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Baetis rhodani (Large dark olive)

Baetis rhodani is one of the most common and widespread Ephemeroptera species.  It can be found throughout the British Isles.  Nymphs of this species typically live in the riffle areas of rivers and streams amongst the sand and gravel on the bed where they swim in short, darting bursts amongst the substrate.  They feed by scraping algae from submerged stones and other structures, or by gathering or collecting fine particulate organic detritus from the sediment. 

 

There are typically two generations a year, one with overwintering eggs that emerge in the spring and another that grows over the summer and emerges later in the year.  In warmer years there may be further generations resulting in adults of this species being recorded in every month.

 


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Emergence of the adults takes place at the surface of the water during daylight hours and at dusk.  The males of this species swarm throughout the day but typically swarming stops before dusk.

 

Once mated, the female flies to the river and lands on a partly submerged stone.  She then folds her wings and pulls herself under the water to find a suitable place to lay around 4500 eggs.  The eggs are laid individually alongside each other to form a contiguous patch of eggs.  Once completed, she will sometimes climb back out of the water and fly away, however more often than not, she will be swept away by the current.