shuts its eyes as the tongue is extended. Although it was formerly debated, more recent research has shown that frogs can see in colour, even in very low light. Hearing Frogs can hear both in the air and below water. They do not have external ears; the eardrums (tympanic membranes) are directly exposed or may be covered by a layer of skin and are visible as a circular area just behind the eye. The size and distance apart of the eardrums is related to the frequency and wavelength at which the frog calls. In some species such as the bullfrog, the size of the tympanum indicates the sex of the frog; males have tympani that are larger than their eyes while in females, the eyes and tympani are much the same size. A noise causes the tympanum to vibrate and the sound is transmitted to the middle and inner ear. The middle ear contains semicircular canals which help control balance and orientation. In the inner ear, the auditory hair cells are arranged in two areas of the cochlea, the basilar papilla and the amphibian papilla. The former detects high frequencies and the latter low frequencies. Because the cochlea is short, frogs use electrical tuning to extend their range of audible frequencies and help discriminate different sounds. This arrangement enables detection of the territorial and breeding calls of their conspecifics. In some species that inhabit arid regions, the sound of thunder or heavy rain may arouse them from a dormant state. A frog may be startled by an unexpected noise but it will not usually take any action until it has located the source of the sound by sight. Call The call or croak of a frog is unique to its species. Frogs create this sound by passing air through the larynx in the throat. In most calling frogs, the sound is amplified by one or more vocal sacs, membranes of skin under the throat or on the corner of the mouth, that distend during the amplification of the call. Some frog calls are so loud that they can be heard up to a mile (1.6km) away. Additionally, some species have been found to use man-made structures such as drain pipes for artificial amplification of their call. The coastal tailed frog (Ascaphus truei) lives in mountain streams in North America and does not vocalize. The main function of calling is for