Female genital dimorphism in a spider
(Published in Amer. Mus. Novitates, 2001, see PDF; and J. Zool., London, 2001, see PDF)
 

Above: male Ciboneya antraia. This genus is endemic to Cuba, and this particular species is unique among arthropods by showing female genitalic dimorphism: females come in two distinct morphs (below), and each female is able to produce offspring of both morphs.

Macrogyne (above) and microgyne (below) females of Ciboneya antraia.