When I lived in Belfast in the early 2000s and needed wildness, a visit westwards to the gently sloping, marshy shores of Lough Neagh was just the thing.
The sky above the lake – a body of shallow freshwater so vast it appeared infinite – was immense.
On my walks, locals would mention the spectacle of swarming ‘Lough Neagh Flies’ which descended in clouds around the lake during late spring and early summer. It sounded dramatic and almost apocalyptic; on seeing the flies gather like a plume of smoke above the water, residents would shut their windows and doors to keep the insects out.
From a distance, when densely packed, they swirl and whirl like a kind of murmuration of starlings; up close, they dart and flit, forwards and backwards. They converge over the lake for one reason: to find a mate within a week before they die.
The ‘Lough Neagh Fly’ isn’t unique. It’s a name given to various chironomid species known as non-biting midges or lake flies. Anglers, who can always be relied on for creative nicknames, call them buzzers, duck flies or hawthorns.
Chironomids are arguably the most critical part of lake ecology; their larvae eat up the debris, while in turn, they form a critical part of the diets of fish like European eels, perch, and pollan, along with some increasingly rare duck species such as Pochard, Scaup and Goldeneye.
We know of 540 chironomid species in Ireland thanks to a handful of scientists who dedicated their professional lives to learning more about them. They include Carmel F. Humphries, professor of zoology at University College Dublin in the 1930s; her student, Dr Declan Murray, now retired from UCD, who first listed the chironomids of Ireland in 1972; and Dr Peter Langton, of Cambridge University, who discovered a new species and now lives in Northern Ireland.
After mating, the female fly will lay 1,000 eggs on the water’s surface but only about two will survive to adulthood. The eggs tumble down to the bottom of the lake and, within seven days, they hatch into slender, worm-like larvae. Some chironomid species produce larvae called ‘bloodworms’ which are cerise red from the haemoglobin, which helps them survive in low-oxygen environments.
The larvae burrow into the base of the lake for up to a year, and it’s here that they act as recyclers of rubbish, hoovering up detritus such as dead fish and rotting algae.
They then pupate – a stage which is like a magician’s hat; what goes in is very different from what comes out just a week later. The cells are rearranged and churned around to create the adult fly. Air collects under the pupae’s skin and brings it to the surface. The pupal skin splits open and acts as a little water raft for the adult fly to dry off before taking flight.
Identifying different species is tricky but, fortunately for scientists, the pupal skin is like a fingerprint with identifiable features. After the adult fly takes off, the pupal skin remains intact in the water for a few days before decomposing, and it’s at this point that scientists get out their nets to collect the skins to find out what species are around.
Chironomids are found everywhere, from Mount Everest to the Antarctic; from deep under the depths of Russian lakes to hot thermal springs with temperatures above 40 degrees Celsius.
They’re in lakes, pools, puddles and ponds; rain-filled barrels in your garden; tree-rot holes, leaves of pitcher plants and marine waters; woodland leaf litter, tillage soils and cattle dung. Dr Peter Langton even found a colony in a water-filled discarded crisp packet.
Because they are so diverse, they have evolved to survive in many different habitats and conditions; some need pristine water quality to survive, while others have found ways to tolerate highly polluted environments where the oxygen is all but gone.
Scientists can test the water for nutrients, such as phosphorous and nitrogen, but it only gives a snapshot in time; understanding which chironomid species live in the water tells us a deeper story of what is happening.
Take Lough Neagh. As the water has become more saturated with nutrients, the species of chironomids have changed over the years from those that thrive in low-nutrient conditions to those that can tolerate high levels of pollution and the limited oxygen that comes with it.
Two chironomids, which thrive in such waters, are dominant in the lake: Chironomus plumosus and Chironomus anthracinus. Scientists familiar with the lake are deeply concerned that species diversity will plummet, wiping out the assembly of life in the lake that has taken thousands of years to evolve. We don’t know the consequences of making a monoculture from nature.
Chironomids tell us about our rapidly warming climate, too. In 1997, Dr. Langton discovered a chironomid species new to science while looking at rockpools in northwest Spain and named it Metriocnemus carmencitabertarum.
Declan Murray spotted it in a drinking trough in the Azores six years later. With climate change, it started to make its way northwards. It was reported in a wheelbarrow in a garden in Somerset, then by Murray in Kilmessan in Meath in 2012 and finally in 2018, in a trough outside Dr Langton’s back door in Northern Ireland.
Lough Neagh is in deep trouble but, with political leadership and decisive action, it is hopefully not too late to bring it back to better health. In future years, the chironomids found there will tell us whether this has been achieved or not.
Meanwhile, if you’re fortunate enough to witness the chironomids clouds over the lake, just remember: they’re harmless and don’t bite. In fact, they can display some very familiar behaviour.
One evening while camping, Dr Langton spotted a chironomid fly which had landed on the neck of a bottle of wine outside his tent. After a few minutes of lapping up the liquid, it fell off drunk and lurched about in the darkness.
