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When you fly fish you're attempting to fool the fish into thinking your fly is its natural prey. Flies are made specifically to imitate the foods fish eat in terms of size, color and shape. They can be homemade or bought in different sizes and patterns. What you choose and use depends on the fish you are targeting, the time of year, where you are fishing, and what the fish are feeding on.
Understanding fly hatches in the area you would like to fish can be very helpful. To find this information, contact local bait shops and guide outposts based on where you'd like to fish.
Surface or Dry Flies
These types of flies float on top of the water. As a result you can see the fish come to the surface and take the fly.
Dry flies imitate adult aquatic insects, such as mayflies, caddisflies, stoneflies, midges or other flying or emerging insects. They are used mostly for trout, but bass and panfish may target them as well.
Terrestrial flies imitate ants, beetles, spiders, caterpillars, crickets, grasshoppers and other insects that live on the land and sometimes fall into water. These types of flies are commonly used for trout, bass and panfish.
Very large flies can be tied to imitate large insects, frogs or mice. They are used mostly for bass and northern pike, but can also be used by the more advanced fly fisher to target very large fish after dark.
Sinking or Wet Flies
These types of flies are made with materials that cause them to sink below the surface of the water and imitate insects, crayfish and small fish.
"Nymphs" are wet flies that imitate immature insects and small crustaceans that live underwater and are popular with trout, bass and panfish.
"Streamers" are wet flies that imitate crayfish and small prey fish. They are used to attract larger predator fish, such as northern pike, muskellunge, bass and large trout.
Check out this Fly Tying 101 document if you're interested in tying your own flies!
Images courtesy of Ann R. Miller.