These spiders are unique to the Greenwood biome and are passive creatures. When attacked, they can shoot up to 3 silk nets to ensnare their attacker.
They can be used as a source of fibers. A Dead Silk Runner can be carried in the inventory and placed in a pot to boil. Boiling yields a Tuft of Spider Silk, which is a type of fiber. Rarely, you can get Spider Egg Sacs instead.
Silk Runners are usually passive unless attacked, however there is a small chance that when foraging an Abandoned Cobweb that a spider will jump out and attack you.
WARNING: Silk Runners have ranged attacks.