Nictitating Membrane Provides Moisture & Protection to Eyes
You and I have two opaque eyelids, one above the eye and one beneath. When we blink, they meet in the middle of our eyes. Some birds, amphibians, reptiles, fish and mammals have three eyelids – two similar to ours, and a third translucent or transparent eyelid, called a nictitating membrane. This membrane moves horizontally across the eye from the inside corner to the outer edge of the eye, much like a windshield wiper, when needed for protection, to clear debris or to moisten the eye. Although this American Crow’s nictitating membrane looks as if it was blinding the crow, it isn’t. Because of the membrane’s translucency, the bird can still see when the membrane is covering its eye.
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Nictitating Membranes
If you look closely, you’ll see that part of this American Crow’s eye appears white. This white (actually transparent) part of the eye is referred to as a “third eyelid,” or nictitating membrane. Many animals, including some fish, mammals, reptiles, amphibians and birds, have nictitating membranes. Instead of moving up or down like the upper and lower eyelids, they move horizontally across the eye, much like windshield wipers. These membranes lie under a bird’s eyelids. When not in use, they are curled up in the inner corners of a bird’s eyes, but can be quickly drawn across the eyes to protect, clean and moisten them without shutting out the light.
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Dragonfly Eyes
Dragonflies (and bees) have the largest compound eyes of any insect, each containing up to 30,000 facets, or ommatidia (house flies have 6,000). Each facet points in a slightly different direction and creates its own image, and the dragonfly’s brain compiles these thousands of images into one picture. This eye structure enables dragonflies to be extremely sensitive to motion. Because a dragonfly’s eyes wrap around its head, it can see in all directions at the same time (though its forward-looking vision is the sharpest). When capturing prey, a dragonfly doesn’t chase it – it intercepts it in mid-air, and it’s successful nearly 95% of the time. This hunting technique entails calculating the distance of its prey, the direction it’s moving and the speed that it’s flying – an impressive feat any tennis, baseball or football player would especially appreciate!
(Photo is looking down on the eyes of a Common Green Darner. Three simple eyes, or ocelli, are located in the black section below (above in the photo) its two tan compound eyes. The short, thin black lines are its antennae, which can detect wind direction and speed. The yellow section is part of the upper half of its face, or frons.)
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