Mayflies - Ephemeroptera

Ephemera danica is the most common of the big mayflies in France. This is the true Mayfly, known under this name because of its familiarity to anglers. From the burrowing larvae, eating plant material and detritus, to the nuptual flights of the adults and the brevity of their life, their story is remarkable and of great interest to entomologists, anglers and ecologists.

Female E. danica sub-adult.
Ephemera spp are typical Mayflies. Their size varies depending on region and habitat. One can find big individuals of over 25 mm (usually females) in the lowland rivers on calcareous substrates, but in the hills, in cold streams on acidic substrate they tend to be smaller. Their 'tails' are as long as their bodies. The wings are transparent and strengthened by many veins. The forewings are much bigger than the hind wings. They are generally yellowish or brownish, with many black markings.

A pair of adult Brown Mayfly E. vulgata (male right, female left),
Aigronne Valley, May, in long grass near a bief (millstream).
The colouration of the abdomen, yellowish to ivory white contrasts distinctly with the dark brown of the thorax and the head. The top of the last 4 abdominal segments are strongly marked right to the sides with very dark motifs in the form of thick commas or blunt pointed triangles. These motifs, if they occur at all on the segments further up are distinctly reduced and are generally missing on the first 3 segments, which are uniformly coloured.

A pair of adult Brown Mayfly E. vulgata
(male on right - note claspers at tip of abdomen).
The variation in size is also found in the aquatic larvae - some are up to 30 mm. The tails, on the other hand, stay distinctly smaller in proportion than those of the adults. The gills are in the form of double plumed filaments, carried high and very clearly visible on 6 of the abdominal segments. The first pair is vestigial. Once again, there are coloured abdominal motifs, and usually these allow the identification to species level.

A pair of adult Brown Mayfly E. vulgata, Aigronne Valley, May.
The larvae possess mandibles, with a long extension out the front, a pair of robust powerful front legs and a small shovel shaped 'helmet', so they are excellent burrowers. A cylindrical body with the gills curved around it aids their progression through the silt. They dig galleries in the shifting substrate of the water course (or sometimes étang) feeding on plant debris and particles of silt.

In the rivers larvae generally live in coarse sand deposits, small gravel beds or silt, in the peripheral currents. In the big lowland rivers they remain close to the banks or around islands. They can also colonise lakes, near banks washed by wavelets.

After a couple of dozen moults the insect becomes a sub-adult, known as a dun because of their subdued colouration. Duns are very similar to adults, but unable to reproduce. They leave the water and fly heavily to the nearest vegetation, where they moult a final time and become a sexually mature adult.

The life cycle generally extends over two years. Depending on the temperature it can be as short as one year or as long as three, so you can find larvae all year round.

The adults will appear sporadically from the end of April, with mass emergences generally restricted to warm evenings in late May or early June. There are stories of late summer emergences, but the closer to autumn the less likely they seem. The males swarm in nuptual flights at variable heights above the trees or grassland bordering the water courses. With the help of their long front legs they sieze females passing through the swarm, mating in flight and dying rapidly. The females lay their eggs in little groups dotting the surface of the water. She can lay 2500 - 4500 yellowy eggs less than a quarter of a millimetre across, then she in turn dies. The eggs are sticky and glue themselves to the substrate. They take several weeks to hatch.

An E. danica dun (sub-adult), Aigronne Valley, May, resting on
a Black Alder Alnus glutinosa on the banks of a bief (millstream) .
Although quite sensitive, like the majority of Ephemeroptera, to the impacts of human intervention on the watercourses, E. danica is relatively tolerant of organic pollutants. However, it should not be forgotten that even widespread species suffer local extinctions, which fragment meta-populations, which in turn can hinder the repopulation of a previous site. In addition, the effects of herbicide residues on aquatic macrofauna remains poorly evaluated. Lastly, the larvae of Ephemera spp, because they are burrowers, are very exposed to strong concentrations of heavy metals in the sediment, but we don't know what the effects are. Their disappearance in a water course nevertheless rings real alarm bells.

There are 4 species of Ephemera in France - E. danica (much the commonest and most widespread); E. glaucops (an eastern species and much the rarest, the abdomen only has 4 fine dark lines and the overall impression is yellowish); E. lineata (a species of the big lowland rivers and recognisable because the dark markings are in the form of pairs of parallel lines); E. vulgata (in strong decline and disappeared from much of its old range, becoming rare in the south-west due to organic pollutants, recognisable because it has dark markings on virtually every abdominal segment, often tinged with red, the darkest overall, and the loveliest).

References and Further Reading:
The above information comes from Mouche de mai et grands Ephémeres de France, an article in Insectes No 148 2008 by Michel Brulin (in French, with photos).

Also of interest in the same publication, No 60 2011, Les mannes d'éphémeres by Nicolas Césard and Michel Brulin (which has a photo supplied by us). The photo, of a road sign warning drivers that dead mayfly bodies can make the bridge slippery can also be seen on our daily blog Days on the Claise.

For illustrations of a small selection of Ephemeroptera and very brief information about those species see Insects of Britain and Western Europe by Michael Chinery.

For photos of an adult Ephemera danica and an empty exuvia (larval case) see Bananas and Mayflies on Days on the Claise.