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Heptagenia longicauda (Scarce yellow May dun)

Recent studies on Heptagenia longicauda have shown that the nymphs are found on submerged bankside vegetation in the riffles and shallows of lowland rivers, particularly in areas of low current.  As a result, mature nymphs are easily collected by hand from aquatic vegetation from May onwards.  The nymphs feed on periphyton and organic detritus that they gather from the substrate or scrape from submerged surfaces.  It was previously thought that populations of H. longicauda and H. sulphurea did not co-exist, however it is now understood that these species do occur together, although H. longicauda tends to avoid watercourses where there are large populations of H. sulphurea.  It is likely that where these species do occur together, they occupy distinct micro-habitats and competition between the two species is negligible. 

 

In Europe, the flight period of H. longicauda is between May and September, although in warm weather, specimens have been collected as late as October.  British specimens have been taken in late May and early June. 

 


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Sub-imagines begin to emerge around sunset and continue to emerge whilst there is any light.  In Germany, swarms of imagines have been observed up to 22:30hrs when darkness prevented further observations.  It is possible that swarming continues after this time.  The imagines form loose swarms of approximately 50 individuals over bridges and other prominent bankside markers.  The adults are can be attracted to a mercury vapour light trap and on calm nights adults have also been observed gathering around bankside lights.

 

Most recent accounts of this species list three British records.  In 1868, the Reverend A.E. Eaton collected a subimago from near the Kennet and Holybrook by Reading.  A female specimen was collected from near the Thames at Staines by E.E. Austen on the 19th May 1904, whilst the most recent specimen, a male subimago, was collected on 28th May 1933 by D.E. Kimmins from a young birch tree, close to the River Wey between Tilford and Elstead.  There is however, a further British record.  J.F. Stephens first described and named Baetis longicauda in 1835 from a specimen he collected near Hertford in mid-June.  This additional record is relatively remote from the other known records being to the north east of London, rather than to the south west.