Potamanthus luteus (Linnaeus, 1761)

the Yellow mayfly

Nymphs: Streamlined nymphs with thick, feathery gills that are held at the sides of the body.

Adults: Large flies with three tails and large hindwings.  Male sub-imagines of this species have a dull yellowish orange body with a distinctive broad yellowish brown stripe along the back of the body.  The body is marked with a pair of pale lines and dots on the upper surface of each segment and a single dark dot on the side of each segment.  The wings are dull yellow and the cross-veins are a dark reddish colour, particularly at the wing tips.  The tails are brown and become progressively lighter further from the body.  The eyes in both sexes are yellowish green.

The imago is similar to the sub-imago however the wings are brighter yellow and the veins vary between dull yellow to greyish yellow.  The tails are yellowish with dark rings and the eyes are yellowish olive.  The female is similar to the male, albeit a slightly brighter shade of yellow and the eyes are dark brown. 

Nymphs: Potamanthus luteus has very distinctive yellowish nymphs, with branched feathery gills.  The only other species with similar feathery gills are the Ephemera, however in Ephemera species the gills are held over the back of the body while in Potamanthus they extend outwards from the body.  In addition, in Ephemera species the mandibles are large and project past the front of the head.

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

P. luteus can be separated from the Heptagenia species by the number of tails present.  In Heptagenia there are only two tails while in P. luteus there are three. 

Nymphs of this species live chiefly in the pools and margins of larger rivers.  They are poor swimmers that typically live amongst stones and sand in side pools.  The nymphs feed by gathering fine particulate organic detritus from the sediment.

There is one generation a year, which overwinter as nymphs and emerges between May and July

Potamanthus luteus is a very rare species with most recent records from the Rivers Usk and Wye in Herefordshire.  It is thought that this species has since been lost from the River Usk and has recently suffered a population crash on the River Wye.  A voucher specimen is required for all records of this species.