Anyone ever tied a wiggle-dun?
Yes, I've tied and used hinged adults/emergers, as well as nymphs and baitfish imitations. However, all mobile elements of a fly (articulations, rubber legs, soft hackles, etc.) generally require an outside influence to make them move (a pull from the line or some interaction with the bottom, for example). If a dry fly is truly dead-drifted, about the only advantage is that the fly will sometimes alight in a variable posture that is not seen in most rigid flies. I say "most" because flies like Kelly Galloup's "crippled" spinners incorporate this idea by building a different posture into a rigid fly.
Matt's mention of the history of "wiggle" flies made me think about another occasional advantage that led anglers to experiment with articulated flies in the past. Matt is right to refer to S&R's
Selective Trout as the work that introduced the "wiggle" concept to many of us. That was certainly true with me, and I have been tying versions of these flies ever since I read the book in the early '70s.
The basic idea is older, however, as are "no-hackle" dries and "emergers." (The dry version of the famous British Hare's Ear was both.) These ideas can be traced--at least--to earlier British fly fishing/tying. Atlantic salmon fly fishers used articulated AS flies well before S&R wrote about "wiggle" nymphs. European hook manufacturers produced articulated AS hooks (such as the "Cebrit" brand) for use in these flies. Although movement was probably one consideration, I believe that the more significant consideration was to eliminate some of the leverage from large or long-shanked hooks that could cause them to work loose during a long fight. This idea is now incorporated into some of the huge articulated flies that modern anglers use for steelhead fishing, as well as in the "rebirth" of interest in tube flies.
Speaking of astute insights into the way trout feed on emerging insects, I have always liked a quotation found in Schwiebert's
Nymphs (1973). It is from John Taverner (another of those insightful Brits) and was published in 1600 (!!):
I have seene a younge flie swimme in the water too and fro, and in the ende, come to the upper cruste of the river, and assay to flie up: howbeit, not being perfitely ripe or fledged, hath twice or thrice fallen downe againe into the bottome: howbeit, in the ende receiving perfection by the heate of the sunne, and the pleasant fat water, hath in the ende within some half houre after taken her flyte, and flied quite awaye into the ayre, and of such younge flies before they are able to flie away, do fish feede exceedingly.