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Ecdyonurus torrentis (Large brook dun)

Nymphs of Ecdyonurus torrentis are typically found in riffle areas of rivers and streams.  They are usually found clinging to submerged plants and stones, although they may swim if disturbed.  E. torrentis feeds either by scraping periphyton from the substrate or by gathering fine particulate organic detritus from the sediment. 

 

There is one generation a year, which usually overwinters as nymphs and emerges between May and September, although in some years adults can be found as early as March.  The flight period is often related to the altitude of the emergence site.  In upstream reaches the flight period can last for up to three months, whereas in the lower reaches the flight period may be as short as one month.

 


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Once mated, the female flies upstream and descends to the surface of the water to release a few eggs by dipping the tip of her abdomen on to the surface at intervals, or by actually settling on the surface for short periods.  After several visits to the water the egg supply of up to 8300 eggs is finished and the spent female falls on to the surface.

 

Due to the problems with identification of nymphs, it is difficult to compile a complete distribution for this species.  E. torrentis is however, thought to occur in suitable habitats throughout the UK and Ireland.