A Dry Fly Introduction by Dave Hughes
“Dry flies are designed to float on the surface of either moving water or still water and to coax the trout into striking up to take them.”
“Dry flies can be broken roughly into two groups. Searching drys, also called attractor or fancy drys, are designed to fish likely holding water and attract trout when they are not feeding in set rise rhythms and no single natural food form is abundant. Imitative drys are designed to resemble a particular insect species when it is present in dominating numbers and trout are feeding on it selectively.”
(From Essential Trout Flies, by Dave Hughes, published by Stackpole Books, p. 17. Used with permission from Stackpole Books.)