The San Juan worm imitates a natural worm found in rivers and streams. It resembles the larvae of the aquatic insect called the aquatic bloodworm. This artificial fly pattern is commonly used in fly fishing to attract fish, especially trout. The San Juan worm is typically made of chenille, which gives it a long and slender shape similar to the real bloodworm. Its bright and vibrant colors, such as red, pink, or brown, mimic the worm's natural appearance. Anglers often fish the San Juan worm by drifting or bouncing it along the riverbed, replicating the natural movement of the bloodworm. This fly pattern is particularly effective in stained or murky water conditions, as fish rely more on their sense of smell than their sight in these environments.