Many animals have a third eyelid called a nictitating membrane that protects the eye.
Many animals, such as diving hawks or polar bears, use nictitating membranes to protect their eyes from water or snow blindness, but few are as spectacularly patterned as that of the red-eyed tree frog (Agalychnis callidryas). It’s speculated that the golden veining provides camouflage to the frog—while the wary frog rests during the day, it is still able to detect the movements of a predator.
Image courtesy of Igor Siwanowicz, PhD, Janelia Farm Research Campus, Ashburn, VA
This photograph was taken using a digital SLR camera and a macro lens plus a set of extension tubes to achieve higher than 1:1 magnification offered by the lens alone.