Artificial flies are designed to resemble all manner of fish prey and are typically used with a fly rod and a fly fishing reel.
The fly is at once the hope of the beginner and the veteran. There are many old-timers who will use nothing else, for wet flies have paid off for them over the years with some mighty big fish that seldom rise to dries. And the novice, providing he can wade to within easy reach of a current where fish are likely to lie and can heave the fly out there to drift in the current, can usually manage to hang a trout, even though his method may involve more dunking than finesse.
Nevertheless, to really fish a fly with the greatest success calls for a knowledge of fish location, their feeding habits and the ability to cast the fly to the greatest advantage. With the exception of those times when a wet fly is played to imitate a moving bit of food, such as a minnow or shrimp or nymph, it seems to me that the one big thing with wets as with dries, to get consistent hits, is the free float, without any semblance of drag. Therefore, the up-and-across cast pays off best. With that presentation, the fly should float along naturally with the current and the trout will rise to it much as they do to a dry fly, thinking it is what it's meant to be, a downed fly, floating downstream.
On the up-and-across cast, I allow the fly to float motionless for almost the complete length of its downward swim until it begins to swing in below me in the current. Then I often manipulate the rod tip so that it will cause the fly to hop forward in a series of short jumps, a couple of inches at a time, and continue that technique until the fly is directly below me in the water, and then I bring it my way for 10 or 12 feet before lifting it as quietly as possible from the water.
This up-and-across-stream cast permits delivery without letting the fish see the angler. Since fish usually lie facing into the current, the angler is far enough below, or off to the side from them that they will not so readily spot him. It's true that a good water can get into the stream and fish a fly on down and scarcely disturb the fish except for the ones he drives off their feeding stations as he wades. But the average angler is usually too noisy in his wading, waves his arms too much, and makes no effort to hide, so the fish can hardly avoid seeing him. Sometimes I wonder that many of them catch a fish at all, because they start at the head of the pool standing on a rock or a bank high above the water, in plain view of any trout within 50 feet, and since the trout's eyes are aimed upward anyway, that wet fly man is at a great disadvantage before he even starts to fish.
But in such a situation a wet fly cast straight upstream, from farther down, would have brought him hits from the very fish that thus spot him. In using the straight upstream cast in fast water, however, a very short line must be thrown in order to be able to retain any control and be ready to strike. A cast of 25 or 30 feet is plenty long enough, and then the line should be stripped at approximately the same speed at which the current is riding the fly down towards the angler and as he raises the rod, he is then ready for the strike or for the pickup for the next cast.
Sometimes when trout seem to be lazy, they can be stirred into striking by throwing the wet fly across the current and then bringing it back at once in a series of foot-long jerks, much as a streamer fly is fished. This is one of the times when I am convinced the fish hits the fly as a minnow, not a fly.
When using the across-current cast, the line must be mended and the fly led down with the rod tip ahead of the line. With that method the angler has good control of the line, avoiding a belly that would delay his strike to the fish; and the fly is presented broad-side so the fish has a better view than if it were hurtling down head or tail first. Then, when the fly starts to swing in below, it can be given motion by imparting short jerks which should attract a trout's attention and cause him to go for it.