The dry fly is designed to match a natural aquatic fly or a terrestrial that has fallen and is floating on the surface of the water. The artificial must look as nearly like the natural as possible, for the trout, and in fact most fish, have eyes that would put the human 20/20 vision to shame. Trout take natural midges that are not much larger than a pin point and a size 12 fly probably looks as big as a dinner plate to them, so that a faulty tie is quickly perceived and passed up.
Therefore the dry fly must be fashioned with much thought to size, color, slant of wings and set of hackle. It should be well balanced so that it will land and sit perkily on the surface of the water, not being pulled this way or that by the weight of the hook so that it rests unevenly on the water. A good dry does not sit on its tail, nor yet lean on its chin. The nearer the parallel to the water, the better, even though many natural flies come down the current resting on their sides, with only one wing sticking upward.
While most anglers' class dry fly as the most demanding way of fishing, it is nevertheless the best way for a beginner to start learning how to fly fish. With a wet fly or a streamer, you are usually fishing the stream by your knowledge of where the fish may lie. But with a dry you are in on all the sights and sounds of the river, going by every signal a fish can flash to you as he eats or swims. The dry fly is also easier to cast than the wet, and the angler can see drag when it occurs and sees the fish strike and is therefore able to set the hook more quickly.
The whole principle upon which dry fly fishing is based is that eh fly should come down the stream exactly like a natural insect stuck in the surface film of the water. Despite this, there are many anglers who fish for years without discovering that their fly is practically never floating free. It may seem to be moving with the current, but look carefully, is it traveling faster than the bubbles and bits of flotsam on the surface? Is it going more slowly? If either of these is true you have drag and chances of a rise to your fly are slim.
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