The secret life of mayflies and caddisflies

By Navigating access rights along English rivers and banks
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27 Jun 2026

The secret life of mayflies and caddisflies

Why the stream's busiest residents are almost invisible

Stand at the edge of any clean brook in Britain and you are looking at the nursery of hundreds of thousands of insects. Mayflies and caddisflies — the two great groups of our running waters — spend most of their lives as larvae on and under the stones at your feet. A mayfly might live a year or more underwater and then fly for a single afternoon. A caddisfly larva toils for months inside a case it has built with its own silk, then emerges for a fortnight of frantic courtship. Most people never notice either, which is a shame, because these insects are among the best indicators of a stream's health and some of the most rewarding wildlife to watch in the British countryside.

Mayflies: a year underwater, a day in the air

Mayflies belong to the order Ephemeroptera, a name meaning "short-lived wings". Adults have reduced mouthparts and cannot feed at all; everything they do as winged insects is devoted to mating. On chalk streams such as the Test or the Dove, the largest of them, Ephemera danica — simply "the mayfly" to anglers — appears from the second half of May. Its nymphs burrow in the silt between gravel and take one or two years to mature.

Emergence is worth planning a spring evening around. The nymph rises to the surface, splits along the back and climbs out as a dull, dusky-winged subimago — a dun. It rests in bankside vegetation for a day or so, moults again into the shiny, long-tailed spinner, and returns to the river at dusk to dance over the riffles and lay its eggs.

Blue-winged olive — small and olive-bodied, hatching in the last hour of light on summer evenings.
Large dark olive — one of the first of the year, often on cold March afternoons.
Green drake — a big, distinctive mayfly of slower, siltier rivers, from late May onwards.

Caddisflies: the stream's little builders

Caddisflies — Trichoptera, or "hairy wings" — are close relatives of moths and far more varied in their habits. Their larvae produce silk from glands in the head and use it in remarkable ways. Some build portable cases of sand grains, small stones or leaf fragments and drag these shelters about the stream bed. Others spin fixed nets across the current to filter drifting algae and small insects. A few are free-roaming predators that stalk the gravel without a case at all.

Adults are moth-like, with tent-shaped wings folded over the body and long antennae. They fly at dusk, often in great numbers. The grannom, a dark little caddis with grey-mottled wings, hatches in April in clouds so thick you can hear the females dipping onto the surface to lay their egg masses. Cinnamon sedges in late summer are a familiar sight along weedy stretches.

How to find and identify them

You do not need a laboratory to get to know these insects. A few simple things will do.

A white plastic tray — stand upstream, disturb the gravel with your heel and let the current carry dislodged invertebrates into it.
A hand lens — ten times magnification shows the gills along a mayfly nymph's abdomen and the three tails that separate it from a stonefly.
A torch — caddisflies and spinners are far easier to watch at dusk than in daylight.
Polarised sunglasses — they cut surface glare so you can see nymphs crawling on submerged stones.

Turn stones over gently and always replace them the same way up. Mayfly nymphs are flattened or streamlined, with pairs of leaf-like gills along the abdomen. Cased caddis larvae look like tiny walking pebbles — until a grey, soft-bodied grub pokes its head and legs out and trundles across the tray.

What they tell you about the water

Both groups are excellent barometers of stream health. Mayfly nymphs need cool, well-oxygenated water and cannot tolerate siltation or pollution. Where you find a good range of them, along with stoneflies, you are almost certainly looking at a healthy brook. Caddisflies are a more mixed bunch: net-spinners are fairly robust, but the cased species that feed on fallen leaves are sensitive to the loss of autumn leaf litter from over-tidied banks and to the runoff that follows heavy rain.

If you are monitoring a local beck, record what you find and when. A simple family-level list kept over a few years will tell you more about your stream than any single visit, and it turns a Sunday walk into something genuinely useful.

Watching the hatch: a few practical tips

Timing is everything. Emergence is triggered by water temperature, so a mild spring can bring flies a fortnight earlier than a cold one. May and June are the peak months for most of the country. Go in the hour before dusk and stay for the last of the light, when spinners gather over riffles. Keep low, wear dark clothing and avoid waving your arms about; mayflies are very sensitive to movement.

Stay out of the channel where you can, and if you do wade, keep your boots off the gravel bars where eggs have been laid. Never disturb spawning beds in cold weather. Above all, slow down. Sit still beside a shallow riffle for ten minutes and you will begin to see the small comings and goings that make these insects so absorbing.

Wildlife

Seasonal changes to watch for in a chalk stream

Small dries, soft hackles and nymph patterns work well for wary wild brown trout in narrow, overgrown streams with clear water.

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5 Comments

Robert Sonny says Jul 21, 2024 at 10:00am

I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
Reply
- John Abraham says Jul 21, 2024 at 10:00am
  
  I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
  Reply
  - Robert Sonnysays Jul 21, 2024 at 10:00am
    
    I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
    Reply
Robert Sonny says Jul 21, 2024 at 10:00am

I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
Reply

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