In their purest form, fishing flies are supposed to fool fish into believing that they are eating something that they normally feed on. Most avid fly tyers put a lot of study into what game fish eat in their natural environment so that they can replicate those organisms with tying materials to exacting detail. One would suppose that the tyer who can construct the most believable counterfeit has advantages in fooling his/her prey. It's called "matching the hatch."
"Attractors" are fly patterns that don't really fit into any of the hatch-matching game. In fact many attractor flies aren't reminiscent of anything that ever lived, or anything that fish would eat, or even anything that trout would regularly encounter in their natural environment. There are attractor flies in nearly every category of flies. Dry flies, wet flies, nymphs, streamers, and bucktails all have their attractor contingent. Attractor flies are regularly used in both freshwater and saltwater environments. Apparently there are attractor patterns designed for nearly every specie of fish that is pursued with fly rods. Some species, such as steelhead and tarpon are often more interested in attractors than they are in flies that look like real food.
Many of us trout fishers grew up reading the works of Skues, Marinaro, Scwiebert, Hafele and Hughes. All these renowned trout anglers have done a great deal of study into what trout eat, and then shared their ideas with the rest of us. Anglers who understand trout, and the environment in which they live have many advantages over anglers who don't.
So one wonders why a trout would eat a stonefly nymph made almost entirely out of purple Mylar. After collecting hundreds of insect samples from dozens of streams and lakes, I have never encountered any living critter which was purple and blindingly reflective. Yet, trout will often eat flies that are purple, or bright red for no apparent reason. Maybe they are the same reasons that trout will take spinners with whirling blades, or lures that are fluorescent chartreuse or fluorescent orange.
Fact is that many trout have been caught on oddball colored flies as well. One of the most popular dry flies, the Royal Wulff, would be hard to fit into any insect family.
The plot continues to thicken. Why would a trout eat anything with a brightly polished brass bead on its head? For that matter nothing lives in or along the bottom of our rivers that have bright white wings, yet thousands of trout have fallen for Bead Head Prince Nymphs which have both bright beads and white wings. It is evident that trout see their environment different than we do. Their eyes see spectrums that we don't.
Many anglers fish to gain this knowledge. It is fairly easy to study aquatic trout foods and attempt to replicate them with flies. As this game is played, we often find that our senses have much in common with our quarry. "Matching the hatch" is a reliable approach.
But, there are apparently many senses we don't have in common with trout. Organ array is different between us and them. For instance our brains are bigger, but we don't have lateral lines. It would be hard to believe that any of us would eat anything which had a brass bead for a head. But, then how many of us have been seduced by a gold watch or a pair of diamond ear rings?
Fact of the matter is, bead head flies work. (And so do diamond earrings.)