Heptagenia sulphurea (Müller, 1776)

= Ephemera sulphurea Müller, 1776

Yellow May Dun

Nymphs: Flattened nymphs with broad heads.  The eyes are large and placed on the back of the head.  Typically found in faster flowing water.

Adults: Medium to large sized flies with two tails and large hindwings.  The wings, body and legs of the sub-imago are, as the scientific name suggests, all bright sulphur yellow.  In addition, both the sub-imago and the imago have very distinctive blue eyes.  The male imago has a bright golden brown body, dark brown wing venation and a grey leading edge to the forewings.  The female imago is similar but has a pale yellow leading edge to the forewings. 

Nymphs: H. sulphurea nymphs are separated from other Heptagenia species by the form of the gills and also by the conspicuous black and white pattern on the body.  Whilst the current key to the nymphs of the British Ephemeroptera (Elliott, et. al., 1988) makes clearer the distinction between H. sulphurea and H. longicauda, the distinguishing feature used, the filamentous part of the gills, is often damaged during collection.  H. sulphurea can also be separated from H. longicauda by the alternating dark-light pattern on the tails and the absence of small protuberances on the pronotum.

Kageronia fuscogrisea can be separated from H. sulphurea by the shape of its gills, which are produced, into a sharp point. 

Heptagenia species can be seperated from Electrogena species by the absence of a long fringe of hairs on the femur (section closest to the body) of each leg.

Adults: Heptagenia sulphurea is a very distinctive fly.  The sub-imago is bright yellow, a characteristic which is shared with only two other species: Potamanthus luteus and Heptagenia longicauda. 

H. sulphurea can be separated from P. luteus by the number of tails present. In H. sulphurea there are only two tails while in P. luteus there are three.  H. sulphurea and H. longicauda are superficially similar however H. longicauda has a pair of flesh coloured rings on the femur of the front leg, and a single black dot on the side of the body above the hind leg.  In H. sulphurea there is often up to three black dots above the middle leg, however there are never any dots above the hind leg. 

Heptagenia sulphurea lives 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.

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.

Heptagenia sulphurea is a widespread and common species, which is found throughout the British Isles.  A voucher specimen is required for records of this species from the South East of England where it is relatively rare.