Many—if not most—mondern nymph patterns use beads in their tying recipes, either for added weight to get a fly down or for aesthetics. Some flies use beads to imitate little air bubbles used by emerging bugs, and some flies use colored beads to imitate various subsurface food sources, like fish eggs, for instance.
Matching a Bead to a Hook
Matching the right bead to the right hook might seem tough, but it’s really not. As Tim Flagler notes above, most bead manufacturers include a little chart (like the one at the top of this post) that tells you what size bead goes best wth the hook you’re using.
— Chris Hunt