Heptagenia sulphurea - Yellow May

The nymph of the Yellow May is a stone clinger and is far more common on spate rivers than on chalk streams. For example there are often prolific hatches in June on the River Teifi in West Wales. The main hatch usually begins in the second half of May and continues through June and well into July.

The dun

The duns (the male is shown above), with their brilliant sulphur yellow wings, hatch in open water from mid morning onwards, and their numbers increase towards dusk. The female is similar to the male, but with smaller eyes.

The spinner

On warm evenings from sunset until dusk the female spinners return to the water to lay their eggs. On some spate rivers the result is a selective evening rise.

The male spinners are similar to their mates, but rather darker; they less frequently end up on the surface of the water.

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