Because of the rhythmic sound they make, they are regularly eaten by sandworms. That’s why 2 or 3 ornithopters (called spotters) flew in the sky above the spice harvester, watching for wormsign (an obvious sign that a sandworm is approaching). Upon detecting wormsign, the harvester would be picked up immediately to clear it out of the way. A sandworm can be large enough to swallow the harvesters.