Nature on our doorsteps: Mayflies

Rosaleen Dwyer is the County Heritage Officer at South Dublin County Council – every week she gives us an insight into nature on our doorsteps and the beautiful biodiversity of its plants and wildlife.

Mayflies are a very ancient group of insects, dating back to the Carboniferous period when the world was covered in swampy forests and coal was in the process of forming.

There are over 3,000 species of Mayfly around the world, with about 33 occurring in Ireland.

Despite their name, adult Mayflies are seen not just in May.  Some species also occur in summer and early autumn.

Mayflies need very clean rivers and lakes in which to complete their life cycles.

As most species do not tolerate polluted water, their presence therefore indicates that these habitats are clean and healthy.

They often occur in very high numbers, making them an important source of food for fish, birds and bats.

Adult Mayflies have four delicate lace-like wings which they hold closed vertically above their long thin bodies when they are at rest.

They have up to three long thread-like bristles at the end of their body, and while these may look like stings, the Mayfly is a harmless insect.

It does not sting or bite.

Some females produce a ball of eggs which they hold under the tip of their bodies before dropping it into the water

Adult Mayflies, in fact, do not feed at all.

Their only role as an adult is to mate and lay eggs.  For this reason, many Mayfly adults do not live longer than a day or two.  Some only live for a few hours.

The young of the Mayfly is called a nymph, and this has a much longer life span than the adult.

Nymphs live underwater, where they feed on plants and algae, and they can take up to three years to mature.

During this time, they shed their skins several times as they grow, before they finally emerge into the air as an adult.

This emerging adult, however, needs to mature a little further before it becomes a true adult.

While it looks very similar to the true adult, it needs about 24 hours before it too sheds its skin and turns into the final adult form that can reproduce.

This additional ‘pre-adult’ stage makes Mayflies unique in the insect world.