
Mayflies come in many shapes and sizes! These are on the small end of what live in my area.
They are also the only ones I have noticed so far that have strongly dimorphic eyes. The eyes of the female (on the underside of the pencil) are what you'd probably expect. The eyes of the male (on top of the pencil) are split into two sections, with the upper section greatly enlarged. See this posting for details of the male's eyes.
--Rik