Ephemeroptera - the Upwinged Flies

There are more than forty species of upwinged flies in the British Isles. They get their common name from the habit of holding their wings upright above the body when at rest. Here are some that are important to trout and therefore to flyfishers:

Caenis Caënis species	Iron Blue Baëtis muticus	Lake Olive Cloeon simile	March Brown Rithrogena germanica	Mayfly Ephemera danica	Medium olive Baëtis vernus
Olive Upright Rithrogena semicolorata	B-W Olive Ephemerella ignita	Yellow May Heptagenia sulphurea	Autumn Dun Ecdyonurus dispar	Lg Dark Olive Baëtis  rhodani	Pale Watery Baëtis  muticus

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.

Nymphs

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.

• 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.
• 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.
• 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.
• 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.
• 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.
• 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 Iron Blue is an agile darter.

After typically one year, the nymphs are ready to emerge as winged insects.

Dun

From the nymphal case a fly called a dun emerges. 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).

Once its wings have dried, the dun takes off for the cover of trees; there it rests until ready to shed one more skin in preparation for its final act of mating.

Egg-laying Spinner

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 Baëtis flies, by attaching them to plants or logs beneath the water.

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.

Baëtis 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.

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