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Light: Emitter organs
Organisms which emit light include some one-celled animals, some molluscs, some insects and a few fish. They are able to produce light because of a special chemical reaction which takes place in certain parts of their bodies. To make light both oxygen and energy are needed.
The colour of light which is given out, from fireflies for example, varies from green to bright yellow. It has been estimated that a flash of 1 nanosecond uses 2520 kilojoules of energy. (One nanosecond = 0,000000001 seconds.) This can be compared to the 1600 kilojoules of energy available in 100g of sugar.
Light: Receptor organs
The simple eye is able to detect light and dark. It may even be able to focus an upside-down image, but with very little detail. The scallop, a sea-water mollusc, has 100 simple eyes on the part of its body lining the edge of the inner surface of its shell.
Slugs and snails also have simple eyes at the ends of their large antennae. Spiders have up to eight simple eyes arranged in two groups on their heads. These two groups are called the main eyes and the secondary eyes. Scorpions can have up to five pairs of simple eyes.
Caterpillars have five or six simple eyes on each side of the head. Adult insects have three simple eyes at the top of their heads but they also benefit from a pair of compound eyes.
Compound eyes are only found in arthropods. These eyes are made of thousands of tiny lenses or facets. Each facet acts as an individual lens. This means that the image is seen as a mosaic. The more facets the insect has in its compound eyes the better the detail of the image will be. The compound eye of most crustaceans has over 10000 facets.
The table shows the number of facets in each eye of three insects:
The table shows that the dragonfly is able to form a clearer mosaic image than that of the housefly or the bee. Some water beetles have eyes which are split into two parts. The upper section of the eye is adapted to see up into the air while the lower section is adapted for underwater vision.