Eyes are multi-tasker, not just for vision, in the animal kingdom
Nov 28 2012
Eyes are not just used to see. In the case of frogs, eyeballs are practically used to swallow food. When noticed carefully, the frog keeps blinking while eating. It is because it cannot chew nor use the tongue hence, it uses its eyes to push food. The muscles around the eye are very strong and help in the process. The eyes would sink into the skull and the food gets pushed into the throat.
Some crab species living around the seashore or riverbanks, which are usually muddy, keep themselves hidden to avoid danger. But they keep their eyes outside the mud as they have super position eyes. In superposition eyes, there is a gap between the lens and the rhabdome. The function of the rhabdome is that of converting the light procured by the lens into electrochemical impulse, which then travels to the neuron. This is also observed in shrimps and prawns.
The spider is a creature with not two eyes but eight; some species may have six or fewer eyes. However, despite these many eyes, their vision is not as good and most of the threat detection, hunting and navigation happen through vibration. The jumping spider is an exception with very good vision. The retina in its eyes move, hence the spider can look in different directions. The retina is the darkest part of the eyes, so when you look into the eyes of a jumping spider you can actually see the changing colours of the eyes.
In insects, thousands of individual photoreceptors combined make a compound eye. These photoreceptors focus the light down a central structure inside the eye, called the rhabdome, onto a bundle of nerve fibres, which are behind the eyes. The nerve fibre then sends the message to the brain. Hundreds of tiny lenses probably do not create the detailed, focused image produced by the human eye. However, they can pick up colours and shapes, and are very good at detecting fast movements. They also produce a very large view angle, sometimes even a 360-degree angle.
In chameleons, the eyes are the most unique with their upper and lower eyelids joined leaving little space for the pupils to open. Interestingly, these creatures can see in two different directions at once, as their eyes are capable of focusing on two different angles. They have good eyesight, which helps in catching small insects.
Now you can imagine the diversity, in terms of the eyes. Many animals are much more advanced than us humans, which is a key to their survival in the animal kingdom.
(The writer is a conservation biologist at Tiger Watch, Ranthambore)