To elaborate on my earlier post: With my Mayflies, I try to "get a feel" for what is happening on the stream. I have some smaller fly boxes organized as a "Life Cycle" type system. Mid-summer, I know I might see PMD's in NE YNP so I pack some PMD nymphs, bead head PMD nymphs, two types of PMD dries, thorax and a comparadun and a spinner pattern into one or two smaller boxes. In the Smokies, I carry a similar set-up for Quill Gordons another for Hendricksons, etc, in early Spring then switch out to Light Cahills, Sulfurs in early June. When fishing in the National Forest outside of YNP, (with less selective fish) I'll switch to attractor type pattern boxes- Trudes, Wulffs and Humpies and only a few realistic dries.
Randall