All animals have eyes (or similar structures) to see the world in their own way. Even the simplest invertebrates, the smallest insects, even jellyfish, 99% made up of water, have cells that are sensitive to light and are used by eyes. How do they see? Although many animals have eyes similar to ours, today we know that our way of seeing is unique in nature, shared only by anthropomorphic monkeys and a few other species.
In fact, each species has evolved by developing the visual characteristics most suited to its habitat and lifestyle. We cannot say that they see better or worse: they see in the best way for them. The moles see well up close and in complete darkness of their burrows; dogs do not recognize colors but see well in the twilight and at dusk. Flies and insects in general do not clearly distinguish the forms, but they see a greater number of fixed images per second, about 200 against the 18 of the man: for this reason, a movement that appears to us rapid for a fly is instead composed from single still images.
The insects' visual system is therefore ideal for surviving predatory traps (including men with fly swatter), or for catching prey on the fly.
The eye is a camera : the cornea and the lens (in the eyes of vertebrates) are lenses that, like a lens, capture images and bring them into focus. The images are then projected onto the retina, similar to the sensitive elements of a video camera. In the retina of man there are two types of light-sensitive cells, called photoreceptors: cones and rods . The first ones (which in man are about 6 million) are found mainly in the center of the retina and are suitable for daylight vision: they adapt to light and allow us to perceive colors and distinguish details.
The rods (which in man are about 100 million) are mainly on the periphery of the retina and serve for night vision: they are much more sensitive than the cones to the light but they "saturate" quickly when it increases and do not allow to perceive colors or distinguish good details.
The more sensitive the central part of the retina, the better the image sent to the brain. Men, in whose eyes cones and rods are well integrated, see better in daylight. Wolves and dogs instead have a retina very rich in rods and therefore more suited to the darkness of twilight and night, moments in which they hunt.
High resolution. Birds of prey have a central point of the eye that works like a telephoto lens and magnifies details of what it sees. The magnification function is ensured by a particular area of the retina, called fovea, where the cells of vision are very concentrated. If in the man there are about 200 thousand cones per square millimeter, in the fovea of the eagle there are 1 million and this means that the image perceived by the eye of the bird of prey has a central part enlarged by 2.5 times and to very high definition.
Furthermore, several animals have eyes that are sensitive to ultraviolet light : this is the case with some butterflies - which thanks to this feature recognize males from females - and bees, which are thus attracted to flowers in which some structures are seen only by ultraviolet rays. Other animals, on the other hand, are sensitive to infrared rays, such as snakes, which see prey in warm blood thanks to thermal receptors placed under the eyes and which the brain associates with eye vision.