I will take a stab at this LOL. Emergers are insects that havent completely broken through the surface film or havent transformed into adult insect just yet. If you see fish feeding on bugs just under the surface but dont see any adult insects on the sruface, then this is a good indication that they are feeding on emergers. frequently emergers are tied with a tailing shuck since the insects havent yet completely shed their "layers" that have as nymphs before turning into adults. As often as not I tend to catch more fish during bwo hatches using a sprout emerger even though I see lots of duns (adult) on the surface. Emergers are an easier target than the duns since the duns could fly off at any second (depending on how long it takes for their wings to dry). An example of an emerger would be the previously mentioned sprout emerger while a good dun/adult pattern for bwo would be a thorax blue wing olive. And if what I just said makes sense, then more power to you. Probably a bit off in the true definitions but thats how I view it.
Attractors are flies that can indicate a number of insects(but frequently are used when certain insects are hatching). My favorite example of this is the stimulator..it could imitate a stonefly, caddis, or in larger sizes a grasshopper (this is my goto fly during the salmonfly hatch and the october caddis hatch). Other examples of attractors are trudes,royal wulffs, royal humpys, and for nymphs the prince nymph. Also a charactersitic of the above flies are that they are brightly colored (though not all attractors are brightly colored such as Gold Ribbed Hairs Ear Nymph or the Adams which can be used for various mayfly hatches or even a caddis hatch). Each of these can be very productive if used in the right sizes for a variety of different insects.
Now just waiting for someone to come along and give a better explanation of both. LOL
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Take care everyone and cya around. Mark
Take care and cya around,
Mark