The arthropod (e.g., insects, crustaceans) eye is built quite differently from the vertebrate eye (and mollusk eye).
Arthropod eyes are called compound eyes because they are made up of repeating units, the ommatidia, each of which functions as a separate visual receptor.
Each ommatidium consists of
The pigment cells ensure that only light entering the ommatidium parallel (or almost so) to its long axis reaches the visual cells and triggers nerve impulses. Thus each ommatidium is pointed at just a single area in space and contributes information about only one small area in the field of view.
There may be thousands of ommatidia in a compound eye with their facets spread over most of the surface of a hemisphere. (The photo, courtesy Carolina Biological Supply Company, shows the compound eye of Drosophila melanogaster.)
The composite of all their responses is a mosaic image — a pattern of light and dark dots rather like the halftone illustrations in a newspaper or magazine. And just as in those media, the finer the pattern of dots, the better the quality of the image.
Arthropods that are apt to be active in dim light (e.g., crayfish, praying mantis) concentrate the screening pigments of their ommatidia into the lower ends of the pigment cells. This shift enables light entering a single ommatidium at an angle to pass into adjacent ommatidia and stimulate them also. With many ommatidia responding to a single area in the visual field, the image becomes coarser. The praying mantis probably can do little more than distinguish light and dark in the evening.
The shift in pigments does, however, make it more sensitive to light than it is in the daytime as more ommatidia can detect a given area of light.
Some insects are able to distinguish colors. This requires two or more pigments, each of which absorbs best at a different wavelength.
In the honeybee,
This system should enable the honeybee to distinguish colors (except red) and — as the image shows — behavioral studies verify this.
Why ultraviolet vision?
Television camera tubes are also sensitive to ultraviolet, as well as visible light, but their glass lens is opaque to ultraviolet. (This is why you can't get tanned — or synthesize calciferol — from the sunlight passing through window glass.)
Using a special ultraviolet-transmitting lens, Eisner and his coworkers at Cornell have demonstrated that many insect-pollinated flowers appear to the honeybee quite different from the way they appear to us. The sharp contrasts between flowers that appear similar to us partly explains the efficiency with which honeybees secure nectar from only one species of flower at a time even when other species are also in bloom.
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The photos (courtesy of Dr. Eisner) show a blackeyed susan photographed in visible light (left) and under ultraviolet light (right).
Monarch butterflies, which can migrate as much as 2500 miles (> 4000 km), navigate by ultraviolet light from the sun. When their view of the sun is through a filter that blocks out only its ultraviolet rays, their flight path becomes disoriented.
Ultraviolet vision is not limited to animals with compound eyes. A few marsupials, rodents, a bat that feeds on nectar, and many birds have also been shown to have ultraviolet vision.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CompoundEye.html#Flicker_effect
Ants can detect small movement through 5 cm of earth; can see polarized light.
Bees can see light between wavelengths 300 nm and 650 nm [300-380:
ultraviolet]. Have chemoreceptors (taste receptors) on their
jaws, forelimbs and antennae.
Worker honey bees have 5,500 lenses ("ommatidia") in each eye. Worker honey bees
have a ring of iron oxide ("magnetite") in their abdomens that may be used to
detect magnetic fields. They may use this ability to detect changes in the earth's
magnetic field and use it for navigation. Bees can see polarized light.
Butterflies have chemoreceptors (taste receptors) on its feet. The butterfly has hairs on its wings to detect changes in air pressure. Using vision, the butterfly Colias can distinguish two points separated by as little as 30 microns. (Humans can distinuguish two points separated by 100 microns.)
Buzzards have a retina that has 1 million photoreceptors per sq. mm; can see small rodents from a height of 15,000 ft.
Cockroaches can detect movement as small as 2,000 times the diameter of a hydrogen atom.
Dragonfly eyes contain 30,000 lenses.
Falcons can see a 10 cm. object from a distance of 1.5 km. Their visual acuity is 2.6 times better than human (Garcia et al., Falcon visual acuity, Science, 192:263-265, 1976.); and can see sharp images even when diving at 100 miles/hr.
Some fish can see into the infrared wavelength of the electromagnetic spectrum.
Penguins have a flat cornea that allows for clear vision underwater. Penguins can also see into the ultraviolet range of the electromagnetic spectrum.
http://faculty.washington.edu/chudler/amaze.html