Between two worlds
Hatched in rivers, mayflies rise to the surface and unfurl new wings, the final phase of their precarious and astonishing lifecycle.
At dusk, on the upper Waiau River under the swingbridge entrance to the Kepler Track, the mayflies were hatching. Each insect rose from a dimple on the water’s surface, then took a hyperbolic flight path, vanishing into the glowing Southland sky.
The mass emergence of mayflies attracts trout, which I had come here to fly-fish, but such was the explosive beauty of the spectacle before me, I put my rod away and just watched.
Mayflies spend almost all of their lives underwater among rocks on a streambed usually a year, sometimes two in the case of the largest species. Then, when conditions are right, they ascend to the surface to hatch. There, they struggle through the viscous membrane that separates the two worlds and climb out of their nymphal shucks—think of a whitewater kayaker, adrift in a current, pulling herself out of a tight cockpit. Then they fly off, keeping their bodies vertical in flight, tails trailing like long legs, giving an overall impression of dainty ballerinas carried on gossamer wings.
Once in the air, they live only a day or two, to mate and procreate, to die and to fall back into the water. This phenomenon, and a misunderstanding of their complete life cycle, has given rise to their Latin name Ephemera—living for a day to describe the fleeting nature of their existence.
Though trout hunt mayfly nymphs among the river gravels all year round, they are especially attuned to the calendar of the mayfly hatching. Ascending insects, briefly trapped in the surface film, are at their most vulnerable—away from the safety and shelter of riverbed crevices, caught out in the open and silhouetted against the sky. No surprise then that, as I watched, the weaving currents of the Waiau were roiling with feeding trout, the fish slashing and punching from beneath the surface, their jaws snapping like so many pairs of wet hands clapping shut.
But the mayflies’ strength is in their numbers and the brevity of their emergence. Some of the hatches in the United States have been so huge and dense, they registered on radar systems at local air traffic control.
One species, Hexagenia limbata, erupts from the waters of the Mississippi in hatches totalling some 18 trillion insects—more than 3000 times the number of people on Earth. The newly emerged mayflies are attracted to lights in riverside towns and descend on them like a blizzard. Local authorities use snow-clearing vehicles to sweep up in the aftermath.
Above the Waiau, thousands of fluttering insects streamed skywards until night fell and the spectacle ended as if by the flick of a switch.
The trout were gone just as quickly and I turned on my headtorch, climbed up the steep rainforest bank and headed back to my camper, pausing for one last look at the river from the swingbridge. Only now I realised that, faced with this extravaganza of mayflies and trout, I had completely forgotten about fishing. This, it later turned out, was not an uncommon reaction, as many careers in entomology began with a fly rod. “You start by fly fishing for trout but after a while you stop bringing your rod,” one mayfly researcher told me. “The invertebrates are a lot more interesting.”
At its more refined levels, fly-fishing is an art of imitating natural streamside insects with artificial ones, and offering those to the fish. Some of the most enthusiastic anglers go as far as setting up freshwater aquariums and stocking them with mayfly nymphs collected from the stream. They invest in aeration pumps to recreate the stream environment, and 100–300x microscopes to view their charges up close. They observe the metamorphosis of the insects through their stages of development, their swimming motion and how, just prior to hatching, a tiny bubble of air forms between the new body and the outer shuck, allowing the nymphs to be ‘airlifted’ to the surface.
Along the way, these anglers would acquire an almost encyclopaedic knowledge of the insects, their orders, families, genera and distribution. They no longer speak of them as bugs brown, black or grey, or refer to them by the names of the flies that imitate them—Twilight Beauty, Kakahi Queen, Grey Ghost—but describe them by their scientific names: Zephlebia and Deleatidium, Ameletopsis and Nesameletus, Ichthybotus and Oniscigaster, or the Coloburiscus humeralis I saw on the Waiau.
