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Heptagenia sulphurea (Yellow May dun)

Heptagenia sulphurea is a widespread and common species, which is found throughout the British Isles.  Thenymphs of this species live chiefly in the riffle sections of larger rivers although it has also been found along the wave lashed shores of calcareous lakes.  It feeds either by scraping periphyton from the substrate or by gathering fine particulate organic detritus from the sediment.  The nymphs usually swim in short bursts, interspersed with periods of clinging to submerged plants and stones.  There is one generation per year which often has a group of fast growing individuals which emerge in May or June and a slower growing group that emerges in August and September.

 


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Emergence of the adults typically takes place during daylight or at dusk and the males swarm from afternoon to dusk.  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 of the water at intervals, or by actually settling on the surface for short periods.  After several visits to the water the egg supply is finished and the spent female falls on to the surface.