How fish see a surface fly..

     The eyes of trout and salmonoids are set flat on each side of their heads. Their vision upwards is conically shaped and called their cone of vision or the Snell Circle, which decreases in size as fish near the surface and increases as they submerge. As a trout's line of sight penetrates the surface, the cone evolves into an oval-shaped window or its window of vision. However, ten percent of each side of the window is not visible, and fish cannot clearly see the fly until it reaches the outside edge. At this stage, the entire fly is not immediately seen, and the first glimpse of the fly is of the wing, next the hackle and then the body. Fish will make their decision to strike at this moment, if the artificial closely resembles the natural’s correct size and shape. The tail has little, if any, influence on the actual take, but it is important in assisting the pattern's floatation and balance.