Ephemeroptera

Insects in the order Ephemeroptera are known as up-wing flies. The term mayflies is also used interchangeably, although the ‘true’ mayfly is the species Ephemera danica. There are 51 species known to be present in the British Isles, and they are unique in that they are the only insects to have two adult stages: the sub-imago (or dun) and imago (or spinner).

With fossils dating back over 300 million years, mayflies are some of the oldest winged insects known.

Their larvae are found in a wide range of freshwater habitats including rivers, lakes, canals and ditches. Adults are often seen dancing alongside the water on warm summer days — however, some species can be seen in flight throughout the year. Their wings are held upright, butterfly-like above their body.

Conservation

The list of Species of Principal Importance in England features two Ephemeroptera species: Southern Iron Blue (Nigrobaetis niger) and Yellow Mayfly (Potamanthus luteus). Both of these species are also on the list for Wales, and the Southern Iron Blue is on the Scottish Biodiversity List. Seven other species of mayfly are on 2023’s Northern Ireland Priority Species list. Inclusion on these lists means that public authorities have a duty to support conservation measures for these species.

Life Cycle

Up-wing flies begin their lives as nymphs, living underwater and undergoing several moults, before eventually emerging from the water as a dun/sub-imago. They then shelter in bankside vegetation before transforming into a spinner/imago. Male spinners form a swarm, and females fly into the swarm from above. They mate in flight, then the female descends to the water to lay her eggs. Whilst the larval stage may last for two years, the adult stage is usually very brief.

Riverfly Monitoring

Volunteers taking part in the Riverfly Monitoring Initiative collect samples of freshwater invertebrates and record the numbers of individuals found within eight easily identifiable groups. Four of these groups are the larvae of Ephemeroptera families: Ephemeridae (‘true’ mayflies), Ephemerellidae (blue-winged olives), Heptageniidae (flat-bodied stone clingers), and Baetidae (olives).

In the Riverfly Extended Scheme, these families are also counted, along with two other Ephemeroptera groups: Angler’s Curse and Prong Gilled.

There is also an Ephemeroptera Recording Scheme that collects records of mayflies — see here.