The cicada is a block/animal that can be found on giant twilight oak trees.
They make a loud chirping noise when approached and can be broken and replaced in the overworld or just the Twilight forest itself. Also, there is a small chance that Cicadas will spawn on blocks near a twilight forest portal in the overworld. It is unknown if this is a bug at this time.
When broken from the tree, the chirping noise may still occur for as long as the chunk is loaded.