Hackled flies are meant to float on the water’s surface, imitating all sorts of adult aquatic or terrestrial insects that fall into the water. The hackle fibres closely mirroring the imprint a real insect would create on the surface film.
Traditional winged dry flies use the surface tension of water to float. The fly will ride on the hackle and tail. The wing creates a silhouette taking on the appearance of a natural insect.