Long Tailed Mayfly (Palingenia longicauda): a Few Hours of Life on the Danube

On a few evenings at the start of summer, clouds of large, pale mayflies rise from the water of the Danube, dance in the low light and vanish within hours. The long tailed mayfly is the largest mayfly in Europe, one of the rarest, and a sign of clean, living rivers. In the Danube Delta it is back.

Three years underwater, a few hours in the air

Almost the whole life of the long tailed mayfly happens out of sight. The larvae dig U shaped burrows in the clay banks and bottoms of large rivers and live there for about three years, feeding on organic particles. Then, on a few evenings at the start of summer, they rise to the surface all together and turn into winged adults.

The adults have no working mouthparts and cannot feed: their only task is to mate and lay eggs. Within a few hours it is all over, and the water is covered with their bodies and wings, a feast for fish and birds.

The wedding flight

The males emerge first and fly low over the water, searching for the females. After mating, the females fly upstream, sometimes for kilometres, before laying their eggs on the surface of the water. This flight against the current makes up for the drift of the eggs and larvae, which the river carries downstream.

Description

The long tailed mayfly has a long, slender, yellowish brown body and broad, smoky, slightly yellowish wings with a fine net of veins. The males carry two very long tail filaments, longer than the body; the females’ tails are much shorter.

A survivor

The species once lived in almost all the large rivers of Europe, from France and Belgium to the Black Sea. Pollution and the regulation of rivers wiped it out from most of them, many already by the 1930s. For decades its last strongholds were in the Tisza basin, in Hungary, where its mass emergence in June is famous as the “Tisza blooming”.

In the Danube Delta it was considered possibly extinct in the 1990s and was added to the Red List of the Danube Delta Biosphere Reserve in 2000. Then, in 2007 and 2008, researchers documented swarms on the Sfantu Gheorghe branch, near Murighiol, and later near Tulcea. Studies of the riverbed found larvae along the Lower Danube, with up to about 125 larvae per square metre on the Sfantu Gheorghe branch. For some years now, swarms have been filling some lakes and channels of the Danube Delta again at the start of every summer.

Interesting facts

• Mayflies are among the oldest winged insects on Earth, older than the dinosaurs.
• Researchers consider the Lower Danube one of the refuges where freshwater species like this one survived the Ice Ages.
• Because the larvae need clean water and stable riverbeds, the mayfly is a living indicator of the health of a river.
• In Germany, scientists collected eggs on the Tisza and released millions of young larvae in the Lippe and the Oder, hoping to bring the species back.