Large Brook Dun Mayfly Taken at Baron's Haugh on 9th July 2016 using Panasonic Lumix LX5 in macro mode. Large Brook Dun Mayfly. Species: Order: Family: Ecdyonurus torrentis. Ephemeroptera. Heptageniidae. Larval Food: Adult Food: Length: Habitat: Flies/Found: Feature: Organic detritus from sediment. Approx: 2cm Riffle areas of rivers and streams. May - September. 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 at intervals.  After several visits to the water, the egg supply of up to 8300 eggs is finished and the spent female falls on to the surface.