Shai'Hulud, the Great Maker

The sandworms of Arrakis rule the desert. Sandworms can grow in excess of 400 meters. They live to great age unless killed by another sandworm or drowned in water.

Sandworms detect all rythmic vibrations on the surface. They detect and attack all vibrations. The sandworm is territorial and will not invade another worm's territory. The only known instances of sandworms crossing territorial lines have been when body shields were activated on the desert floor. Multiple worms have been known to come to body shields. It is for this reason that no Fremen use shields.