Country diary: A mass hatching rises from the river | Environment

The arboreal corridor around the River Wharfe is bright with the sharp, tangy colors of April: the fresh lemon-and-lime fuzz of hawthorn budburst; the icy white of wood anemones; the acidic yellow of willow catkins. There is a gauzy delicacy to it all, with the sunlight diffused through a sheet of high cloud. Much of the wider landscape is still thin and pallid from winter, but these first stabs of color have a vitality that startles and surprises me, as they somehow do every year.

After a few weeks of cold, volatile weather, serenity and sunshine have returned, suddenly uncorking the energy of spring again. I take a closer look at the river and, sure enough, what I hope to find is there: a constant flow of small brown insects flying upstream, traveling just above the surface of the water, like a soft horizontal snow.

I have noticed this phenomenon on April days like this in previous years. It can only be seen during a window of around a fortnight, and it seems to require the air to be windless and the water still. I’m no entomological expert, but I have consulted people who are, and it seems likely to be the mass hatching of grannoms (Brachycentrus subnubilis)a type of caddisfly.

These small, moth-like insects have a fascinating infancy within the river. As larvae, they wrap themselves in a protective tubular silk case, which they augment with scraps of vegetation and debris, arranged in a specific ornate pattern. Once ready, a rise in water temperature triggers the pupae to float to the surface in vast numbers and emerge into the air as adults. Some species of caddis use their discarded shells as a raft, floating on the surface until they’re dry enough to take flight.

Individually, these might be inconspicuous creatures, but the vast scale of their emergence is quietly spectacular. For days on end, hundreds stream past every minute, and slacker sections of the water simmer with ripples as the bugs take flight, falling upwards into spring.

With thanks to Matt Shardlow and Craig Macadam

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