Dry flies are intended to float on or in the water's surface. Many, particularly the modern ones, are tied with buoyant or water-repellent materials such as polypropylene, antron or deer-hair to prevent them sinking. A large proportion of dry fly patterns, especially those designed for use on rivers, are imitative rather than impressionistic, tied to represent specific aquatic insects such as Mayflies, Caddisflies and various species of Olive.