ODWS icon

The Open Door Web Site


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
Touch 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.


Drawing of an octopus © Shirley Burchill


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.


Section through the complex eye © Shirley Burchill


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.


Hawk, Devon, UK © Shirley Burchill


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.


Owl, Chestnut Centre, Derbyshire, UK © Shirley Burchill


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!



Custom Search

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.


Glow worm © Paul Billet

A female glow worm attract a male glow worm using bioluminescence.
At night, in woodland areas, the and of the female's abdomen
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


Drawing of a scallop showing its simple eyes © Shirley Burchill


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.


Dragonfly © Paul Billiet


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:


Insect Number of facets in each compound eye
housefly 4000
bee 15000
dragonfly 28000


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.




The Open Door Web Site is non-profit making. Your donations help towards the cost of maintaining this free service on-line.

Donate to the Open Door Web Site using PayPal







© The Open Door Team 2017
Any questions or problems regarding this site should be addressed to the webmaster

© Paul Billiet, Shirley Burchill, Alan Damon and Deborah James 2017

Hosted By
Web Hosting by HostCentric