Maneka Sanjay Gandhi
Human eyes are among the least useful and become weaker with age, genes, or bad nutrition. The animal kingdom on the other hand has the strangest, most complex eyes in it. From six eyed spiders to geckos that can see 350 times better than humans. Some animals see infrared, and ultraviolet, parts of the spectrum not visible to humans. Some animals even use their eyes to frighten predators or, like butterflies, imitate the eyes of larger animals with completely different parts of their bodies.
Here are some of the strangest eyes in nature-
Guitarfish don’t have eyelids. But they have muscles that pull back their eyeballs almost 1.6 inches into their head. This is important because the guitarfish hunts for prey on the ocean floor and this protects the eye from sand.
Chameleons have cone shaped eyes protruding from their head. Their eyelids join in a circle round the eye leaving only a pinhole for them to look through. Their eyes move independently of each other, so they can look in different directions at the same time. Each eye can move a full-360 degrees, which allows a chameleon to watch an approaching object while simultaneously scanning the rest of its environment. When the chameleon sees prey, it focuses both eyes in the same direction and shoots out its tongue at high speed, a technique that requires a very precise distance and depth perception. Chameleons have very sharp eyesight, being able to see an insect several metres away, and they can see ultraviolet light.
The colossal squid has the largest eyes of any animal on the planet. They are nearly 11 inches across — about the size of soccer balls, with lenses the size of oranges. Each eye has photophores (light organs that work similar to headlights) that produce light for the squid to see its prey in the dark, useful for an animal that hunts 2,000 meters below the surface.
Dragonflies have eyes that consist of thousands of thousands of tiny hexagonal eyes, giving them nearly 360-degree vision. Their eyes are so big that they cover most of their head. These eyes are made up of 30,000 visual units called ommatidia, each one containing a lens and a series of light sensitive cells. They can see ultraviolet and polarised light. They have three smaller eyes, named ocelli, capable of detecting movement, allowing them to react within a fraction of a second.
The leaf-tailed gecko has vertical slit-like eyes without eyelids. Its vertical pupils contain a series of ‘pinholes’ that widen at night, allowing for amazing night vision, even seeing colours at night. The pattern on the whites of their eyes is part of the full-body camouflage. Their eyes are protected by a transparent membrane and geckos clean this with their tongue.
Goats have rectangular pupils which give them a 320 – 340 degree field of vision (compared to humans’ 160 – 210 degrees), and better peripheral and night vision. They can see virtually all around without having to move.
The nocturnal ogre-faced spider has six eyes. It has amazing night vision, 100 times better than humans, not only because of its large eyes, but also because of a light-sensitive layer of cells covering them. This membrane is so sensitive that it is destroyed at dawn and a new one is produced every night.
Mantis shrimps, aggressive predators, have the most complex eyes of any animal. They are two googly eyes, like muffins mounted on stalks compound like a dragonfly, but they have 12 colour receptors (humans have three), as well as ultraviolet, infrared and polarised light vision. Each of the mantis shrimp’s eyes is divided in three sections, allowing the creature to see objects with three different parts of the same eye. In other words, each eye has ‘trinocular vision’ and complete depth perception, meaning that if a mantis shrimp lost an eye, its remaining eye would still be able to judge depth and distance as well as a human with his two eyes. While they have only 10,000 ommatidia per eye, in the mantis shrimp each ommatidia row has a particular function. For example, some of them are used to detect light, others to detect colour, etc. The eyes are located at the end of stalks, and can be moved independently from each other, rotating up to 70 degrees. Interestingly, the visual information is processed by the eyes themselves, not the brain.
Spookfish have four eyes. Most animals have eyes that use lenses to focus light and to see. Spookfish are different. Each eye has a swelling called a diverticulum, separated from the main eye by a septum. While the main part of the eye has a lens and functions in a similar way to other animal eyes, the diverticulum has a curved, composite mirror composed of many layers of guanine crystals. This ‘mirror’ reflects light and focuses it onto the retina allowing the fish to see both up and down at the same time.
Sea Stars, or starfish, have five eyes capable of sensing lightness and darkness — one on the end of
Tarsiers, squirrel sized primates, have the largest eyes on Earth, relative to body size. They can’t move their eyes around but they can turn their heads 180 degrees in either direction to scan for prey or predators. With each eye weighing more than its brain, the tarsier has extremely acute eyesight and superb night vision, even seeing ultraviolet light.
Four eyed fish, found in fresh or brackish water, feed on insects, so they spend most of their time swimming at the surface. Despite their name, four eyed fish have only two eyes. However, these eyes are divided by a band of tissue and each half of the eye has a pupil of its own. This bizarre adaptation allows the four eyed fish to see perfectly, and at the same time, both above and below the waterline, scanning for both prey and predators. The upper half of the eyeball is adapted to vision in air, while the lower half is adapted to underwater vision. Although both halves of the eye use the same lens, the thickness and curve of the lens is different in the upper and lower eye halves, correcting itself for the different behaviour of light in air and water.
Stalk eyed flies have long stalks on the sides of the head with the eyes and antennae at the end. Male flies usually have much longer stalks than females and females prefer males with long eyestalks. Males, during mating season, often stand face to face and measure their eyestalk’s length; the one with the greatest ‘eye span’ is the winner. Male stalk eyed flies also have the extraordinary ability to enlarge their eyestalks by ingesting air through their mouth and pumping it through ducts in the head to the eyestalks. They do this during mating season.
These are just a few. Next time I will explore why and how eyes have evolved to be what they are.