This arrangement enables detection

The eyes of most frogs are located on either side of the head near the top and project outwards as hemispherical bulges. They provide binocular vision over a field of 100° to the front and a total visual field of almost 360°.[61] They may be the only part of an otherwise submerged frog to protrude from the water. Each eye has closable upper and lower lids and a nictitating membrane which provides further protection, especially when the frog is swimming.[62] Members of the aquatic family Pipidae have the eyes located at the top of the head, a position better suited for detecting prey in the water above.[61] The irises come in a range of colours and the pupils in a range of shapes. The common toad (Bufo bufo) has golden irises and horizontal slit-like pupils, the red-eyed tree frog (Agalychnis callidryas) has vertical slit pupils, the poison dart frog has dark irises, the fire-bellied toad (Bombina spp.) has triangular pupils and the tomato frog (Dyscophus spp.) has circular ones. The irises of the southern toad (Anaxyrus terrestris) are patterned so as to blend in with the surrounding camouflaged skin.[62]

The distant vision of a frog is better than its near vision. Calling frogs will quickly become silent when they see an intruder or even a moving shadow but the closer an object is, the less well it is seen.[62] When a frog shoots out its tongue to catch an insect it is reacting to a small moving object that it cannot see well and must line it up precisely beforehand because it shuts its eyes as the tongue is extended.[38] Whether a frog sees in colour is debatable but it has been shown that it responds positively to blue light, perhaps because that colour is associated with bodies of water that can provide refuge when the frog feels threatened.[63]

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.[64] 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.[65] Because the cochlea is short, frogs use electrical tuning to extend their range of audible frequencies and help discriminate different sounds.[66] 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.[65] 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