Why is the winged subimago stage sometimes mistaken for a separate mayfly species?

The winged subimago stage is often mistaken for a separate mayfly species because it closely resembles the final adult stage, or imago, in its overall appearance.1

However, the two stages are distinct developmental phases. The subimago, also known as a dun, is a winged form that emerges from the nymphal skin. Compared to the mature imago, the subimago is softer, duller in color, and possesses shorter legs and tails. Furthermore, the wings of a subimago are generally opaque and often tinted with gray, blue, yellow, or olive, sometimes featuring a mottled appearance due to heavy pigmentation along the veins that does not persist in the adult. Because mayflies are the only insects that molt after developing functional wings, the subimago must undergo one final molt to reach the imago stage, leading to the common, yet incorrect, assumption that these two winged forms represent different species.12