University of California Berkeley biologist Robert Full analyzes centipede motion by observing the insect's movement across a glass plate covered with "photoelastic" gelatin. On either side of the gel are thin polarizing filters that together block all light coming through the glass. When the centipede's feet contact the gel, they temporarily deform it, altering the way light goes through it and allowing some to pass through the filters. In the test above, one group of legs works on one side of the animal's midsection while two other groups work near its head and tail. UC Berkeley (California. From the book Robo sapiens: Evolution of a New Species, page 94 bottom..