2 days ago
Country diary: A stonefly in the sun – this is a sign of clean water
A brilliant, golden-green day by the River Horner, sunlight flaring between alder leaves, fast-flowing water sliding through a series of clear, shallow pools.
Minnows (Phoxinus phoxinus) are leaping for mayflies, their sides mirror-flashing as they fall back into the river with a soft, confidential plop. There are five or six of them working a sunny patch near the bank where tree roots clutch the riverbed. White bellies below a dark-olive stripe show them to be females; the males' undersides turn rosy in summer. Some have plain, stone-coloured backs, but others are flecked and mottled, imitating the gravel below.
Their restless grace is interrupted by a blundering insect, crash-landing on a twig in the middle of the stream. It's a stonefly the size of a pencil stub, long wings folded cloak-wise against its elongated body. It makes agitated attempts to reach the bank. Having spent its larval stage underwater crawling among rocks and crannies, it doesn't seem to understand why it can no longer move through water with ease.
After much fussing, it launches into a desperate, flailing swim to a bankside tree stump, where it grips moss and runs upwards to bask in the sun, before flying off in a clumsy blur of bark-coloured wings, only to plummet and disappear into the cracked earth.
There are 34 species of stonefly in the UK, found in and around upland streams. They can be difficult to distinguish, but this one had dingy orange patches at the back of the head and top of the wings, indicating that it was probably a common medium stonefly (Diura bicaudata).
Widespread in rivers, stonefly nymphs are particularly sensitive to pollution, especially organic pollution from agricultural runoff. They are one of the first species to disappear if oxygen levels drop, and their presence is a gauge of water quality. It's heartening to see the species here, just above where the river meets the sea, but the Horner is a short river, as is the Aller, which joins it at Bossington. It's no coincidence that the Aller is part of an ambitious habitat restoration project that includes reintroducing beavers and water voles.
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