Marvellous eyes in nature !!!

Think of a car key and an ignition switch. As soon as you turn the key, the engine starts. Now instead of you, light (yes, light) needs to start the car. The light falls on the key and turns it, the engine starts. This metaphor can be replicated to understand the basics of how eyes sense light.

We all know that every animal sees differently, and this can be attributed to the fact that they all vary in size, shape, placement, number etc. But one thing is universally common among all the eyes-opsin- a protein that works by tightly embracing a molecule called chromophores.

Every animal also has light detecting cells called photoreceptors, which vary dramatically from species to species. Chromophores are present in these cells.
Light partials, also known as photons, fall onto the chromophores.
Chromophores then absorb energy from the photon and change their shape.
This shape shift forces opsins to change their shape as well.
The transformations in opsin triggers a chain reaction that ultimately results in neuron firing, enabling the brain to create an image.

Although this process of sensing light is universal, all species see different. Some cannot see the visible-light-spectrum, while others can see UV rays. Some use the sense of sight dominantly (humans), while others use it passively (scallops). Let us see some amazing eyes to in nature.

The following animals do have eyes on their heads, but also posses photoreceptors on other parts of their body to sense light, and do actively use them.

Olive sea snakes

In addition to having eyes on their anterior end of the body, they have photoreceptors at the tips of their tails to look out for predators.

Cephalopods

Octopuses, cuttlefish, and other cephalopods have photoreceptors dotted throughout their skin.

Japanese yellow swallowtail

This butterfly has photoreceptors on its genitals, which helps either of the sexes during coitus. Female uses them to position her egg-laying tube on a plant.

The following animals display a brilliant variety of true eyes.

Scallops

They are a very primitive mollusk, but their eyes are really complex and advanced for such a lower species. Each one sits at the end of a mobile tentacle and each has a little pupil. The brain is also complex. Research suggests that they do not use eyes predominantly. The eyes only sense motion. Think of a security room with hundreds of cameras in it. The camera in which motion is detected is the one that beeps. It is like what touch is to humans. We are constant in physical contact with so many objects, but we only pay attention when it is needed.

Honeybee

Honeybees and almost 98 percent of all other insects have the blurriest vision of all. They see the sight of a legally blind person. Their visual acuity is just 1 cycle per degree.

Wedge-tailed eagle (of Australia)

Its photoreceptors are some of the narrowest in the animal kingdom, which allows them to be densely packed within their retinas. It can spot a rat from a mile away.

Vultures

These birds can see on their sides completely, but have large blind spots above and below. So while scanning the ground, they can also see other vultures flying next to it, without having to turn. The blind spots are the reason why they crash into electricity and light towers even though they seem to fly straight.

Heron

Their visual field covers 180 degrees in the vertical; even when standing upright with its beak pointing straight ahead, it can see fish swimming near its feet.

Mallard duck

Their visual field is completely panoramic, with no blind spot either above or behind it. It can see the entire sky without moving.

Cock-eyed squid

Its left eye is twice the size of its right. The small eye points downward to spot bioluminescent flashes and the big eye points upward to spot silhouettes. The bigger eye is the size of a football and they are the largest eyes in the world.