Boy, I fished in Maine, totally immersed in blackflies, but never tried the nymph. I believe they are a type of larva that rappel along the stream bottom by silk tether. Don't some folks simulate this effect by coloring their tippet white? I think LaFontaine mentions a similar event with caddis and his company sells the correct magic marker for the job. (Nymphmeister: please fill in the gaps in my explanation, thanks.)