Subimago

Subimago, Fem.—Dull piceous: antennæ black: abdomen more than twice the length of the thorax: setæ brownish testaceous, mutilated: legs dull testaceous; fore-legs brown: wings gray; veins black, not strongly marked.

The subimago (a winged, but usually sexually immature stage) in the life cycle of mayflies is unique among living insect orders. The Ephemeroptera have two winged-instars, the subimago and the adult (or imago).

Subimagos emerge at the surface, fly to a support, molt immediately to the adult stage, live only a few hours, swarm, and mate shortly after ecdysis.

Subimagines flutter away immediately and rest at protected sites near the stream for up to 2-3 days, until the molt to the very short-lived adult occurs.