Well, for lack of a better title I'll call this page Fly Tying Theory. The basic focus of this page is to understand why concerning fly tying. Why are certain materials are on certain flys, and what to add to gain particular effects. When I was first starting I was trying to tie all kinds of tiemco flys at a local shop until the shop owner must've pitied me and let me in on a not so little secret. He said, "there are flies that catch fish, and there are flies that catch fisherman."
I will never forget what he said, and my interest is in catching fish. I respect hobbyists who tie flys for a frame, but I tie them purely to fish. This simplifies things for me, but then requires some materials knowledge and some entomology knowledge to determine why we use what we use.
Tying flies for fish...
There are basically 2 types of flys. Imitations and attractors. Imitations have to have some vague resemblance to natural trout food while attractors don't really look like anything except sometimes obnoxious. This is sometimes not even black and white but gray. As a fly tyer be creative and flexible.
To the left this green drake fly imitates the general form and color of the green drake mayfly. It will be beneficial for you to observe the insects you want to imitate because it may eliminate a tying step. For example catskill dry flys like the one to the left usually use a wing of real or imitation wood duck. This wing is then split as if to make open wings. If you observe the natural you'll see that the real fly doesn't sit with split wings. The real fly keeps it's wings together like a sail. If wanted, this observation could eliminate a tying step of splitting a wing saving tying time.