The fly is the bait you use for fly fishing. Unlike traditional casting bait, the fly is as light as a feather and relies on the weight of the fly line to cast it out onto the surface of the water, where it imitates the skirting motions of a natural insect to attract fish. Since they are designed to look like the insect there is a fly for virtually any insect you can think of, which means there are thousands of them to choose from; so how do you know which one to use?