Do you see the Yellow May Dun on your river?
The Yellow May Dun, Heptagenia sulphurea, is a beautiful bright yellow mayfly, the adults of which are easily recognised by their distrinctive colour and two long tails.
The Yellow May Dun is the focus of the Riverfly Partnership 2010 Species Survey.
YOUR RECORDS COUNT - Please let us know if when you see the Yellow May Dun on your river. The information will contribute to a study by the Ephemeroptera Recording Scheme on both the current distribution and flight period of the species.
The distribution map indicates that the species could be found across mainland Britain, with lower abundance in the south east and eastern areas.
How to send in your records
On line at: www.brc.ac.uk/mayfly/recording.php
By post: Print the postcard below and return by post.
Note that colour copies of the postcard are available direct from the Riverfly Partnership email: info@riverflies.org or telephone 0207929 6966.
Yellow May Dun © Stuart Crofts
» Yellow May Dun postcard – PDF-File, 896.7 KB
Is it a Yellow May Dun?
Is it a Yellow May Dun? The sub-imago (or dun) and imago are bright yellow, a characteristic shared with only two other species: the Yellow Mayfly (Potamanthus luteus) and the Scarce Yellow May (Heptagenia longicauda). Adults of the Yellow May Dun can be readily separated from the Yellow Mayfly by the number of tails present. The Yellow May Dun has two tails, whereas the Yellow Mayfly has three. The Yellow May Dun and the Scarce Yellow May are superficially similar. However the Yellow May Dun has up to three black dots on the body above the middle leg, and never any dots above the hind leg. Whereas the Scarce Yellow May 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.
Distribution map of Yellow May Dun
Red - widespread Pink - localised
Life cycle and habitat
The flat bodied nymphs of the Yellow May Dun live predominantly in the riffle sections of larger rivers although the species has also been found along the wave lashed shores of some calcareous lakes. The nymphs feed either by scraping algae from the surface of submerged stones or by gathering fine particulate 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, commonly with a group of fast growing individuals which emerge in May or June and a slower growing group that emerge throughout the summer, peaking in August and September. There is anecdotal evidence that the ratio of individuals in each of these groups is changing. It is thought that fewer individuals are emerging en-masse in the early part of the year and instead there are more hatches in trickles throughout the summer.
Emergence of the adults typically takes place during daylight or at dusk and the newly emerged sub-imago characteristically flies upwards towards the tree tops. The species can often be identified by its bright yellow colour together with its upward direction of flight.
As imagos the males of the Yellow May Dun swarm from afternoon to dusk. This often happens well above the river and it may be difficult to see. Once mated, the female flies upstream and descends to the surface of the water to release 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 depleted and the spent female falls on to the surface.
