Ephemera danica - copyright Allen BeecheyMayflies are an ancient group of insects and the only group of true flies that have two adult stages; the dun (sub-imago) and the spinner (imago). The larval stage of mayflies is aquatic. There are 51 species in the UK and they range in size from the tiny Caenis (body length approximately 4 mm), also known as the Anglers' Curse, up to the classic chalk stream mayfly, Ephemera danica (approx. 25 mm in length), sometimes known as the Green Drake. Mayflies are a good indicator of the quality of a river with many of the species only able to survive in the cleanest of rivers.  The presence of species such as E. danica and Serratella ignita (the blue winged olive) on the River Chess is indicative of the good quality of the water in the river. However, these populations are under threat from pollution, such as run-off from towns and sewage. You can read more about the threat to the river's water quality here.