Saturday, September 10, 2011

The Stunning Variety of Spectacular Eyes in Nature

The Eagle has eyes quite different from humans, the backs of the eyes being larger and flatter, and thus providing an excellent vision. In the center of the Eagle fovea – focusing point on the retina – there are approximately 1,000,000 cones per mm, compared to the measly 200,000 that humans have, so that objects or prey can be seen much more clearly and sharply, even from great distance. Eagles and Owls also have three eyelids, with the bottom eyelid being larger than the top one, and this explains why they always blink up instead of down.

The beast in the above photo never actually “cries crocodile tears”, because they do not have tear ducts, and cannot weep. The eyes can produce secretions and these fluids are secreted behind the third eyelid, or nictitating membrane, to both clean and lubricate it as it travels across the surface of my eyes, and inhibiting bacterial growth. The edge of this nictitating membrane is visible in the corner of the eyes when opened widely, and ‘tears’ can always be seen forming within it.

Do you know why snakes like the Python shown above always keep the eyes open? They do not have movable eyelids as most lizards do, which help them to lubricate and protect their eyes. This is why snakes always stare at prey. The eyes are protected by tough, immobile, transparent scales. While snakes do not have very good eyesight, their sense of smell, the reason why their tongues are always so active, is the best in the animal world. Some of snake species have a reddish brown ring around the left edge of their pupils, and even green, orange, gold, yellow or brown eyes.

This is the assassin fly, which is often mistaken for abumblebee. Living in grasslands all over the world, it has excellent vision that enables it to determine the distance and speed of objects or prey moving in front of it. When it spots food, it will fly close to investigate. The head is very large with two large compound eyes, each containing several thousand separate lenses that provide remarkable vision, particularly good for spotting small insects from a long way off.

The most striking eyes among the reptiles have lower and upper eyelids joined together, leaving one large hole for the pupil to see through. The Chameleon has the ability to roll or rotate each of the eyes independently, to see two different objects from two different directions at the same time. This perfect visual ability gives it a 360 degree field of vision, unique in the natural world.

Dragonflies are amazing creatures in that each has a pair of large multifaceted compound eyes which occupy almost all of their heads. Each compound eye has up to 30,000 individual lenses called ommatidia (a separate light-sensing organ), and thus giving these incredible beasts 360 degrees perfect fields of vision.

Scallops like the one pictured above have many eyes positioned along the right and left valve mantle margins. Each of eye is very unique, functioning like a reflecting telescope to bounce incoming light rays back to theimage-forming retina with an angular resolution of around 2°. The crude image then is formed on light-sensitive cells located in the middle of the eyes. These fantastic shellfish can afford to lose an eye, too, as they are able to regenerate them.

Eyes, as you can see, come in many strange and beautiful forms. Many of these creatures see the world in ways we never could, because human visual ability is really quite limited. The wonders of nature never cease to amaze and delight. The more you think you know, the more you realize how much there is still to learn.