The eyes are the windows on the world. Vision is found widely in many different classes of animals and may have evolved independently at different times. Vision, which involves perception of light and dark, is distinct from simple light sensitivity, such as that displayed by germinating plant sprouts that respond to the sun's direction.
The complexity of eyes varies markedly in different groups of animals. Nonfocusing eyecups are found in the planarians, the medusas (jellyfish) of cnidarians, some snails, and some other invertebrates. Light enters a depression lined with pigment-containing, light-sensitive cells. Neurons connected to these cells carry messages to the rest of the nervous system. Because there is no focusing system, the general direction and intensity of light can be detected, but there can be no perception of form or image.
Most adult insects and crustaceans, as well as the horseshoe crab and the extinct trilobite, have compound eyes, constructed of as few as one (in some ants) to as many as thirty thousand (in some dragonflies) individual units called ommatidia. Each ommatidium is covered with a cornea, formed from the insect exoskeleton , and has its own crystalline cone within. Both structures focus light on the retinula (light-sensitive) cells at the base. The amount of light entering the ommatidium may be controlled by increasing or decreasing the amount of screening pigments within. The individual ommatidia do not usually cast clear images on the retinula cells, rather just a spot of color. The individual retinula cells then send this information into the brain, which puts all of the spots together to form a mental image.
Although the details of insect visual processing are unknown, there appear to be multiple levels of processing, as there are in vertebrate visual systems. Finally, insects usually have three ocelli, non-image-forming simple eyes, on the tops of their heads. These seem to awaken insects for their daily activities.
Vertebrates (including humans) and cephalopods (such as the octopus) have so-called camera eyes. Camera eyes have muscular rings called irises to control the amount of light that can hit the light-sensitive cells in the back of the eye. The ability to control the amount of light is called visual adaptation. Human eyes have a cornea on the outer surface that provides about 70 percent of the eye's focusing power, and they have an adjustable lens that provides the rest of the focusing power and allows accommodation, or change, of focus for near or far objects. Light entering the eye passes first through the cornea, then past the iris, through the lens, then the vitreous humor, which is a clear jellylike substance that gives the eye its shape. Light is absorbed by the retina, the layer of light-sensitive cells lining the back of the eye.
Despite the differences in structure, eyes generally use the same set of biochemical tools to transduce light into a neural signal. A carotenoid compound (such as the chemical relatives of vitamin A), linked to a protein in the retinal cell membrane, captures the light energy. The light alters a chemical bond in the carotenoid, which then changes its shape, causing the membrane to alter its electrical state. The change in electrical state then will cause the retinal cell to release a chemical (called a neurotransmitter) which will excite an adjacent nerve cell. The carotenoid plus an associated protein is referred to as the visual pigment. (Interestingly, carotenes are also used by plants to help them capture the energy of the sun in photosynthesis.)
The visual image detected by the retina is not recorded whole and passed unchanged to the brain. Instead, the image is processed, with highlighting and integration of some features along the way. The degree of image processing varies among different types of animals. For example, toads have a "worm detector." When the optic nerves send signals to the visual-processing area of the brains to form a linear pattern, the brain says "worm" and the toad aligns to the worm and snaps it up.
The eyes of some animals have fields of vision with little or no overlap between the two eyes, giving them a 360-degree view of the world. Such wide fields of view are seen often in prey animals, allowing higher vigilance against predators. Some ground birds, for example, have eyes that have absolutely no overlap. In contrast, other animals have eyes with highly overlapping fields of vision. This allows stereoscopic vision, in which an object is viewed from two different points. Integration of these images, along with information about the relative direction in which the two eyes are pointing, allows depth perception, a critical tool for predators. It is also important for monkeys and other tree-dwelling primates, for instance, in order to know how far that next branch is so that they do not fall out of their trees!
Ultraviolet and Polarized Light
The visual spectrum of all animals goes from around 350 nanometers (ultraviolet) through all the colors most humans see to the infrared, around 800 nanometers (one nanometer equals one-billionth of a meter). In the vertebrates, elaborate color vision is found in the primates (including humans), birds, lizards, and fish. Most other mammals lack the ability to see red or other colors (including bulls).
Insects are less well able to see the red than humans can, but they do see colors, and some insects can detect ultraviolet light. Bees can see the hidden ultraviolet color patterns of black-eyed susans and other flowers, for instance, allowing them to hone in on these flowers more easily.
Another unusual light quality that insects can detect is the plane of light polarization. Light polarization means that all of the rays arriving at the retinal cells are vibrating in the same plane; light typically becomes polarized when it is reflected off surfaces. Insects' retinas are arranged so that they detect changes in polarization. This makes it possible for honeybees to determine the direction of the sun even on cloudy days. The sun's direction in the sky is a critical piece of information communicated in the bee dance that a scout bee will do to communicate the location of nectar or pollen sources to other bees in the hive.
SEE ALSO Eye
David L. Evans
Drickamer, Lee C., Stephen H. Vessey, and Elizabeth M. Jakob. Animal Behavior, 5th ed. Dubuque, IA: McGraw-Hill, 1996.
Romoser, William S., and J. G. Stoffolano, Jr. The Science of Entomology, 4th ed. Boston: McGraw-Hill, 1998.
Saladin, Kenneth S. Anatomy and Physiology: The Unity of Form and Function, 2nd ed. Boston: McGraw-Hill, 2000.