 All salamanders lack middle ear cavity , eardrum and <unk> tube , but have an opercularis system like frogs , and are still able to detect airborne sound . The opercularis system consists of two ossicles : the columella ( equivalent to the stapes of higher vertebrates ) which is fused to the skull , and the operculum . An opercularis muscle connects the latter to the pectoral girdle , and is kept under tension when the animal is alert . The system seems able to detect low @-@ frequency vibrations ( 500 – 600 Hz ) which may be picked up from the ground by the fore limbs and transmitted to the inner ear . These may serve to warn the animal of an approaching predator . 
