the air. Known as “spinners” and technically named “imagos”, this final stage of the mayfly’s life is fleeting, sometimes lasting only a few hours. In times of abundance, adult mayflies emerge in synchronised hatches in May in such huge numbers that they rise in dense clouds above the water. Years ago, over the Corrib, I watched so many lift off at once it looked as though smoke was drifting across the water from an unseen fire. On my recent walk along the Boyne, only a few hundred were dancing in the air, their wings like transparent sheets of rice paper, carrying their slender bodies upwards.
Mayflies are easy to spot by the long, hair-thin tails trailing from the base of their abdomen, which appear to weigh the mayfly down, as if dragging it back into the water.
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For the likes of trout and salmon, mayflies are protein-rich power snacks, devoured at every stage of their life cycle. As underwater nymphs (think of it as mayflies’ childhood phase) this insect clings to stones or burrows into the riverbed, living unseen for years. In their teenage stage as sub-imagos, known by anglers as “duns”, they rise to the water’s surface, shed their outer skins, or “shucks”, to emerge as hairy-winged adults. Finally, as fully mature imagos, they take to the air.
Adult mayflies don’t have mouths and so cannot feed. So they must use all their stored energy to complete their final job: to reproduce. During this burst of aerial activity, males and females rise for their first and only nuptial flight, seeking one another to mate. The females lay eggs in the water before joining the males and falling lifeless on to the surface.
As developing eggs, mayflies are vulnerable to water pollution. Sediment can smother and block the flow of oxygen to the eggs, which is vital for survival. Excessive phosphate levels in the water will fatally disrupt their development due to excessive algal growth. Yet even under immense pressure from human activities, mayflies can adapt.
In 2023, fisheries scientist Dr Ken Whelan led a study of mayfly populations in the western lakes – Carra, Conn, Corrib and Mask. They discovered profound changes in the ecology of these lakes. Where once there were two distinct peaks of mayfly emergence (in May and August), mayflies now appear continuously throughout the summer.
The deeper parts of the lake beds of Corrib, Mask and Carra are becoming increasingly unsuitable for nymphs to survive. However, in Lough Sheelin in Cavan, where mayfly numbers have grown recently, Dr Whelan discovered nymphs burrowing in the empty shells of invasive zebra mussels on the lake bed – a striking example of adaptation.
“No matter what Nature or Man throws at them,” he writes in this month’s Trout & Salmon magazine, “warming water, storms affecting egg-laying adults, the appearance of invasive species, new predators – apparently, our beloved mayfly can adapt, survive, and thrive.”
The pressures are many. Pollution and record-high water temperatures in rivers are already taking a toll. Two weeks ago fisheries authorities closed multiple fisheries including the iconic Erriff and Moy fisheries after water temperatures exceeded 20 degrees Celsius twice within 24 hours - an alarming situation. For some mayfly species, the warmer waters lead to stunted growth and leave them more susceptible to early death.
Unlike many other European countries, Ireland still relies heavily on rivers and lakes as a source of drinking water – a practice that can worsen rising water temperatures. On top of this are other threats: invasive species such as non-native shrimps and snails that feast on juvenile mayflies; lethal pesticide use; and physical disturbances such as dredging, which can devastate the delicate habitats these insects depend on.
So, how are Ireland’s 33 mayfly species faring overall? By the end of this year, we will have a much clearer picture when Dr Jan-Robert Baars, an insect scientist at University College Dublin, publishes his nationwide assessment. With a bit of help from citizen scientists, from anglers to schoolchildren, Dr Baars is gathering data to track mayfly populations across the country.
There are small signs of hope: last summer, a pupil from Scoil Bhríde in Kilcullen discovered a mayfly species in the Liffey that hadn’t been recorded there in nearly a century.
