An artificial fly that floats on the surface of the water. The traditional requirements for a dry fly are:
The authoritative definition of the dry fly comes from Vincent Marianaro's In the Ring of the Rise (1976)."We begin with the proposition that no matter how dry the fly is, it must touch the water and be exposed to the air at the same time. If this idea is carried out to its logical conclusion, all must agree that if the smallest portion is exposed tothe air no matter how deeply submerged the fly may be, it is still a legitimate form of the dry fly." Thus, low-floating emergers and swamped spinners are "dry" when they touch air. In short, Marinaro conceded that "any fly, natural or artificial, touching the surface film, whether it be on the film, in the film, or mostly under the film" is a dry fly.
John Waller Hills dates the 1851 edition of G.P.R. Pulman's The Vade mecum of Fly-Fishing for Trout as the first published observation on the dry fly. He also adds that dry-fly fishing was probably practiced on the river Itchen in England during the 1840's. By 1865, the practice of the dry fly was prevalent on the southern Hampshire streams in England. In Ogden on Fly Fishing (1887), James Ogden establishes 1839 as the date that he invented the dry fly. Some forty years ago, when I introduced my floating flies...."
The modern dry fly may include patterns that "swim" in the surface - such as the so-called flynymphs "damp" patterns, emergers, and parachute patterns - as well as patterns that pentrate the water surface with the body or hook, such as the Swisher-Richards no-hackle patterns. : A dry fly may be any pattern that is visible above the water surface, hence any pattern that extends above the water surface. Thus, spinners may be considered a dry pattern. The distinction between the dry fly and the wet fly is not as clear as it was during the last century. A "wet" fly may include the traditional wet fly as well as any pattern that is entirely submerged. With the advent of fishing the various stages of a particular insect, fly patterns and fly hooks have become more specific. Some patterns, in fact, may be fished as dry, emergers, and wet in a single cast or drift. At the end of a dry float, the pattern may be tugged beneath the surface to imitate a sunken nymph and then, on the retrieve, pulled close to the surface to imitate an emerger. Thus, the manner of presentation and line technique may define or determine the pattern type.
A few "flutter" patterns are technically dry; though they are "damp" or "moist" when scraped along the water surface. Usually, nymphs or larvae are specialized wet patterns, and floating emergers are dry patterns. Generally, any patterns without a completerly submerged hook may be considered a dry fly. A completely submerged or saturated dry fly is a wet fly; a dry fly fished as a wet fly is a wet fly.
This information can be found in The Fly Fisher's Illustrated Dictionary, authored by Darrell Martin, copyrighted in 2000, and published by The Lyons Press. This is an excellent dictionary of fly fishing terminology and would be a great addition to any fly fisher's library.