FOR a few days during May, in several places along the river Wharfe (near Ilkley), there were explosions of Mayflies (Ephemeroptera). Mayflies can emerge any time from early spring to late autumn – but these were true to their name. Clouds of the insects formed over the water. Depending on the species (there are approximately 50 in Britain), Mayflies have one or two pairs of wings (typically upright at rest), and either two or three tails. Generally, an abundance of Mayflies is regarded as a positive sign, insofar as it is thought to indicate that a river is in relatively good health.

At first sight, movement within the swarms of Mayflies seemed random – but, on closer examination, I thought there were some areas of synchronisation, like a murmuration, with many insects having a common direction of travel, swirling around. Perhaps it was the effect of a slight breeze, but I don’t think so. It seems more likely it was related to mating activity.

These congregations are a vital part of the Mayfly’s life cycle. The vast majority of their life, sometimes as much as two years, is spent as a nymph, underwater. Uniquely, when the time comes to emerge into the air, most Mayflies make two transformations in quick succession. First, to a subimago (or ‘dun’), before moulting once more to become an imago (or ‘spinner’). This final part of a Mayfly’s life is brief, lasting a few hours, or a day, but not much more. Adult Mayflies are not physiologically equipped to eat so, without sustenance, they soon perish. Before their eventual fate, they take to the air, where they mate. The female then returns to the water to deposit fertilised eggs. In time, the submerged eggs hatch and the whole cycle begins again, with a new generation of nymphs. (The following link is to a short BBC film of the Mayfly lifecycle, with some great photography: https://www.youtube.com/watch?v=dzbLuLzMWE4)

The emergence of swarms of Mayflies provides a food bonanza for many animals, including fish, birds, bats, amphibians, and other invertebrates. Indeed, in some countries, Mayflies are harvested for human consumption.

Of course, predation of Mayflies results in a valuable transfer of nutrients, both within the river, and from the river to the terrestrial area. In this respect, Mayflies provide a valuable service. They are part of a food chain, converting underwater detritus and periphyton (growths of algae, etc., on underwater surfaces) to energy for a wide variety of other species.

wharfedale-nats.org.uk