This is the biggest and best known of the British up-winged flies, and its appearance accounts for great excitement among trout and trout fishers alike.
In Britain and Ireland the term Mayfly has traditionally been reserved for this species and two other members of the same genus that are very similar in size and appearance. Anglers in the USA refer to all members of the order Ephemeroptera as Mayflies (although 'Dayflies' might be more appropriate as a reference to their brief lives as adult flies).
Living in tunnels in the beds of rivers and lakes, mayfly nymphs have specially adapted breathing filaments which they wave back and forth to create a current over their backs. The body of the male nymph is generally little more than two thirds as long as that of the female.
The developing nymphs, which can grow to over 30 mm long, normally take two years to reach maturity; however there is evidence that some mature in just one year and that others, particularly in colder upland and northern rivers, may live for up to three years in their nymphal form.
Mayfly nymphs burrow beneath gravel and silt. In spring they emerge from the bed of the lake or river and migrate towards shallower water. These large, slow-swimming insects are at their most vulnerable when they begin ascending to the surface to 'hatch'. (The technical term for this process is eclosion.).
In Britain and Ireland the main hatch usually begins somewhere between mid May and the first week of June, but there are regional variations. The hatch generally begins early in the afternoon and continues for several hours.
Duns are readily identified not simpy because of their large size but also because their wings are dull and yellowish with brown veining, whereas at the spinner stage the wings are brilliantly translucent and this makes the veins appear to be almost black. Forelegs of a dun are shorter than those of the spinner, but even more noticeably the tails of a dun are very much shorter than those of the spinner. (Males can double their tail length during the process of transposing from dun to spinner. Needless to say, from mid morning when the first duns emerge from the water until mid-to-late afternoon when the first of the egg-laying spinners return to lay their eggs, it is the dun stage that flyfishers need to copy.
Swarms of male spinners form cloud columns, usually within a stone's throw of the waterside. There they climb and dive until a female flies in to the swarm and mates.
Apart from the obvious differences in their behaviour (males do not lay eggs; females do not swarm) it is relatively easy to differentiate between males and females simply from their appearance. males are smaller and noticeably darker, especially at the dun stage but also as spinners. The eyes of a male are noticeably larger than those of a female.
Egg-laying spinners, larger and paler than the males, fly a foot or so above the surface touching down periodically to release a batch of eggs. It can be very frustrating for trout as they rush to seize a surface dun or spinner only to see it fly off - something that cannot happen in the final phase of a mayfly's existence!
Once all of their eggs have been expended, the spinners tire and fall to the surface, where they flutter a while before dying in the 'spent gnat' position. Dead or dying female spinners, sometimes referred to as 'Spent Gnats', drift downstream on the current unable to escape the jaws of feeding trout.
Rises to these spent spinners soon become leisurely affairs as the fish learn that there is no benefit in rushing to grab a morsel. It also means that if a flyfisher's artificial mayfly is a poor imitation of the real thing, then the trout has plenty of time to spot the deception and reject the offering... and they certainly will, especially if there are plenty of dead mayflies to choose from. Any reasonable copy of a mayfly may serve as a credible dun or egg-laying spinner, but you need a good Spent Gnat.
The artificial fly known as the Grey Wulff is often cited as a good Mayfly pattern. It works well in fast water, and when few flies are hatching it can even pass muster on slower, streamy glides on chalkstreams; however, in my experience a closer imitation gives better results when the trout are being choosy.
The detached-body tying shown here is particularly effective as a dun or an egg-laying spinner. It was devised and tied by a brilliant flydresser, Peter Masters, and I have used this pattern to great effect not only on the River Test, River Itchen and River Avon (chalkstreams) but also on demanding limestone rivers in Ireland including the Fergus, Suir and Boyne.
Tying details for this supern dry fly are included in Matching the Hatch, together with techniques and tactics for nymph and dry fly fishing on rivers, streams and lakes in the Mayfly season.
Trout can get very difficult when a lot of mayflies are hatching at once, and even the best of artificial fly patterns can be let down by its Achilles' Heel - the hook. Seen from beneath the surface, a traditional fly has the hook point clearly visible below the surface.
Many years ago I devised my No-hook Spent Mayfly pattern to solve this problem. The deer-hair body provides all the buoyancy that is needed to represent a Spent Gnat, and the hook lies parallel to te water surface and concealed within the wing.
Is it easy to tie? Yes. Does it spin when cast and cause leader twist? No. Okay, so what's the snag? Only one... you can't buy them in fishing tackle shops. The materials list and tying details are in Matching the Hatch, so if you have a copy turn to page 91 and give it a try; you won't be disappointed.
Ephemera vulgata and Ephemera lineata also occur in Britain and Ireland, but the latter is quite rare and so it is unlikely to be of much interest to flyfishers. Separating Ephemera danica from Ephemera vulgata is fairly straightforward, provide you can get a good look at the markings on all of the segments of their abdomens. On segments 7 to 9 of Ephemera danica the markings are elongated, solid and and very obvious, although on the other forward segments the markings are much less distinct. In Ephemera vulgata there are dark triangular markings on each segment of the abdomen, although on segments 1 and 9 these markings are sometimes rather indistinct.
If your fishing takes you to mainland Europe or to the USA, you are likely to encounter other mayflies in the Ephemera genus; all can be copied using the fly patterns mentioned here.
Trout are sometimes wary of large duns that suddenly fall from the sky, so it usually pays to cast well upstream of a rising fish so that your fly drifts across the trout's window in the same way that most of the natural duns will be doing.
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 withy 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