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Cloeon dipterum is a species of mayfly with a Holarctic distribution. It is the most common mayfly in ponds in the British Isles and the only ovoviviparous mayfly in Europe. Males differ from females in having turbinate eyes.
In common with other members of the genera Cloeon and Procloeon, C. dipterum has a single pair of wings. This is also reflected in the specific epithet dipterum, which is from the Latin di-, meaning two, and the Greek pteron, meaning wing, and in his original description, Carl Linnaeus stated Inferiores alæ vix existunt ("smaller wings hardly present").
The compound eyes of C. dipterum show a striking sexual dimorphism, whereby females have lateral apposition eyes, while the males' eyes have an additional dorsal "turban-shaped" parts that function as superposition eyes. These extra eyes are thought to enable the males to locate isolated females in the mating swarm.
Cloeon dipterum is unusual among mayflies in being ovoviviparous, and is the only ovoviviparous mayfly species known in Europe. Females lay eggs 10–14 days after mating, and the eggs hatch as soon as they hit the water. The larvae can survive for months in anoxic conditions, an adaptation which is necessary to survive the winter in ponds which freeze over and therefore contain little dissolved oxygen. In captivity, adult females have been kept alive for up to three weeks.
Cloeon dipterum is widespread across Europe and Asia. In the British Isles, C. dipterum is the commonest mayfly in ponds, with around 40% of all ponds containing C. dipterum, rising to 70% in the south. In 1953, a single female Cloeon dipterum was discovered in Illinois, having not been previously recorded in North America, and was found near Lucas, Ohio in 1960. The species is now known to have a wide distribution in North America. Individuals of C. dipterum from Madeira are now placed in a different species, Cloeon peregrinator.
