Éventail présent au bout de la patte médiane d'un patineur d'eau ou punaise d'eau

Isolated feathery fan-like structure on the tip of the middle legs of a water strider, rhagovelia. The fan was dissected and mounted between a slide and cover slip with the aid of a transparent viscous solution. It was observed using an optical microscope at a 20X magnification. Water-repellent hairs on the legs of these insects enable to move across water. They are also able to travel upstream along rivers, with the aid of fan-shaped structures on the tips of their second pair of legs, which they use as swimming fins. As these fans exist in rhagovelias alone, they offer a good model for studying the emergence of so-called "new" structures (evolutionary innovations). These water striders have two hitherto unknown genes that must be expressed in order for the legs to develop with an intact fan. One of these genes is of early origin, as it is present in the common ancestor of all heteroptera bugs, whereas the second gene is more recent, existing in rhagovelia alone. The striking similarity between the two genes suggests that a genetic mutation may have given rise to the more recent gene by duplication of the ancestral one.