We saw clusters of Mayflies flying above the Great Ouse on our circular route around Stony Stratford Nature Reserve on Tuesday evening’s site visit, 13th May 2025.
Watching Mayflies in May is an extraordinary and brief event. We watched them ‘dancing’ above the river, in the shadow of the A5 viaduct, with the fallen lying still on the surface of the water but we saw no fish gobbling these up. Although they are known as Mayflies, some appear at other times throughout the year, though May is a good time to watch them.
There are 51 British Ephemeroptera or ‘up-wing flies’ species. These are not True Flies, of the much larger Order of Diptera which has over 7,000 British species. Mayflies are one of a group of species, together with Stoneflies (Plecoptera) and Caddisflies (Trichoptera), informally known as ‘Riverflies’. Alderflies (Megaloptera) also frequent rivers and other water bodies but the Riverfly Partnership do not include these three British species in their surveys. Although Alderfly wings look similar to Mayflies, they do not have the double or triple tails of Mayflies.
It is not only naturalists who take an interest in Mayflies: they are of particular importance to fly-fishers who use imitations of real Mayflies as their lure at the end of their fishing line. Each of these has a popular name such as: Purple Dun, Turkey Brown and Large Green Dun.
The life of an adult Mayfly is brief, but it is only the last of four stages in its life-cycle. The first stage is the Egg from which the Nymph hatches. Nymphs live longest, up to two years, feeding at the bottom of the river on algae and river plants, dead organic matter and detritus, and pass through several sub-stages (Instars) at each of which they moult before finally emerging as adults. Emergence as an adult is in two stages: first as a ‘Sub-Imago’, or in fishing terms a Dun, then as an ‘Imago’ or Spinner.
The Sub-Imago stage is when the mature nymph frees itself from its ‘Shuck’, its nymphal skin, and emerges onto the surface with wings lowered. In its dark adult shape, it finds a shrub on which to dry. Soon it sheds its skin again to emerge in brighter colour as an Imago, the complete adult with wings upwards, but without mouthparts, so it never feeds and its adult life is brief. Mating starts with a cluster of mature males swarming above a river and the females flying into the swarm to be mated while in flight. The fertilised eggs are dropped into the water to stick to stones or plants, and the cycle starts again.
One aspect that looks distinctive about Mayflies is the reason for their other name as ‘up-winged flies’. Their wings look huge and are held close together and upwards. Many have dark, thick and prominent veins. They are also distinguished by having either a pair of ‘tails’ (Cerci) or three, used primarily as sensory organs. Other features that are important for identification are the sets of external gills by which they ‘breathe’. Mostly these stick outwards from the body. Their number and shape are key features for identification.
There are excellent sources from which to identify British Ephemeroptera and their larvae:
- A Pictorial Guide to British Ephemeroptera
by Craig Macadam and Cyril Bennett (an AIDGAP Guide)
Field Studies Council (FSC) 2010 - There is a brief key in:
Guide to Freshwater Invertebrates
by Michael Dobson, Simon Pawley, Melanie Fletcher, Anne Powell
Freshwater Biological Association 2012 - Mayfly Larvae (Ephemeroptera) of Britain and Ireland: Keys and a Review of their Ecology.
Freshwater Biological Association 2010. - Britain’s Insects (Wild Guides): A field guide to the insects of Great Britain and Ireland
by Paul D Brock
Princeton University Press 2021
This has a key to the ten families and photos and distribution maps for nine of the 51 species.
© Mike LeRoy, 15th May 2025