The Open Door Web Site
Part XX: How Organisms Communicate
The Effect of Stimuli on the Behaviour of an organism
Some plants, such as mosses and liverworts,
In flowering plants, the pollen grains which reach
Some sharks are able to detect these odours
Marine slugs produce a strong acid to defend themselves
The spawning of mussels in a mussel bed is
Chemical signals are used by many different types of plants and animals. The chemicals are released into the air, or in some cases into water, in very small quantities. These chemicals are extremely specific stimuli. The receiving animal or plant will only react to the presence of a precise chemical. Animals use chemical stimuli for different activities such as mating, marking territories, setting trails and identifying young.
Plants also use chemical stimuli in different ways. Flowers attract bees with their scent. The scent is caused by a complicated chemical which the petals release into the air.
Some plants, such as mosses and liverworts, use chemicals to attract the male sex cells to the female sex cells. In these plants fertilization can only take place when they are wet and the sperms swim towards the egg cells guided by the chemicals which the egg cells release into the water.
In flowering plants, the pollen grains which reach the mature stigma of a flower will only develop a pollen tube if a certain chemical is present.
This means that only the stigma of a flower of the same species as the ones which produced the pollen grains will stimulate the pollen tube to grow towards the ovule.
Releasing chemicals into the air has one major disadvantage for animals. The scent does not disappear immediately and its presence could attract predators as well as members of the animal's own species. This is why only very small amounts of the chemical are produced.
The advantage of using chemical stimuli is that they are able to travel long distances, up to 50km. Moths, which are active at night, use chemicals to attract mates. It is the female moth which produces the chemical. The males are capable of detecting extremely small amounts of the chemical. They use their antennae to pick up its presence in the air. Of course, the strength and direction of the wind determine where and how far the chemical travels.
There are two types of chemicals used to stimulate a response from other organisms. There are the chemicals which are released into the air and are detected by the olfactory organ of the receiver, such as the perfumes produce by flowers to attract insects.
There are other chemicals which do not produce scent but are passed from one individual to another by contact, such as the chemical produced by the queen bee. This uses the sense of taste rather than the sense of smell.
The spawning of mussels in a mussel bed is regulated by a chemical stimulus so that all of the mussels liberate their eggs and sperms into the water at the same time.
Some chemicals are used as alarm stimuli. Ants and bees become alarmed and excited in the presence of formic acid. Agitated and injured fish give off warning odours that alarm other fish of their species.
Some sharks are able to detect these odours and are attracted by them. In this way they move towards the injured fish.
Some animals produce repellent chemicals to keep predators away. These can be extremely odorous, harmless substances or very toxic chemicals. Stinkbugs, millipedes and skunks produce strong-smelling, bitter-tasting secretions when they are disturbed.
Marine slugs produce a strong acid to defend themselves, and bombardier beetles eject a spray of fluid which has been heated by chemical action to about 100°C!
Some of the repellent chemicals produced by animals, (those which are not too odorous or harmful), have been identified by scientists and manufactured in large quantities. They are used in the insect-repellent sprays and solutions used by humans.
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