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Siphlonurus lacustris (Summer mayfly)

Siphlonurus lacustris is the most common of the British Siphlonuridae.  It occurs in localised pockets throughout the British Isles, including Ireland.  Nymphs of this species typically live in the pools and margins of rivers and streams, lakes, and pools at high altitude.  The large nymphs are good swimmers and typically swim in short, darting bursts.  They feed by gathering or collecting fine particulate organic detritus from the sediment. 

 

There is one generation a year, which overwinters as eggs and emerges between May and September, although the main flight period is from June to early August.

 


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Emergence of the adults probably takes place partially or entirely out of the water on a stick, stone or plant stem during daylight hours.  Males of this species can be found swarming throughout the day, including at dawn and dusk.  Once mated, the female returns to the water to release a few eggs by dipping the tip of her abdomen into the water at intervals, or by actually settling on the surface for short periods.  After several visits to the water her egg supply of up to 2500 eggs is finished and the spent female falls on to the surface.