Note: strictly the genus name is Cloëon, but as people searching for information on the internet do not generally include accents (diacritical marks) such as the umlaut above the character ë, we have used the easier-to-type Cloeon in the main text of this page.
The nymphs of Cloeon dipterum, the Lake Olive, and Cloeon dipterum, the Pond Olive, are so similar that to tell the difference you have to examine the gill structures, preferably with a magnifying lens.
Any slim-bodied olive-coloured artificial nymph fished in the surface film seems to work well when Pond Olives are hatching. When there is no hatch a weighted nymph, such as a bead-headed Gold-ribbed Hare's Ear, is necessary to get down to where the trout are feeding.
As always when flyfishing, matching the hatch can make a big difference, but it's not merely a matter of making your fly the right size, colour and shape so that it looks like the insect you are trying to imitate. Most important of all, you need to make your fly behave like the natural insect. This is a subject I cover in detail in Matching the Hatch, and in my experience behaviour is the most important topic in flyfishing. Put simply, to be a really good flyfisher you need to know how aquatic insects behave, how trout behave, and how you need to behave in order to make your fly behave like a credible insect... and without scaring the fish!
The Pond Olive is the most common and abundant up-winged fly on most small lowland stillwaters in Britain and Ireland; it also occurs in the shallow margins of large lakes and reservoirs and in slow stretches of some rivers and streams. This fly is not often found in mountain lakes and peaty moorland pools, where other Ephemeroptera species such as the Claret Dun, Leptophlebia vespertina, the Sepia Dun, Leptophlebia marginata and the Purple Dun, Paraleptophlebia cincta, are more likely to be encountered.
The Pond Olive dun leaves the surface in open water, always near the shore in steeply shelving deep lakes; however, if you are boat fishing on a shallow lake you can find them hatching a long way from the shore. When Pond Olive duns are hatching a dry fly can give good sport.
A good general imitation for all sorts of olive duns is that old favourite, Greenwell's Glory. A Rough Olive is equally effective, I find.
Of course, a lot depends on how windy the weather is, because without a surface riffle the fishing can be very tough indeed, especially on bright summer days. That is when a closer imitation of the Pond Olive dun really can be all the difference between success and failure.
You won't need to differentiate between male and female duns when choosing an artificial fly, but if you would like to tell males from females it's very easy: just look them straight in the eye.
Males of the Pond Olive, in common with several other closely related up-winged flies, have what are termed 'turbinate' (turban-shaped) eyes. Having these specially adapted eyes is thought to help male (at the spinner stage, actually) to locate females that are not yet paired with another male in a swarm. It seems to work, because pond olives are (apart from tiny Caenis flies) probably the most abundant of all the up-winged flies in British stillwaters.
To separate these two species at dun or spinner stage in their life cycle you would have to examine the wing veins closely. Cloeon dipterum has between three and five cross veins at the tip of each wing - I could have written 'forewing' but neither of these species has any hindwings - while Cloeon simile has between nine and eleven cross veins at the tip of each wing. This is not something a trout's brain can take in, and it is also way beyond the skills if any flydresser I know to tie artificial flies in this degree of detail.
Incidentally, the only other British up-winged fly that has no hindwings is Procloeon bifidum, the Pale Evening Dun.
The Pond Olive female spinner lays her eggs on the surface from late afternoon on into darkness. Uniquely among British up-winged flies, as soon as the eggs touch the water young nymphs emerge and begin swimming down to the bed of the pond or lake or towards the stream margin.
The Pond Olive is the only up-winged fly in Britain and Europe whose nymphs leave the eggs as soon as the eggs leave the female dun - the technical term for this is that it is an 'ovoviviparous' insect.
A Tups Indispensable is a pretty good representation of the egg-laying spinner, and a size 14 is about right to match the size of the natural insect. Spent spinners, on the other hand, deserve a closer imitation, because trout have more time to inspect them. A spent-wing tying style is much better than a hackled fly.
In my book Matching the Hatch, which I wrote for flyfishers of all abilities, there is plenty of helpful information, advice and tips for experienced anglers as well as newcomers to the sport. There you will find large pictures and tying details for some superb patterns (a few are my own but most have been contributed by the world's top flyfishers including Charles Jardine, Brian Clarke, Bernard Venables, Jon Beer and many others).
There are some very good matches for both duns and spinners, artificial flies that have been proven to give great results, especially on days when the fishing is tough. (Its all written in jargon-free plain language that won't give a beginner a headache.)