In New Zealand, this approach was pioneered by the late Norman Marsh. Marsh migrated from Lancashire in 1953, settled in Invercargill and found his angling paradise in Southland, especially on the Clinton River, along which the Milford Track was built. He fished every spare minute, at least when he wasn’t studying streamside insects. The result of his decades-long enquiry into the lives of our river bugs is Trout Stream Insects of New Zealand, which remains a bible for angler entomologists. Thanks to Marsh’s popular work, you can now run into a farmer and a roading contractor lunching on a riverbank discussing with some authority the differences between Deleatidium myzobranchia and Deleatidium vernale, speaking of cerci and variegated wing patterns, all the while keeping an eye on the river to see what might alight above it. Over anglers and scientists alike, mayflies have a way of casting their special kind of magic.
Unlike other insects, hatching mayflies undergo not one metamorphosis, but two. The first adult form emerging from the nymphal shuck on the water’s surface is a dull-coloured sub-imago, or dun, which flutters into the air and immediately seeks shelter in riverside vegetation. After a few hours, the dun once again sheds its skin, this time to transform into a bright and shiny imago, or spinner. A mating dance takes place next, with columns of insects shimmering above the river and hopping upstream flights of egg-laying females.
After, the mayflies—then known as spent spinners—fall back into the water, their lustre gone, their wings no longer set like sails but spread-eagled flat upon the surface like a propeller.
Entomologists are unsure about the reason for this two-step transformation, but it’s thought that there is too significant a change from an aquatic gilled nymph to an airborne and sexually mature adult to be accomplished in a single transition. Whatever the reason, mayflies have proven extraordinarily successful. They are present throughout the world—some 2500 described species, more than 60 of them in New Zealand—and have endured virtually unchanged for 350 million years.
The world’s largest known mayfly, Bojophlebia prokopi, had a wingspan of 450 millimetres. It’s known only from fossil imprints in the Carpathian Mountains dating back to the Upper Carboniferous period 310–280 million years ago. It would have cut the silhouette of a hawk in the air, and the notion of their prolific hatch would have been frightening. Many smaller species have been found preserved in amber in Siberia and the Baltic, their slender and fleeting beauty frozen in time.
“It’s easy to fall under their spell,” Ken Whelan tells me. “As a species, mayflies must be a contender for the world’s most elegant insect.”
An Irish ecologist recognised as one of the world’s foremost experts on mayflies, Whelan was visiting New Zealand, and I arranged a rendezvous with him on a trout river in the Mackenzie Country.
“As a child, I had a severe case of polio and it was questionable if I’d ever walk again properly,” he says. “So while other boys ran about playing soccer and rugby, I stayed with my father, who worked the peat bogs, and started wandering the banks of rivers and creeks. I was besotted by flowing water and the life within and around it: the trout and salmon, and the insect communities, the entire cold-water ecosystem.”
Later in life, Whelan would also become a leading authority on the restoration, protection and management of freshwater fisheries for trout and salmon, but his signature project, and the subject of his PhD, was to re-establish mayfly species into several limestone loughs in the Irish moorlands.
“No one had ever attempted anything like that before, and you must know that in Ireland, angling is huge and deadly serious, and mayflies are something of a national insect,” he says. “I secured the use of an old fish hatchery, then collected hundreds of insects in the wild. The locals became suspicious that I was poaching their mayflies for sale in a Chinese restaurant.”
Collecting insects was precise and delicate work, requiring a stocking net atop a long fishing pole as the females needed to be captured after they had mated but before they laid their eggs. “I found out that mayflies are triggered into laying eggs by contact with water,” says Whelan. “If you hold them by their folded wings and lightly touch their abdomen to the water surface, they release a glue-like droplet of eggs.” Once in the water, the glue dissolves and the eggs disperse, and the invisible part of the mayfly life cycle begins among the underwater gravels and stones.
In this way, one egg-laying insect at a time, Whelan hand-planted and restored mayflies to several lakes. Once they saw how his work benefited their sport, even the taciturn Irish anglers forgave his apparent transgressions.
“I wanted to figure out how to do it, and to prove that it could be done,” he says, “but it’s painstaking work, and it seems a much wiser course of action to never let the water quality deteriorate to the point where the mayflies disappear from the aquatic environment.”
Mayflies are a barometer of the health of riverine ecosystems, their presence and wellbeing direct indicators of the quality of water.
