Animals Eye

Almost all animals can perceive and respond to light, but eyes are as varied as the animals that posses them.

Eyes that form definite images are found only in some mollusks, mainly squids, octopuses and cuttlefish; in a few worm; in most arthropods, including insects, spiders, lobster, and most crabs; and in vertebrates. Except for most insects, these animals have eyes that are similar in structure and function to a camera. The eye uses a single Lens to focus a picture on a surface of densely packed cells called photoreceptors. The receptor surface, called the retina, functions like a piece of film. An external object is pictured on the retina like the points of newspaper photograph. The picture later received in the Brain, however is not the same simple point by point image. Exactly what this picture is remains unknown, but perception is a process that takes place in the brain not in the eye. Information from the eye, like the piece of a puzzle, is analyzed in the brain and fitted into meaningful forms.

Most insect eyes are built on an entirely different principle from the described above and are called compound eyes. Thousands of densely packed lenses are spread like a honeycomb over a spherical surface so that a mosaic image is formed. Each lens is associated with relatively few receptor cells, and the entire unit is called an ommatidium. No structure, therefore, is strictly analogous to the retina of other animals. What kind of image this arrangement conveys to the insect depends on the complexity of the structure.