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Baetis niger (Southern Iron Blue Mayfly)

Baetis niger is a widespread, though localised species found in running waters throughout the British Isles with the exception of Ireland.  Nymphs of this species typically crawl amongst in-stream vegetation in riffle areas of rivers and streams or swim in short, darting bursts swims 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 two generations per year - a slow growing winter generation and a much faster summer generation.  This results in a fairly long flight period, with adults being present between April and October.  Recent work on the River Test has found that the summer generation is considerably reduced, if not absent.

 


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Emergence of the adults is thought to occur at the surface of the water during daylight hours.  The males of this species can be found swarming throughout the day, and often swarming continues until dusk.

 

Once mated, the female fly either pulls herself under the water surface to lay her 1200 eggs directly on a partly submerged stone.  In some cases she will fly to the river, where she descends to the surface of the water and releases her eggs in several batches by dipping the tip of her abdomen onto the water surface.