So he starts to get a little bit of gas in his casing to make him a little buoyant, floats to the film, pops out of his case and after a moment or two flies away. Or gets eaten by a hungry trout. Which is where our fly comes in.
A great way to fish these is to bounce them along the bottom, then before casting slowly upstream, slowly raise the rod so that the fly lifts off of the bottom but stops and hesitates in the film. The rising fly will grab the trouts attention and they will make a quick grab for the fly before it "flies" away.
Hook: size 20 caddis style
Thread: olive 8/0
Wing Casing: z lon yarn fibers
Abdomen: light olive dubbing