Species Profile – Yellow Mayfly

I wrote this profile as coursework for the course I did at Centre for Alternative Technology in North Wales, and thought I would share it here for anyone interested!

Species: Heptagenia sulphurea

  • Common Name: Yellow May
  • Family: Heptageniidae (Stone Clingers)
  • Order: Ephemeroptera (Mayfly / Upwinged Fly)
  • Trophic Level: 2 – Primary Consumers (herbivores that eat plants)

Physical Features

The Heptageniidae family are stone-clingers.  They have flattened broad bodies and with sturdy legs [3].  With eyes on top of their head, each of their gills consists of a single plate with a feathery tuft [2]

Like all Mayfly nymphs the bodies of Heptageniidae terminate in three distinct caudal filaments, which are particularly long on nymphs of this family [4].

The Yellow May is pale in colour, often with yellow pigmentation.  Unlike some other species in the same family, the Yellow May’s pronotum – the plate-like structure covering the thorax – lacks backwards-facing lateral projections.  Its femur lacks a dark spot, and its first pair of gills is small [3].

Life Cycle

Like all Mayflies, the Yellow May undergoes incomplete metamorphosis, in which the insect hatches from an egg and then goes through four stages – egg, nymph, subimago and adult (imago) [4]. In each nymphal stage the insect looks like a small version of the adult, moulting its skin to allow further growth.  At the final nymphal stage the skin splits down the back and a winged form – the subimago or “dun” – emerges and leaves their empty nymphal “shucks” on waterside or emergent boulders [3].

The subimago then flies to rest nearby for a short time, after which the skin is shed for the last time, and the adult imago stage – sometimes called a spinner – emerges.  It is interesting to note that Mayflies are the only insects that moult after developing functional wings [5].

The adults typically emerge during daylight or at dusk and the males swarm from afternoon to dusk.  Once mated, the female flies upstream and descends and dips the tip of her abdomen on to the surface of the water at intervals to release a few eggs.  Once the egg supply is finished the spent female falls on to the surface [2].

There is one generation of this species per year.  It often has a group of fast-growing individuals which emerge in May or June, as well as slower-growing group that emerges in August and September [2].


Stone-clingers are perfectly adapted to adhere to and move around the solid surfaces of stones, even in the fastest sections of mountain streams.  Their substantially flattened bodies reduce drag from the current, and their strong legs are spread to the sides with ending with a single tarsal claw [4].

Well-developed eyes are placed on the back of their flattened head, allowing them to look away from the stone to spot predators.

Nymphs scrape algae and other microorganisms by using their adapted mouthparts, on which their labial palps possess setae, thereby forming brushes [4].


Like all Stone-clingers, the Yellow May occurs on the surface of submerged rocks and logs, where it searches for food [4]

They usually swim in short bursts, interspersed with periods of clinging to submerged plants and stones [2].  They quickly hide themselves under stones when disturbed.  This has been known to lead to fish turning over stones in order to find them [4].

Food and Feeding

Stone-clingers feed either by scraping periphyton – microscopic algae and other organisms – from the substrate or by gathering fine particles of organic detritus from the sediment [2]

UK Distribution

Yellow May nymphs are common in flowing waters of streams and rivers throughout the British Isles [2,3,4]. Occurrences have been officially recorded in the locations shown on the map [1].  It is often abundant in limestone and chalk streams as well as along the wavy shores of some calcareous lakes.

Their presence is often an indicator of good water quality, because they are relatively intolerant to pollution [4].

UK Distribution of Yellow Mayfly


  1. https://species.nbnatlas.org/species/NHMSYS0000066973
  2. http://www.riverflies.org/heptagenia-sulphurea-yellow-may-dun
  3. Freshwater Life (Collins Pocket Guide) Paperback – 2007 – Greenhalgh & Ovenden (Authors)
  4. https://lifeinfreshwater.net/mayfly-nymphs-ephemeroptera/
  5. https://www.britannica.com/animal/mayfly#ref266454

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