Just a note about the manner of the emergence of the Callibaetis mayfly: It has been my observation that the
Callibaetis nymph positions itself horizontally on the surface film to emerge from its nymphal shuck. The nymphal shuck splits along its dorsal surface and the adult pushes itself forward and out of it. Sometimes the empty shuck itself might, in this process, be pushed down and below the film, but normally the shuck (which will remain floating on the surface after the dun has fully emerged) trails on the surface. The pattern, as it was originally shown to me, featured a wire rib, apparently to sink the hook shank (in the manner of the Klinkhammer). One of the first changes I made to the pattern was to eliminate the wire and, in fact, I dress the entire fly, including the sparkle yarn representing the shuck, with floatant to try to maintain its horizontal attitude.
It probably doesn't matter a great deal since all sorts of mishaps can occur during the process of emergence but I'm reluctant to meddle with what has proven to be a highly effective pattern.