Large Dark Olive Mayfly

• Mayflies are the only group of insects that moult after having wings: they emerge from the water as a subimago, a dull, matt winged stage, and a few hours later moult again to become the definitive adult.
• The adult has no functional mouthparts and no working gut: it lives only to breed and dies the same day.
• Its abundance is a classic indicator of water quality, and the biotic indices used to assess European rivers rest in good part on mayflies like this one.
• Unlike most European mayflies, which have only one generation a year, it produces a second generation in the same season.
• Its nymphs go on taking up oxygen normally even with their gills removed, which is not the case in burrowing mayflies such as Ephemera.
• It is one of the key species in the biological water-quality indices used on European rivers.

Skeleton type

Invertebrate - chitin exoskeleton (insect)

Habitat and environment

Fast-flowing, well-oxygenated stretches of rivers and streams

Sources consulted: First Nature, Ephemeroptera of Britain and Ireland · estudios comparados de respiración en ninfas de efemerópteros · First Nature, ficha Baetis rhodani · guia de emergencias de Ephemeroptera europeos

Best time: Adults visible mainly between late autumn and early spring; the main emergences are in March-April and in September-October