“Many mayfly species are sensitive or intolerant to pollution, and so by looking at their populations—numbers, diversity and distribution—you can take a quick but descriptive picture of water quality in a particular river or stream,” says Whelan.
Chemical analysis can identify specific pollutants and the conditions of the river at the time of testing, but biological assessments—such as surveying macroinvertebrate populations—are a measure of the long-term effects of pollutants on aquatic organisms, a truer indication of the river’s state.
To make such biological assessments, ecologists such as Whelan sample a fast-flowing section of a river and collect the specimens found in the area into a mesh net to be counted and identified. Though all present macroinvertebrates are taken into account—stoneflies, shrimps, snails, bivalves—the biodiversity of mayflies is particularly telling. They are abundant, well studied and found in bodies of water throughout the world, providing baseline data for comparisons and conclusions. And because of their limited mobility, they represent a reasonably accurate representation of the health of a particular waterway.
This health is expressed as a ‘biotic index’. Because most of New Zealand’s macroinvertebrates are not found in other countries, our scientists, beginning with water-quality surveys in Taranaki streams in the mid-1980s, have developed their own system known as the Macroinvertebrate Community Index (MCI). The MCI has a value range from 0 to 200, and also differentiates between hard-bottom waterways, such as trout streams, and soft-bottom environments, like the mangrove creeks of Northland that are a home to entirely different communities of macroin-vertebrates.
The river I fished with Whelan was flowing straight out of the mountains and it teemed with trout and invertebrates. It could top any water-quality charts, ace any biotic index, and I for one never hesitated to drink from it. But further downstream, past a complex of dairy farms, it’s a different river entirely.
“Considering your current water-quality crisis, I think New Zealand needs to pay a lot closer attention to its freshwater invertebrates, like mayflies, and the story they tell,” says Whelan. “You really need to learn the lessons from Europe, where reductions in loadings of phosphorous and nitrogen have resulted in a dramatic improvement in water quality and restoration of natural ecosystems.
“It’s no longer about trout fishing or preserving species of insects. It’s about water, and water is life, their qualities directly proportional to each other.”
I left Whelan to his fishing. He was a boy again—wandering the riverbanks, looking under stones, enthusing about mayfly hatches. I travelled out of the Mackenzie and down the Waitaki Valley to visit a pioneering mayfly research project.
The foothills of the Kakanui Mountains, inland from Oamaru, are a Hobbiton-like landscape of pastoral prosperity, cut through with pine windbreaks and bands of weathered Otekaike limestone and drained by innumerable creeks, all tendrils of the Waitaki. On one such creek, I met Otago University freshwater ecologist Andreas Bruder, who is conducting an experiment to find out how aquatic ecosystems function under environmental stress.
“Our main objective is to find the balance between land use and ecological integrity,” Bruder tells me, “to what intensity you can farm the land while maintaining its water quality and ecosystem health. And what are the ecological consequences of this balance?”
The site of the project, the first of its kind in the world, is poised on the threshold: above it are unpopulated mountains and low-density sheep farming, immediately below a brand-new dairy conversion. The experiment examines how agricultural stressors such as nutrient levels, fine sediment inputs, reduced flow, and chemical pollutants affect ecosystem health.
“We are particularly interested in how these stressors combine and to what effect,” says Bruder. “So we’ve devised a system in which we can manipulate the key individual stressors and see what effects they have on macroinvertebrate communities and trout.”
The equipment for the project is made almost entirely of materials you’d find in any hardware store. Over a scaffold skeleton there’s a spaghetti of garden hoses, with tanks and valves feeding fresh water from the creek into 128 identical aquariums containing insect communities and fish. The aquariums themselves are made from microwave cake tins and are covered with hair-nets to contain the wildlife. Each container and its inhabitants are subjected to a different mix of agricultural stressors, simulating a scenario in a stream or a river.
“We’ve designed the experiment so it not only covers all possible combinations of stressors, but also so it’s not a lab exercise but real-life, applied ecology,” says Bruder.
