source: They are rivaled as biological materials in toughness only by chitin.
target: They are rivaled only by chitin in toughness.
references: They are rivaled only by chitin in toughness. They are as tough as insect skeletons. The only thing tougher that is living items is chitin. CHitin is the only biological material to compare to their toughness. Their own rival as biological materials in toughness is chitin. The only biological material as tough as them is chitin. The only difference in the toughness of the materials is the presence of chitin. Only chitin are more tough than them. The only thing more tough in biological materials is chitin. Only chitin is a tougher biological material.