Every fly box needs this pattern in all sizes. My favorite is a yellow size 8. I know, I know, a lot of fly guys and girls will say this is too large. But I think larger makes a better attractor unless you are fishing a hammered stream. Larger floats longer using a dropper. Larger is better for Giant Yellow Stones that are, well, giant. I've seen aggressive spawning male bows attack larger Stimulators in the dead of winter, on the surface. I have seen large yellow Stimulators work exceedingly well as a hopper pattern, although the smaller sizes will work also. In fact, in my opinon, a yellow Stimulator can be a better hopper pattern than a hopper pattern. One of my best memories is the giant sucking sound a big fish makes when it sluuuuurps in a large Stimulator on still waters. Orange Stimulators work very well during the October Caddis hatch as these orange Caddis, are, well, orange. I use #10s for the Octobers because where I live that is the size they are. I have even seen a huge sucker hook up on a #10 Orange Stimulator during the October Caddis hatch, on the surface! I wholly agree with Mr. Collett's review below. Although I like to fish the larger Stimulators, I routinely use # 16 on rivers and creeks as a very effective caddis pattern. In fact, one of the biggest fish I ever lost hooked up on #16 yellow Stimulator. Buy 'em. They are good.