The Fremen’s ability to ride the monstrous worms of Arrakis is one of the great conceits of the novel Dune. Just imagine: the rider stands still in the desert, while a “thumper” draws the worm to roar past him. At the key moment, the rider uses hooks to grab the passing worm and jumps aboard. (Herbert even explains how the rider can steer a worm.) The image of boarding a great worm spurs the imagination, and Paul’s first ride on a worm is one of the great scenes in the book. I read the entire novel back in 1965 without realizing how impossible it would be to board a moving worm.
Frank Herbert was especially good at imagining sci-fi technology, so I believe that he understood what I am about to explain to you. The novel is quite coy about how fast the great worms moved.
Internal evidence in the book suggests that the worms traveled 50 to 100 miles per hour. A lookout flying in the air warns spice miners when worm-sign is seen, and the workers have just minutes to clear out before that worm attacks them. Therefore, worms must be able to close in on workers from six to ten miles away, in minutes.
A journey from the northern to southern hemisphere of Arrakis is a “ten worm” journey. Arrakis must be similar in size to the earth, because its gravity is similar. A worm can be worn out in a few hours by a skilled rider. Obviously, these worms have to move very fast when they traverse the hemispheres.
Stilgar warns Paul not to stand too close to the worm before boarding it, because the sand spray kicked up by the fast-moving worm is dangerous.
Now imagine yourself catching a ride on a car that drives past you at 50 mph. (Do not try this at home!) If your arms aren’t yanked off, your body will be mangled. You might hook a car and jump on top of it, if it was moving less than ten mph. The Fremen could not possibly hook a ride on those fast worms.
By the way, there’s a website devoted to photos of baby animals. It's devoted to a serious cause, and it is impossibly cute.