The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
I'm not sure how a griffith's gnat is supposed to imitate such a thing
Schwiebert's theory was that when it was awash in the film, the herl and halo of hackle suggested the loosening pupal shuck around the dark body of the emerging midge. Others have speculated that it imitates a cluster of midges. Both theories are reasonable, I suppose, depending on the size of the Griffith's Gnat that is effective relative to the size of the actual midge. Like many anglers, I just know that it does work. :)
Anglers have long been analyzing this fly as to the reasons for its effectiveness. The rationale you offer certainly sounds good to me, along with many others I've heard over the years. All I know for sure is that there are times when it works very well during midge activity - and sometimes even when midges are nowhere to be seen.
Gary Borger in his "Naturals" speaks of the Griffith's Gnat as the "finest imitation of emerging midges I've ever seen."
At many locations (especially the popular ones) it's now even less so, Allan. Analogous to "Yeah, the fishing was great there until it hit the fishing magazines ad nauseam"?
Are you suggesting that the fish refuse a pattern that has worked because they see it too much, or that increased angling pressure causes fish to refuse previously successful patterns?