The worm was a type Fremen called a “growler.” It frequently dug in its foreplates while the tail was driving. This produced rumbling sounds and caused part of its body to rise clear of the sand in a moving hump. It was a fast worm, though, and when they picked up a following wind the furnace exhalation of his tail sent a hot breeze across him. It was filled with acrid odors carried on the freshet of oxygen.
Possibly the first time that Frank goes into detail about the adult sand worm's biology. Other than that they create the spice melange.
