Mirkwood Centipede

The Mirkwood Centipede is a large ghostly-white Centipede that lives below the trees in Mirkwood. They live underground and emerge through Trapdoor Spider-like lids, covering up their burrows. They have two sets of mandibles for mouths, have no eyes, and scavenge for either what may fall from the trees, such as a Giant Spider, or what may cross it's path from the ground. In 'The Desolation of Smaug', when Bilbo Baggins fell from the trees when killing a Giant Spider, he ended up losing the Ring. As he was going to retrieve it, a Mirkwood Centipede came emerging out from under it's trap lid, and came over towards Bilbo, walking over the Ring as it goes. But before it had the chance, Bilbo with his sword, and from the effects of the Ring, was already attacking it, severing a mandible, and stabbing it's underside that caused it to collapse on the ground, before Bilbo gives it the finishing blow by stabbing the head.

Trivia

 * The Mirkwood Centipede scene may have been intended to show the first signs of the Ring's effects on Bilbo.