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After emerging from the water to molt into winged insects, the larvae of mayflies undergo two distinct stages before lying the eggs and dying. The dun (sub-imago) is the phase right after the emergence, the insect is characterized by duller colors and cloudy wings, fringed with hairs. This stage can last from few hours to some days, after that the insect is molting a last time into a spinner (imago). The colors are turn brighter and wings transparent. Left female dun of Serratella ignita , the BWO. Right the same insect as spinner.