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| Sunday, 10 July 2005 |
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Junior Observer |  |
| News Business Features Editorial
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Eyes extraordinary
If you take a look at the extraordinary visual powers some inhabitants
of the animal kingdom are endowed with, you will be amazed... Startling
facts about animals that have evolved with We generally believe that the pair of eyes in animals such as birds, mammals and other vertebrates are identical, but according to new research, it may not be so. Would you believe that the two eyes of a starling bird could be performing two different functions - one detecting colour and the other, movement? It has been discovered that the retinal cellular composition of this bird's two eyes differ, with the left eye retina having more single cones - photosensitive cells that respond to colour, and the right having double cones, which detect movement. So, if you observe which eye it's looking from, you could know whether it's looking for colour or movement. Then, there is a fish known as Anableps, anableps with a unique optical construction which is native to Caribbean lagoons. It is commonly known as the four-eyed fish because its pair of eyes are actually split by horizontal partitions into two halves.
Each half has its own iris and retina and this enables the fish to swim at the surface of the water, with the upper half of each eye scanning the upper region for any predators, while keeping an eye for small fish to feed on, on the surface, with the help of its lower eyes. Now isn't that really fascinating? Even though this fish has two sets of eyes, scientists have discovered that each eye has only one oval-shaped lens with the lower portion of the lens thicker than the upper portion. Now, why do you think the eye lens is of two different thicknesses. Because, in order to see through water, the lens needs to be thicker than normal. You may be even more amazed to learn that there is another fish that beats this four-eyed fish - a six-eyed fish discovered in 1958. A slender pike-like fish, the Bathylychnops exilis is also called spookfish. Dwelling at depths of 100-1000 m (330-3280 ft) this fish has paired spherical organs pointing downward within the lower half of its huge eyes.
These organs which were originally believed to be nothing more than light-producing organs are not accepted to be accessory eyes. These organs are also known as secondary globes and they possess their own lens and retina. Scientists believe these globes enhance the spookfish's sensitivity to light (photosensitivity) within the dark, murky waters, in the depths of the sea in which it lives. The third set of 'eyes', which makes this fish the champion among animals with unique eyes, are located behind the accessory eyes. They are tinier and less sophisticated than the accessory eyes, and lack retinae. What purpose do these third pair of 'eyes' serve? They direct incoming light into the spookfish's principal pair of eyes.
Could you think of another creature in the animal kingdom which has extraordinary eyes? The creature is not a fish but a reptile - it's the chameleon.In most higher animals, the eyes have a lens for focusing incoming light onto the retina to create an image. The chameleon however uses the cornea for this purpose. This enables the creature to avoid drawing attention to itself when trying to check out the distance of a potential prey. Most creatures do so by moving their heads from side to side, causing closer objects to appear to move more quickly than those at a distance. It is called the parallax effect. But the chameleon can do so with only its eyes. **** Compound eyes
Our eyes can see 50 images per second in good light, less in dimlight. Insects, however, have paired compound eyes made up of many different light-focusing lenses which are known as ommatidia. These are clustered together on each side of the insect's head. These ommatidia are very small and can collect light only from a small portion of the insect's field of vision. Because of this, an insect sees the world not as a single image like we do, but as a mosaic made up of dozens or even thousands of small images. The more ommatidia there are, the sharper the picture. Not all insects are endowed with equal vision. Some such as the worker ants have only around ten ommatidia and have very poor vision, while dragonflies have up to 30,000 and have acute vision.Insects have almost 360-degree vision because of the distinctive curved surface of their eyes. However, as they do not possess shape-changing lenses, they are unable to adjust their focus between near and far objects, and need to get closer to examine something better. So, although insects cannot see as sharply as we humans, they certainly can sense movement much better than we could. That is why when we try to swat a fly or a mosquito it flies away even before our hands are half way through. |
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Are you aware that flies and other insects can detect movement up to
six times more quickly than we can? This is because the composition of
their eyes is different to that of humans. Humans have a single lens,
which focuses light to produce a single image. 
