Mayflies are small, fragile, short-lived – the adults live for a day, sometimes less. They do not bite. They do not sting. They do not eat crops or spread disease. They simply emerge from rivers, dance in the sun, mate, lay eggs, and die. They are, in the grand scheme of things, unremarkable. But when they are gone, you notice. The river feels empty. The air feels still. The summer mornings feel quieter. Size and spectacle made the giant mayfly special. A 3-centimetre nymph is a chunky, muscular creature – armoured, heavy, built for clinging to rocks in fast current. It grazed on algae and diatoms, scraping the surface of stones with its specialised mouthparts. It breathed through feathery gills along its abdomen, waving them constantly to extract oxygen from the water. It spent a year – sometimes two – growing, moulting, hiding from fish and eels. Then, on a trigger no one fully understands – water temperature, day length, something in the chemistry of the stream – it crawled out of its final nymphal skin and became an adult. Wings unfolded. Bodies lengthened. The giant mayfly took to the air, clumsy and glorious, with a wingspan that would make a dragonfly glance twice. It participated in the greatest mating swarm in the insect world. Mayfly emergences are timed for synchrony – thousands, millions of individuals emerging on the same morning, flooding the air with bodies, overwhelming predators by sheer numbers. The giant mayfly would have been the star of that show – the biggest, the brightest, the most spectacular. Its nymphs were also a cornerstone of the river food web. Fish ate them. Eels ate them. Birds ate them. The giant mayfly turned algae into protein, and protein into kingfishers and trout. It was the river's engine. Sediment and poison destroyed it. When Europeans cleared forests for pasture, the soil ran off the hills and into the rivers. Mayfly nymphs cannot breathe in silt. Their gills clog. Their movements slow. They starve, suffocate, die. At the same time, farm runoff – cow manure, fertiliser, sheep dip – poisoned the water. Mayflies are exquisitely sensitive to pollution. They are the canaries of the stream. When the canary dies, the stream is already dead. The giant mayfly, being larger, needed cleaner water and more oxygen than its smaller cousins. It was the first to go. The smaller mayflies held on – some still survive in our cleanest streams – but the giant is gone by the 1950s. The river shimmer is a name for something you will never see. A summer morning, a gravelly riffle, and the air above the water thick with giant wings. It used to happen here. It does not happen now.