The Squirmy Wormy is built around one material: a stretchy silicone-rubber "squirmy" filament lashed to a hook, usually over a colored bead or a thread thorax. In the water that rubber pulses and wiggles on its own, which is what sets it apart from the stiffer San Juan Worm. It reads as an aquatic worm or a drowned earthworm washed in off the bank.
Tie and fish it in sizes 10-14. Red and wine cover most situations, pink is the high-water attractor color, and tan plays it more natural. Lean on the brighter colors when the water is dirty, since this is a dirty-water fly: it fits stained, off-color, and muddy flows, and it shines when rain has the river running normal to high.
The best window is water in the 40-65F range, though it earns its keep anytime worms are getting flushed in. Fish it dead-drifted under an indicator or on the point of a nymph rig, bumping bottom. The classic time to tie one on is the day after a rain, when the river is up, off-color, and trout are looking down for an easy meal.