Woodlice are passive arthropods (they're crustaceans, not insects!) that may spawn when destroying the hollow logs found in Subterranean Savannahs and the Ulterior Outback, as well as from breaking Rotten Wood in Submerged Swamps and Fungal Forests (or any biome that you place the wood in).
Upon spotting a player, they will run away from them at insane speeds, even faster than the Solifuge. Players with good reflexes and good weapons might be able to take it out right as it spawns, but otherwise you are in for a chase.
Upon right-clicking them, they turn into a Woodlouse Ball, a throwable item. Once thrown, the Woodlouse will unfurl and run away again. The balls do no damage.