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What happens when a mayfly nymph becomes a subimago?

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A mayfly (Ephemera vulgata) with delicate, veined wings and long tail filaments clings to a green blade of grass against a blurred green background. — mayfly An adult mayfly (*Ephemera vulgata*) in Europe.

When a mayfly nymph reaches maturity, its skin splits down the back, and it emerges as a winged form known as a subimago (or dun).1

Once this transformation occurs, the subimago flies from the water's surface to a nearby sheltered resting place. Mayflies are unique among insects because they continue to molt even after developing functional wings. After an interval ranging from a few minutes to several days—most commonly overnight—the subimago sheds its exoskeleton one final time to emerge as the imago, or adult stage (sometimes called a spinner).1

The subimago stage is characterized by a softer body and duller, more opaque wings compared to the final adult form.1