Materials.

[Fly Tier]

Materials create a fly's animation (action),realism (look and feel),and other specific properties.As an example,you cannot tie a standard style dry fly (Adams) with soft, webby hackle and expect it to float above or on the waters surface.Conversely,you cannot expect a needlefish imitation to portray the transparent and iridescent colors and the quick-paced animation of the natural if it is construced with chenille.You would not try to imitate the eye of an adult caddis,but the eye is of paramount importance for a squid pattern.In other words ,size,shape,silhouette,color,animation,visual realism,general  attractiveness,castability,and fishability are all the direct result of which and how materials are incorporated into a fly.