A quiet hour at the water's edge

There is a particular kind of peace in kneeling at the shallow edge of a brook with your sleeves pushed up and a white tray of stream water in front of you. The current braids and chatters, the light shifts, and after a minute or two you start to see life everywhere — under stones, in the trailing weed, on the underside of a single leaf. Learning to identify what you find turns a pleasant hour into something closer to a conversation.

You do not need much kit, and none of it is expensive:

• A 10x hand lens, the single most useful thing you will own. Get into the habit of holding it right against your eye and bringing the creature towards it, rather than the other way round.
• A white tray or shallow dish. Dark invertebrates show up beautifully against a pale background.
• A small net, sieve or kitchen strainer, for gently disturbing a handful of gravel so that whatever lives among it drifts downstream into your tray.
• A soft watercolour brush, for moving creatures about without crushing them.
• Clean wellies, and a sense of restraint. More on both below.

Keep the tray in the shade and only for a few minutes at a time. Warm water is as dangerous to a stonefly as a dry bank is to a slug.

Start with the tails: the simplest key there is

The quickest route into stream invertebrates is to count the tails — properly called cerci — at the rear end, and then to check whether there are gills running along the sides of the abdomen. That pair of questions will get you to the right group almost every time.

• Three tails, or two tails with leaf-like or feathery gills along the sides of the body — a mayfly nymph.
• Two tails, and no gills along the sides of the abdomen — a stonefly nymph.
• No trailing tails, usually caterpillar-shaped, often inside a case — a caddisfly larva.

It is worth saying plainly that the key is a beginning, not an ending. There are mayfly nymphs with only two tails, and there are small stoneflies that will test your patience. That is part of the pleasure.

Mayfly nymphs: gills like little leaves

Hold a mayfly nymph in the lens and the gills are usually the giveaway: a row of delicate plates or filaments along each side of the abdomen, flickering gently as the insect breathes. Bodies vary enormously. In slow, silty stretches you will find slim, active swimmers that dart with a flick of the abdomen. In fast water you will find flattened, clinging forms pressed tight to the stone, with broad heads and legs splayed out like a climber on a rock face. There are also burrowing sorts in muddy beds, with tusks curving forward from the head.

Look closely at the legs too. Many mayfly nymphs have flattened, plate-like gills tucked behind the leg bases, and some have a single gill plate forming a kind of shield over the underside.

Stonefly nymphs: the cold-water specialists

Stonefly nymphs are the ones that tell you a stream is well. They need cold, clean, oxygen-rich water and they are not tolerant of pollution or low flows, so finding plenty of them is genuinely good news. Place a stonefly nymph beside a mayfly and the differences sharpen quickly: two tails rather than three, no gill plates running down the sides of the abdomen, and wing buds on the thorax that look like two small flat pads.

Where stoneflies do have gills, they sit in tufts at the base of the legs or beneath the head — not in a neat row along the body. Their movement is different too. Set one down in the tray and it tends to walk rather than swim, deliberately, with a faintly crab-like purpose.

Caddisfly larvae: cases, caterpillars and silk

Caddisfly larvae are the most architecturally interesting things in the stream. Most build a case from whatever the water offers: fine sand grains, tiny gravel, fragments of dead leaf, even small twigs, all glued together with silk. The cases alone are often enough to suggest a species. A stout, square-cut tube of coarse grit in fast water points one way; a soft, curling case of leaf pieces in a sluggish reach points another. Look for the pair of hooked prolegs at the rear that hold the larva anchored in its case, and the single pair at the front for hauling itself along.

Not all caddis build. Some are free-living and greenish, with a distinctly predatory look about them, and these are among the most active hunters in the tray.

Other regulars, and putting everything back

Once your eye is in, the rest of the community starts to appear. Freshwater shrimp, curved and translucent, are easy to spot as they flick sideways. Hog lice, the freshwater woodlice, crawl slowly over dead leaves rather than swimming. Flatworms glide across the tray with a strange, unhurried smoothness. Blackfly larvae sit glued to stones in the fastest flow, wafting fan-like mouthparts at the current, and alderfly larvae are unmistakable, with a long segmented body and a fierce pair of pincers.

Then comes the important part. Put everything back exactly where you found it, in the same patch of stream, out of the sun. Never carry water or weed from one stream to another — the mud on your wellies can move disease and invasive species between catchments — and rinse and dry your kit before you visit somewhere new. Leave the stones as you found them, disturb as little as you can, and take only the memory of what you saw. The brook will still be there next week, quietly getting on with it, and so will its residents.