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sub-imago (dun) In Ephemeroptera (and unique among the Insecta), a pre-adult, winged stage through which the insects pass. The aquatic nymphs emerge on to the surface of water or on to rocks, and take from a few seconds to several minutes to change into sub-imagos. Emergence times vary from species to species, and the sub-imago may last from minutes to a whole day before moulting to the adult stage. Some exotic species breed and die in the sub-imaginal stage.