Millions of insects in search of a mate That's why so many mayflies are buzzing around the Rhine at the moment

Bonn · Millions of mayflies are once again looking for a mate in Bonn at dusk. They are particularly fond of the Rhine. This also gives an indication of the quality of the water.

  Lots of mayflies are again buzzing around Bonn this summer.

Lots of mayflies are again buzzing around Bonn this summer.

Foto: Kristina Dyck

Keep your mouth closed, put on your glasses and, if possible, wear a tight-fitting scarf between your neck and your T-shirt If you don't take such steps, a romantically planned walk at dusk can quickly turn into a blind flight through a large swarm of small insects.

Mayflies, too, go in search of a mate on balmy summer evenings. But the tiny creatures don't have much time to do so. After a short wedding night, the males fall dead to the ground - a tragic end to a life just for love.

On the banks of the Rhine, on the bridges and now especially in the Rheinaue, millions of them are currently on the move. "Mask on," warns a Facebook user, posting a video from the Rheinaue in Bonn.

The natural spectacle has actually been happening for a few days. "This phenomenon is typical in high summer," explains Matthias Schindler from the Bonn/Rhein-Erft Biological Station. For the agricultural engineer and insect expert, mayfly swarms are always a good sign.

"They are an indicator of good water quality," he says. They are also protein-rich food for other creatures, such as bats. He suspects that the animals have lived in the Rhine so far and not in the lakes of the Rheinaue. The flies' love dance will last another week or two, he estimates.

In the flight stage, mayflies often live only a few hours or even minutes. They have to use this time to reproduce. Shortly after mating, the males drop dead to the ground, which can lead to traffic chaos. In August 1990, there were so many dead insects that the road on the Kennedy Bridge was as slippery as soft soap and had to be cleaned by the fire brigade.

Older than the dinosaurs

Mayflies are older than dinosaurs and have inhabited the earth for about 355 million years. Their German name (Eintagsfliege or one-day fly) is more than deceptive. As flying insects, they die after about two to four days at the most, but their complete life cycle lasts about two years. Before they set out in huge swarms on warm summer nights in search of a suitable mate, they have gone through various developmental stages in the water.

The eggs laid by the females in rivers and streams hatch into larvae that grow for at least a year (some species even two or three years) and during this time they moult several times before they come to the surface of the water and rise into the air to reproduce.

To have enough energy for growth and moulting, the mayfly offspring must eat a lot. The young mayflies must not be choosy about what they eat. They graze on algae beds and often supplement their diet with leaves and dead fish or other creatures. Even water fleas are not safe from them.

Mayflies are also a delicacy for other pond dwellers. Even the larvae that have just hatched from the egg are eaten by dragonflies and diving beetles. As soon as they have grown a little larger after a few moults, small fish come after them. And when they turn into flying insects after their penultimate moult and come to the surface, hungry birds are already waiting on the shore.

The fish are already waiting

If the mayflies have actually survived until then and take off, they are hunted by swallows, dragonflies and bats. If a mayfly finally falls into the water, dying after mating, fish are guaranteed to be right there. All they have to do on such summer nights is open their mouths.

Original text: Gabriele Immenkeppel

Translation: Jean Lennox

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