These massive worms will emerge from the cavern walls during combat guided by the chants of the Kantus. However, their large size makes it easy to hide behind them as they slither about. They love to eat the red glowing fruits that are in the Hallow. So they are almost like portable cover. Also, if you are to get in the way of any of these harmful creatures, they will turn harmful by first rising, and crushing itself on you, which will kill you instantaneously. It's also stated later on in the campaign that the Locust eat them for food. The new locust in Gears of War 2 Kantus can take control of rock worms with the wierd noises it makes.