Ephemeroptera - the Up-winged Flies

There are at least forty-five species of up-winged flies in the British Isles. They get their common name from the habit of holding their wings upright above the body when at rest, rather as a butterfly does. Here are some up-winged flies of Britain and Ireland that are of greatest importance to trout and grayling - and therefore also to flyfishers:

Caenis species
Caenis horaria, Caenis species
Iron Blue
Iron Blue, Alainites muticus (Baetis muticus)
Lake Olive
Lake Olive, Cloeon simile
Medium olive
Medium Olive, Baetis verna
Olive Upright
Olive Upright dun, Rhithrogena semicolorata
B-W Olive
Blue-winged Olive, Serratella ignita (Epherella ignita)
Dusky Yellowstreak
Dusky Yellowstreak, Electrogena lateralis
Autumn Dun
Autumn Dun, Ecdyonurus dispar
Large Dark Olive
Large Dark Olive, Baetis rhodani
Pale Watery
Pale Watery, Baetis fuscatus
Pond Olive
Pond Olive, Cloeon dipterum
Sm Dark Olive
Small Dark Olive, Baetis scambus
Pale Evening
Pale Evening mayfly Procloeon bifidum
Small Spurwing
Small Spurwing Centroptilum luteolum

Matching the Hatch

Of all the groups of aquatic insects, it is the up-winged flies that have most intrigued and captivated the interest of flyfishers down the ages. The reason is more to do with practicality than with the undeniable beauty of these ephemeral winged insects. Trout and grayling rise avidly to upwinged flies, and at times they feed on one species, sometimes even selecting just one particular lifecycle stage, to the exclusion of all else. If you cannot match the hatch with a reasonably good artificial fly, your fishing is much less productive and a lot less fun.

That's why I wrote Matching the Hatch, and so far many tens of thousands of flyfishers have found it useful.

In any case, in the individual species pages on this section of the First Nature website I have included at least some basic information to get you started in choosing artificial flies of the right size and pattern to match the natural insects fairly well, and you can always go into the subject in more detail later.

Flyfishing tips and tactics

Using the right kind of artificial fly is only the first step in matching the hatch. A crucially important topic (and one that is covered extensively in Matching the Hatch) is that of making your fly behave like the insect it is meant to represent. Inappropriate behaviour will give the game away even more quickly than using a fly of the wrong size, design of colour (and I should add that colour is often the least important of these key factors). Again in each species page on this section of the First Nature website you will find some basic information on this fascinating and very rewarding topic.

Life Cycle of the Ephemeroptera

All upwinged flies go through four distinct phases: egg, nymph, dun and spinner. The first two phases are spent under water, while duns and spinners are the winged stages in the life cycle.


Nymph of Alainites muticus, the Iron Blue
Nymph of the Iron Blue, Alainites muticus

The nymphs, which have three tails, feed mainly on algae and rotting vegetation. They are categorised according to how they have adapted to suit different habitats.

  1. Burrowers: nymphs that live in tunnels beneath the mud, sand and gravel, venturing above the bed of the river or the lake only when ready to hatch. In the British Isles, the Mayfly alone falls into this category.
  2. Silt crawlers: a group of tiny nymphs which crawl upon the silty beds of rivers and lakes, feeding among the detritus. The Caenis nymphs are included in this group.
  3. Stone clingers: a third group of relatively inactive nymphs which spend most of their time clinging to the undersides of stones, where they feed by grazing on algae. The nymphs of the March Brown and the Yellow May dun are stone clingers.
  4. Moss creepers: a very feeble-swimming group of nymphs, which feed among mosses on the river bed. The blue-winged olive belongs to this group.
  5. Laboured swimmers: slightly flattened nymphs which spend most of their time in the margins and can swim from place to place, but only slowly. The claret nymph is one of the laboured swimmers.
  6. Agile darters: torpedo-shaped creatures of special interest to anglers because they swim rapidly from place to place in search of new feeding spots and are therefore available to trout even when there is no hatch of flies. The nymph of the Iron Blue is an agile darter.

After typically one year, the nymphs are ready to emerge as winged insects. (Some of the up-winged flies of springtime produce a second generation later in the year - the Large Dark Olive, Baetis rhodani, for example.)


female dun of the Yellow May
A female dun of the Yellow May

From the nymphal case a fly called a dun emerges. The scientific name for this stage in the lifecycle of an up-winged fly is 'subimago', but anglers use the term dun because many of the olive flies have dun-coloured (olive-brown) bodies at this stage in their development. the Yellow May, shown here, is an example of a fly where the term dun is far from accurately descriptive!

The dun's wings are dull and fringed with tiny hairs, and its tails are much longer than those of the nymph from which it 'hatched' (an imprecise angling term, since strictly the nymph hatches from the egg and later transposes into dun and then spinner forms). (In the next stage the tails will grow even longer.)

Once its wings have opened up fully, which can take several seconds, the dun flies to a nearby tree; there it rests until ready to shed one more skin in preparation for its final act of mating.

Egg-laying Spinner

BWO spinner with eggs
A Blue-winged Olive with her ball of eggs

After anything from a few seconds to a few days, the dun splits its skin and out crawls the adult spinner. Gone are the hairy fringes and most of the colour from the wings, and the tails are noticeably longer than those of the dun. At last the insect is ably to fly off in search of a mate.

After mating, the female heads off back to the water to lay her eggs, either on the surface or, in the case of the Baetis flies, more often by crawling down plant stems of semi-submerged boulders and attaching them to rocks, plants or logs beneath the water.

Spent Spinner

Mayfly spent spinner

Once the female spinner has released her eggs, she falls exhausted onto the water, flutters awhile and then drifts inert with her wings outstretched. Trout can sip them leisurely from the surface.

Baetis spinners that have crawled down below the surface to lay their eggs approach the surface from the other direction and can become trapped just below the surface.

Etymological note: We use the terms flyfishing and flyfisher throughout our pages, but some people still write fly fishing, fly fisher and fly-fishing techniques.

Entomological note: The diversity and particularly the abundance of flylife (or if you prefer fly life) on rivers and streams in Britain and Ireland has declined dramatically over the past forty years. We support the Riverfly Partnership's campaign to raise awareness of this issue, to involve anglers and others in scientific monitoring of water quality via aquatic invertebrate diversity, and to promote actions to halt and reverse this decline.


In identifying up-winged flies to species level we make extensive use of two Freshwater Biological Association publications:

A key to the Adults of the British Ephemeroptera with notes on their ecology; J M Elliott and U M Humpesch (1983). FBA Scientific Publication No. 47.

Larvae of British Ephemeroptera: a key with ecological notes; J M Elliott, U H Humpesch and T T Macan (1988). FBA Scientific Publication No. 49.

For flyfishers on stillwater, river and stream there is an illustrated guidebook with advice on corresponding artificial flies plus fishing tips:

Matching the Hatch, Pat O'Reilly; Quiller Publications (1997, latest reprint 2010) ISBN 978 1 85310 822 8

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