Part XX: How Organisms Communicate
The Effect of Stimuli on the Behaviour of an organism
The Effect of Chemical Stimuli
The Effect of Sound Stimuli
The Effect of Light Stimuli
The Effect of the Touch on Stimulus
The Effect of Stimuli on Behaviour Summary
(useful for revision)
The Effect of Stimuli on Behaviour : Questions
Chemical Emitters and Receptors
Sound Emitters and Receptors
Emitters and Receptors Summary (useful for revision)
Emitters and Receptors : Questions
Communication inside the Organism
The Vertebrate Central Nervous System
Topic Chapters Index
More about Light
The vertebrates and some of the larger molluscs, such as the squid, have developed a camera-like eye with only one large lens. This eye gives a more precise image
than the compound eye. It has been estimated that the compound eyes of a honey bee have only 1% of the efficiency of human eyes.
The camera-like eye is able to control the amount of light entering it. It does this with a structure
called the iris. In the light the iris is large and the light can only enter the eye through a small, black circle in the
centre of the iris, called the pupil. In the dark the iris becomes smaller and the pupil becomes larger. This allows more
light to enter the eye. Nocturnal animals, such as the owl, have very large pupils.
The positions of the two camera-like eyes on the head are not the same for all vertebrates. Humans have
three-dimensional vision, also called binocular vision, because the right and the left eyes see the same things but from slightly different angles. Birds, however, only have two
small areas of binocular vision but, because of the position of their eyes, they are able to see all around them at the same time.
The camera-like eye contains two kinds of light receptor cells called rods and cones. The rods are used in night vision and form a black
and white image. The cones are used for daylight vision and, in many animals, form a coloured image. The sharpness of the image is dependent on the number of cone cells present in the eye.
The hawk's eye has many more cone cells than the human eye and its vision is very sharp; it is eight times sharper than human
vision. The elephant's eye, however, has very few cones. The elephant can only manage to see blurred objects which are near to it and it has very poor distance vision.
The large eyes of the owl cannot be moved up and down or from side to side. The owl has to move its whole head to see to the side.
Fortunately, its head is able to move more than 1800 in either sideways direction, so the owl can turn its head to look directly behind it!
LIGHT EMITTER AND RECEPTOR ORGANS
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.
A female glow worm attract a male glow worm using bioluminescence.
At night, in woodland areas, the and of the
glows with a bright green/yellow light.
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:
||Number of facets in each compound eye
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.
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