Function
I feel that function tends to be produced by the other aspects previously addressed, but it is worth mentioning that the most important part of a fly is whether or not it functions as the tyer intended it to. The classic adage of, “Form Drives Function” comes to mind. The design of the pattern should always be geared towards how the fly should be fished. Although there is much to be said about color, proportion, aesthetic appeal, and intricacy, if the fly fails to fish as it has been intended to, then it is a complete waste in fly design terms. A great example is a perdigon pattern. The entire point of the perdigon tying platform is to produce a fly that will drop quickly in the water column because they typically involve a heavy bead, and don’t possess characteristics that produce drag, such as lots of hackle, dubbing, or rubber legs. If the tyer is looking to tie a pattern that will fall quickly and have little drag, but they cover it in dubbing and hackle, then they have failed in the design of their fly. Likewise, if they set out to tie a very bulky fly that intentionally produces a lot of drag, that will hold its position in a seam because of the incorporated drag, but they tie a very slender and drag-free nymph that relies on a synthetic body wrap and a UV coating, they have failed to construct the fly to their own fishing specifications. A good fly tyer is always designing around fishing conditions, rather than aesthetics or already established tying methods. These methods may or may not be helpful and applicable, but they should not be the first angle of attack when designing new patterns, or seeking to tweak and alter old ones. Remember, “Form Drives Function”.