Frogs use their regular set of eyelids to keep their eyes moist and clean. They have a bottom lid that stays still and an upper lid that blinks. Frogs also have a third eyelid that serves a different purpose. This one, called the nictitating membrane, is a semitransparent lid that covers the eye completely, helping the frog see underwater and hide from predators.
Swimming in a pond isn't quite the same as your neighborhood pool. Ponds are full of floating debris, much of which is suspended below the surface of the water. Frogs' nictitating membranes provide a protective layer over their eyes, letting debris bounce off harmlessly as the frogs swim underwater. The membranes keep the water out of their eyes, letting them watch where they're going while keeping their eyes protected. Frogs usually have excellent vision, but engaging the third eyelid blocks some of their vision. They can still see -- but the membrane is only partially transparent, so it limits their sight. When they swim, many frogs use their sense of smell to guide them as much as they use their eyes.
Keeping Moisture In
Frogs use their semitransparent third eyelids on land as well. While blinking their regular eyelids can distribute moisture across their eyes, raising their nictitating membranes helps keep frogs' eyes moist. They can raise the transparent eyelids halfway to keep part of their vision sharp, or cover their eyes completely. This is especially handy on windy or hot days, when their eyes can quickly dry out without the membranes.
Although not all species of frog hibernate in cold weather, those that do tend to use their transparent eyelids to protect their eyes while they rest. The eyelids can keep water, debris and mud out of the frogs' eyes, depending on whether they winter underwater or burrowed into the soil. They typically pull their bulging eyes closer into their bodies to conserve heat and protect the eyes even more.
While serving as powerful hunting weapons, a frog's eyes can give away its position to predators. Frogs have good night vision, but the mirrored rear surface of their eyes can reflect nearby light and pinpoint their position in the dark. Some frogs have brightly colored eyes that lead to the same problem. The nictitating membrane can help hide their eyes, even though it limits vision. Red-eyed tree frogs, for example, have a nictitating membrane that's covered in thin stripes, similar to a tiger's stripes. This helps camouflage their eyes so the bright red orbs don't give away their location to predators.