Lids and Lashes for Protection

Any sudden movement in front of a human eye causes it to blink. The single eyelid snaps shut in a swift reflex action to protect the eye. In many other types of animals the eye’s protective system is far more elaborate. Some creatures–including most birds–possess three eyelids to help guard their eyes against dust, sand and twigs, as well as to carry cleansing tears across the eye surface. The third eyelid is a semitransparent tissue–the nictitating membrane–that flicks across the eye from the inside to the outside corner. Birds seldom close their upper and lower lids except in sleep, but use their nictitating membranes to blink. Tiny feathers on the inside surfaces of the membrane act as miniature brushes to “dust” the eyes. In ducks and some other waterfowl, the third lid serves another purpose. It houses a clear gogglelike lens which improves the ability of the eye to focus underwater while searching for food, a task for which these birds’ normal vision, adapted for flying, is too farsighted.

Snakes and most fishes have no lids at all; the eyes of both are protected by a tough, glassy coating. When a snake sheds its skin, it sheds the coating too. In fishes, the coating is permanent and the water constantly washes it clean.

Eyelashes of different kinds also protect the eye, shading it from glare and filtering out dust. Long, thin feathers act as lashes for birds; similarly rows of scales circle the lids of lizards. Desert animals, which must protect their eyes from the whirling fury of sandstorms, often grow unusually long lashes. A camel’s lashes may measure as much as four inches.

Life Science Library - Light and Vision