We have talked a bit about the human visual system. Let’s have a look at the way some other animals do it.
Consider the case of the bee. The bee is a very small animal and therefore it has got a very small brain. The bee’s brain weighs just one gram and it has only got 10^6 neurons. So contrast that with the human brain, which has got one and a half kilograms of grey matter and 10^11 neurons. The bee brain is much smaller, much simpler, yet it is still able to do very complex tasks based simply on the information that comes from its eyes.
Now we have two eyes and the bee has two eyes. There is nothing that says that all animals have to have two eyes. So this is a close up of a spider and this particular spider has got four eyes and I think there are some spiders that have got five eyes. So evolution has come up with many different designs of eyes themselves, and configurations of eyes, that clearly confer some particular advantage to the organism that is carrying them.
Here is a close up photograph of the eyes of an insect. These are compound eyes, made up of a number of polygonal facets.
Here is another type of compound eye. This one doesn’t have quite the field of view of the one we just looked at.
Here is the eye of a nautilus, which is quite a primitive ocean-going animal. And the eye really is a very simple pinhole camera; it doesn’t have a lens as such.
Here are the eyes of a scallop. So the blue objects that you can see, each of those is a very simple eye and it has got a large number of them around the outside of what is effectively its mouth. Each of these eyes is a bit like a reflecting telescope. The inside of those blue objects is quite shiny and so incoming light rays, instead of refracting in through a lens, reflect off the shinny surface inside that eye and fall onto the photoreceptors. So this scallop, it uses reflection rather then refraction and it has got a very large number of eyes.
This is the eye of a squid and it has got a lensed eye like we have, and it has evolved completely independently of the lensed eye that we have.
And here is a close up of the eyes of a bird.
The compound eye in the insect contains a number of facets; individual light sensing elements called ommatidia and if we look at a cross-section of the ommatidia it has got effectively a lens on top which funnels any light that falls on it down inside this conical cell which contains the light sensitive chemicals, which omit a neural pulse when light falls on it. So each one of these is effectively a single pixel camera, or we can think of it in those terms. And we have a whole array of them pointing in slightly different directions and that allows the insect to create a fairly large field of view.
Vision is a ubiquitous sense and is found in almost all animals, but the number and type of eye is very diverse. We will look at examples such as compound eyes of insects, spiders, and sea creatures such as scallops and squids.