The flies of Britain’s rivers

Shrimps and aquatic insects such as mayflies, caddis flies and stoneflies are important food items for most trout and grayling in rivers all year round. Terrestrial insects such as daddy-long-legs and beetles are also significant – especially from May to September.

Trout and grayling can sometimes become selective towards an insect species. This is usually because that insect is avail- able in greater numbers than others. It’s not the size of the fly or nymph that attracts trout, but the numbers of insects available.


Trout take mayfly nymphs underneath the water, and they feed on the adult duns and egg-laying spinners on the surface.

In some species of mayfly the egg-laying female deposits its eggs on the surface. With others they crawl into the water to lay their eggs. Weeks later the eggs hatch, and the tiny nymphs emerge. Moulting several times to grow, they reach maximum size a couple of weeks before becoming adult flies. Their life-cycle lasts about a year.

Some mayflies thrive in slow-moving water with a silty river bed. Other species require faster water with a bottom of stones or gravel, and still others need moss or weeds in moderate currents.

The body shapes of nymphs have adapted to different environments. Broad-bodied, flat nymphs cling to the sides of stones in fast water; cylindrical, fast-swimming nymphs live among weeds. Slim-shaped, burrowing nymphs only come out from the silt when about to hatch into adults.

Nymphs become more active just before they become adults, as they wait for ideal hatching conditions. Their wing pads darken, and they reach their maximum size. They swim busily about and even make tentative journeys to the surface. Trout begin to take an avid interest in the nymphs at this time.

When hatching at the surface, the nymph pauses while the upper sides of its nymphal some are free swimming. The larvae feature in the diets of trout and grayling for 12 months a year. They are most important in winter and early spring, though, when fish take more of their food from the stream bed.

When a larva matures, it seals the case or builds a shelter with sand and silk, and then pupates. When it’s ready to emerge, the pupa chews through its case and swims to the surface.

With folded wingpads on the underside and long rear-facing antennae, the pupa is humped in appearance. At the surface the winged adult sheds the pupal skin and emerges. They can take flight immediately. skin split. The winged dun struggles out of its nymphal shuck. The surface tension is sometimes too much for the fly, and the trauma of the change can cause many to die in the process of emerging.

Hungry trout feast upon newly hatched adult insects that are sitting on the water’s surface, waiting for their wings to dry. The best time to fish is during a hatch when most trout are feeding. Present a dry fly such as an Adams, making sure your fly drifts in line with the trout’s lie.

The sizes of the adult upwinged flies vary from 8-30mm. Within a few hours the dun sheds a layer of skin to emerge into the final stage, the spinner. At this point the insect is much brighter and has a longer tail than the dun.

The spinners mate, and the females return to the river to lay their eggs. Both the males and females die within 24 to 48 hours after they emerge as adults.

All species have different peak hatching times in the season with preferred times of the day for emerging. Recognizing a major group of mayfly common to a particular river and knowing when it might be expected to hatch are important aspects of fly-fishing on rivers.

Caddis flies

The segmented, grub-like larvae of the various sedges live on the river bed and among weeds. Most species build cases with the materials found on the stream bed, but

The adults’ wings, covered in tiny hairs, are held in a roof-shape when at rest. Many species have long antennae. None has a tail. Adult caddis flies range widely in size from 6-17mm Q4-%in*. Colouring is fairly sober in brown or beige.

The mated females return to the water to lay their eggs, and here they attract the attention of trout. Some egg-laying females stay quite still. Others skitter over the water, and a few dive or swim below the surface to deposit their eggs.

Of the almost 200 British species of caddis flies less than a handful are easy to identify, but the fly-fisher needs only to copy the natural with an artificial of the same overall size and colour.


Most stoneflies prefer fast-flowing, stony-bedded rivers. Only a few species live in slow-moving water. Stoneflies are brown or black in colour and have two tails.

The nymphs vary in size from 8-33mm. Adult stoneflies have four wings. When the insect is at rest, the wings lie flat across the back.

Stonefly nymphs usually stay close to the river bed clinging to stones or debris, and they do not swim freely along the bottom. When ready to emerge, they usually crawl ashore. Imitating the nymph stage is often your best bet.

Midges and smuts

In slow stretches of water midges and smuts can be very important. These black, mosquitolike insects are weak swimmers and poor flyers.

Small dimple rises are one sign that the trout are taking pupae in the surface film. Because of their feebleness, especially when it comes to breaking the water’s surface, midge pupae are taken more than the adult. Emerger patterns fished in the surface film are highly effective during a hatch.