Rock Worms are subterranean creatures that appear in Gears of War 2. These annelids are one of the more benign Hollow-dwelling beasts encountered by the soldiers of the COG; they are unintelligent and they are not affiliated with the Locust Horde. In fact, the Locust hunt rock worms for food.
Rock worms are covered in a rock-solid epidermal layer that cannot be penetrated by gunfire. The only soft parts of their bodies are the pharynx and parapodium. Presumably, when the Locust hunt these creatures, they kill them by attacking these areas.
Rock worms are much more benign than any other known Hollow creatures and tend to ignore any other animals in their vicinity. They feed primarily on luminescent red fruits and fungus that grow within the caverns of the Hollow, but they are capable of eating virtually anything. The only times they have been known to attack other creatures is when they are standing right in front of them. Their jaws are extremely powerful and can tear through a human and even a Locust Drone with one bite.
Since rock worms ignore other creatures around them, Gears have been able to use their stony hides for cover during firefights with Locust. The rock worm typically ignores any bullet impacts as they cannot penetrate its hide, but shots to the tip of the tail will hurt it, causing it to flee.