The subimago is covered with tiny water-repellent hairs that make it almost waterproof.    From Wordnik.com.  [Insecta (Aquatic)] Reference

Perhaps the subimago in this genus of Ephemerids, as in certain others, is the permanent aerial state of the female.    From Wordnik.com.  [Scientific American Supplement, No. 385, May 19, 1883] Reference

At the end of their development, juveniles swim to the surface where they moult into a sexually immature subimago stage.    From Wordnik.com.  [Insecta (Aquatic)] Reference