Musings on mystical mayfly

MAY is the high point of the year for the fly fisherman. People with no interest at all in fishing have heard of the mayfly and the excitement it inspires in fish and fishermen when it makes its annual appearance.

The reason for the excitement is there is a large group of insects from many different families that spend most of their lives under water and then emerge, sprout wings and spend a short period, varying from a few days to a few weeks, flying around and mating, quite a lot of them in May.

Why should so many species of insect, from huge dragonflies to tiny midges, have evolved to spend part of their lives under water and part of them flying around in the air? The answer to the question seems to be that life under water (almost exclusively fresh water) seems to offer great advantages when it comes to feeding and growing bigger. But it is necessarily a rather sedentary life style which is a disadvantage if you want to inspect a large number of potential mates. The mobility of the flying insect facilitates the maximum dispersal of genes.

Of course, this leaves the chicken and egg question. At some stage in their evolutionary history did flying insects develop a taste for the under water life or did under water insects eventually develop the ability to fly? I don’t know the answer.

Some of these insects are called ‘dayflies’, based on the belief they only live for a single day as winged insects. This is almost completely untrue. Some of them lose their mouth parts when they gain their wings, which obviously means they are doomed to death from starvation but, unless they are eaten by a trout or a bird or meet some unfortunate accident they can live for up to a week. Some of them even go through a second metamorphosis, changing from what anglers call a dun into a spinner —- the fully sexually mature form which often differs in colour or shape.

Dragonflies and damselflies, on the other hand, not only retain their mouths they become fearsome airborne predators, the eagles and hawks of the insect world, catching and devouring smaller insects in flight. Most species live at least a year under water and several months in the air.

The word mayfly is a little confusing and tends to be used differently in America and Europe. In Ireland it usually describes a small group of species which are the largest of the ephemeroptera, or ‘up-winged’ flies. It appears at different times on different lakes and rivers but the hatch is almost exclusively in the last two weeks of May.