Just how the results will play out in real life will be known soon—the experiment is in its final stages, conclusions being drawn from massive datasets that will fuel debate around land use, irrigation and dairy intensification. But even the preliminary results point to excessive sediments as the main culprit of the riverine ecosystem degradation.
“Our research has shown that these mayflies are very sensitive to elevated levels of fine sediment deposited on and between the pebbles,” says Bruder, who insists it’s good news for mayflies, because the solutions are relatively simple. “All you need are healthy riparian strips: corridors of vegetation framing the waterway. They act as filters against the sediments, stabilise the banks, keep stock away from the water and protect and connect the habitat.”
We’re interrupted by a cry from Rune Knudsen, a Norwegian salmonid expert who is working with Bruder. “Guys, you’ve got to come and see this!”
In the shallows, where large rocks rise above the water surface like islands, huge, bright-yellow mayflies were hatching. Their nymphs didn’t swim up to the surface, but crawled onto dry rocks and for a few moments sat there resting, yellow buds ready to burst open. Then, in a blink, they were out of their drab nymphal shucks, golden wings spread like hoisted sails and fluttering in the breeze.
We watched transfixed. In two decades of fly-fishing—hundreds of days on rivers around the country—I had never seen mayflies so large and colourful. Bruder and Knudsen were equally perplexed; they couldn’t even identify the species. Ephemeral as always, the hatch was over and the golden mayfly duns vanished, camouflaged against the blossom of broom and gorse, waiting to undergo their final transformation.
“Beyond pure aesthetics, mayflies are such remarkable little animals,” says Bruder.
“They are the link between algae and fish, and so for such tiny things they play a huge role in the ecosystem. In one life, they live in two worlds, water and land, and to me, they connect these two worlds. They connect a river to the landscape.
When you stand on top of a mountain and look down, you see patches: forests, rivers, pastures. But when you watch mayflies, below and above the water level, you realise that it is all one system. Within it, everything is connected and every fragment is necessary.”
The roomy elevator at the Canterbury Museum in Christchurch has secret back doors. Swiping his magnetic pass through a card reader, Terry Hitchings opened them for me, leading the way into part of the museum never seen by the public. The collections rooms—temperature-controlled vaults lit by strips of fluorescent lights contained floor-to-ceiling aisles of books, and framed sheets of insects, each bug pinned and carefully labelled. There were cabinets of drawers with perforated bottoms, each containing a glass vial with a small creature in preserving liquid. This floor was devoted entirely to entomology, and in its centre, three men bent over microscopes looking for glimpses of the unknown, unseen or undescribed.
“There’s no money in mayflies,” Hitchings laughed. “Most entomological research in New Zealand is driven by the needs of agriculture. Mayflies are not a pest but they have no economic value either, so we are all labourers of love here.”
A retired physics and chemistry teacher, Hitchings has voluntarily devoted his golden years to the study and description of the country’s mayflies.
“New Zealand has no still-water mayflies,” said Hitchings. “No one has conclusively answered why, but I’d offer that it’s because, in geological terms, our lakes are still very young, so no species of mayflies has evolved to colonise their littoral zones.
“North Island forest streams seem to have most of the diversity, though our knowledge of mayfly distribution depends more on where people have collected them rather than where mayflies are most numerous or diverse. What we know about them is far from complete, more detailed fragments than the whole picture.”
Like the rest of New Zealand’s native fauna, many of our mayflies are distinct and endemic, found nowhere else in the world, says Hitchings. Some families are represented here by just one unique species, while others, such as the ubiquitous Deleatidium, have at least 16 members and counting. “We don’t know why this is, but it may be because of New Zealand’s isolation and its mountainous nature, which restrict migration and the availability of alternative habitats.”
Mayfly diversity is under threat worldwide, he says. “With the increasing economic value of water, habitat is being significantly reduced and we’re losing species, some of which we didn’t even get a chance to scientifically describe.”
The mayflies, those dainty ballerinas on fairy wings, have survived virtually unchanged for 350 million years, but they are slow to evolve and adapt. In the rapidly changing human landscape, their existence is becoming ever more ephemeral.