The Australian Emu (Dromaius novaehollandiae) has protective nictating membranes on its eyes. The membrane over each eye protects them from dust.
A nictating membrane is a transparent or translucent eyelid. The eyelid membrane not only protects the eye from dust, it also moistens the eye and cleans away any dust or dirt. This is because the emu lives in dry, dusty areas of Australia.
Nictating means blinking. The eye membrane is called a third eyelid, or a haw, or a plica semilunaris, or a membrana nictitans.
The emu has an upper eyelid and a lower eyelid, just like humans and other birds and animals. Because the nictating membrane is an extra eyelid, it is called the third eyelid.
Normal upper and lower eyelids move up and down. The emu’s nictating eyelid moves horizontally, side to side, across the eyeball. It moves from the inside edge of the eye to the outside edge of the eye.
Not all animals have a nictating membrane. Humans don’t, but some reptiles (lizards and snakes), birds, sharks, cats, camels, polar bears, seals, and aardvarks, have full nictating membranes.
Photographer: Martina Nicolls
Martina Nicolls: SIMILAR BUT DIFFERENT IN THE ANIMAL KINGDOM