The nymphs of the Autumn Dun are stone clingers, similar in size and shape to the nymphs of the March Brown, Rhithrogena germanica, but mostly they emerge not at the surface in open water but by crawling onto exposed stones in the shallows. They are, as a result, not readily available to trout and hence of limited interest to flyfishers. (If nymphs are being disturbed from their hiding places beneath stones - for example as a river rises due to rain but before it colours up excessively - then a size 14 weighted Pheasant-tail Nymph should be a perfectly adequate imitation.
It is extremely difficult - and quite unnecessary from a flyfishing point of view - to separate the nymphs of the various members of the genus Ecdyonurus that occur in Britain and Ireland.
The most common species on British rivers are: Ecdyonurus dispar, the Autumn Dun, Ecdyonurus torrentis, the Large Brook Dun Ecdyonurus venosus, the False March Brown.
The nymph illustrated on this page is almost certainly Ecdyonurus dispar, because it was collected in early autumn, by which time the other species mentioned above would have already hatched, and any nymphs from eggs laid in the preceding spring or early summer would be immature and very small. The best times of year for studying Ephemeroptera at their nymphal stage are in late winter and early springtime, because that is when most species are approaching their final instar. (An instar is a growth stage, at the end of which the nymph sheds a skin and grows a new, larger one.)
Autumn Duns, late season flies appearing in the afternoon and early evening mainly in August and September, are most common on spate rivers, although you may see them in modest numbers on most chalkstreams and even in the wind-swept margins of some large lakes.
The duns, which eclode from nymphs that have crawled out onto semi-submerged stones in the shallows, are not generally available for trout to eat; however, on very windy days a few of them get blown off their perches and end up on the surface of the water. At such times a dry March Brown is a very effective pattern, and indeed I have not found any other pattern that seems to work noticeably better. For this reason I don't have an artificial fly specifically for imitating the Autumn Dun.
Apart from thesize of the eyes (the male has much larger eyes than the female), the sexes are very similar in appearance, especially at the dun or subimago stage.
In the past this fly was known as the August Dun, but in Britain hatches appear in late June or early July, and in all but the coldest weather they continue until late October. ('Summer and Autumn Dun' might be a more appropriate name!)
On blustery evenings some spinners (males and females) may get blown onto the water; however, as the female spinner lays her eggs from the vantage point of emergent vegetation or stones in the shallows, it is the spent fly rather than the egg-laying spinner that appears prominently on the trout's menu.
The male and female spinners are sufficiently similar in appearance for a single imitative pattern to suffice. A size 12 or size 14 Pheasant-tail Spinner would be an appropriate general representation.
It is not common to see large quantities of Ecdyonurus dispar spinners on the water during the day, but by the time evening falls, if you are lucky enough to be on the river during a prolonged spell of settled warm weather when there are lovely red-orange sunsets, then there is no doubt that a reddish fly that reflects red and orange light well is a good bet.
In the typically breezy conditions of autumn you might be forgiven for thinking that drag (where a dry fly skates unnaturally across the surface) is not a problem. Unfortunately the stronger the breeze the more a dry fly and its leader tend to be swept along by the wind. Even though natural insect are buffeted by the breeze, if your artificial fly skates like a high-speed wind surfer the wiser and larger old trout are likely to be wary of it.
Learning how to make a 'wiggle cast', where the leader and fly line land on the water in a snake-like pattern is a very useful skill. To do so, make a cast and, while the line is still shooting through the rod rings, gently waggle the rod tip from side to side. It looks difficult when you see an expert make a wiggle cast, but actually it is a surprisingly easy technique to master. Try it!
In tumbling streams a Pheasant tail Spinner works perfectly well as an imitation of either a dun or a live spinner that have been blown onto the surface of the water. Spent spinners do not look much like this hackled dry fly, and once dead Autumn Spinners drift out of the riffles and into calmer water a better imitation is advisable.
Malcolm Greenhalgh has devised a very effective imitation of spent spinners of the Ecdyonurus group of up-winged flies, and he calls it Ecdyonurid Spinner. The rusty orange body of these spinners is well represented by Antron fibres of that colour, and the wings, tied to lie flat on the surface, also use Antron - either white or very pale blue will do. The spread tails are tied using brown cock hackle fibres (or Coq-de-Leon). Like most really effective spent spinner patterns, this fly does not have hackles, and so it lies in the surface film rather than standing up on the surface. More details are contained in Matching the Hatch.