Mayflies are the most primitive winged insects. They pass the majority of their lives as aquatic nymphs in streams and lakes, and are not tolerant of polluted conditions. They are unique among insects in having an extra pre-adult (subimago) winged stage; it lasts only a day or two and is inactive, resting in vegetation. The true adult (imago) stage then emerges, leaving a shed skin, and takes flight, often to join a swarm over water or waterside vegetation.
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