I've been thinking more about Jason's comment about legs above, and recalled a mid-August day on the Delaware where I met a guy who was catching more fish than anyone around him. Although many Delaware anglers use comparaduns and cdc or snowshoe emergers with no hackle, he was using a somewhat sparsely hackled Catskill dry olive pattern with a single synthetic post wing. His explanation was that the hackle did create the illusion of mayfly feet. I also note that several buddies' favorite mayfly duns also use sparse hackle in thorax patterns. However, a number of flies that are commonly used on difficult fish have no hackle, including comparaduns (deer, CDC, and synthetic winged) and various CDC, deer hair, and snowshoe winged emergers, and hence have no illusion of legs or feet. Furthermore, the more heavily hackled Catskill flies that were popular in past decades have generally fallen out of favor for difficult trout on the flat water of hard fished spring creeks and tailwaters, though some still recommend them in riffled water. I'm guessing different elements of different flies produce different illusions or triggers. For some flies it may be the wing, for others the legs, while a shuck or body form may be the trigger. Possibly combinations of these work synergistically at times. In Maylfies Knopp and Corimer discuss this idea in their chapter on "The Mayfly and its Imitation," with a specific discussion of multiple triggers in the R.S. Quad fly design. Scientific studies of these "triggers" seem almost impossible for most of us, as we typically are more interested in catching a fish as we cycle through different flies, and success (or failure) may depend on other variables such as changes in tippet, changes in position, changes in casting, changes in the fish's position or pattern of eating, etc. rather than on fly design, however much we might want to believe the new fly led to a fish in the net--or lack thereof.
"He spread them a yard and a half. 'And every one that got away is this big.'"
--Fred Chappell