Visual Stimuli

The ability of most animals to see their surroundings has made the light stimulus one of the most important methods of communication.

Nocturnal animals have no colour vision but some of them are able to 'see' the part of the spectrum called the infra-red. These animals can receive a picture of warm objects from the heat which the objects give out. Snakes which hunt at night detect warm-blooded prey in this way. The rattlesnake has heat pits close to its eyes.

Even in the daylight not all animals see the surroundings as we do. Bees can see the ultra-violet part of the spectrum as well as yellow and blue light. This means that they see many flowers in a completely different way from the way we do. Some of the animals which live in the sea are also able to detect ultra-violet light.

Some fish which live in deep water where very little sunlight is able to penetrate have light-producing organs on the sides of their bodies. In some cases these light organs help to attract prey. In other cases they are used as a defence to make the fish seem more aggressive than it actually is. (Some scientists think that these light organs are just simply to help some fish to find their way in the dark!)

Monkeys use a variety of facial expressions to communicate with each other. Elephants use their ears, trunks and bodies to show their feelings. When an elephant is annoyed it sways its body from side to side.

Animals and plants which live in the daylight use colour to their advantage. Flower petals an succulent fruits are brightly coloured to attract animals. Male birds develop bright plumage an perform mating displays to attract females. The female stickleback will only respond to a male which has developed a red colour on its underside. Other animals use stimuli to warn predators that they are unpleasant to eat.

Certain animals which are active at night or which live in dark places produce light signals. Male fireflies send out flashes of light while in flight as a signal to the female fireflies. Different species of firefly use different flashing codes, just as every lighthouse can be identified by its sequence of flashes.

Johnny's Fact File No.46 The male firefly of the species Photirus pyrlis flashes spontaneously while in flight. He emits a 0,3s flash every 5,5s at 25°C. On the ground, the female responds after a precise interval of 2s.

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