- The ability to respond to stimuli is a characteristic of all plants and animals.
- The animals respond quickly to the stimuli and their responses can be easily noticed.
- Plants on the other hand, respond slowly and cannot be easily noticed.
- A response is defined as the conscious or unconscious reaction of the living beings to the stimuli, which is mostly an involuntary activity that benefits the organism.
- Stimulus may be physical or chemical change in the immediate surroundings of the living organism. Gravity, light, chemicals, water, sound, temperature etc. are few examples of stimuli.
- Whenever a mobile (moving or motile) organism responds to a stimulus in such a way that movement or locomotion is affected, the response is called taxis.
- This locomotion or movement may be towards (positive taxis), away from (negative taxis) or at a fixed angle to the source of the stimuli. Taxis, on the basis of stimulus is of following types:
- Phototaxis:
- Oriented locomotory movement caused by the light either towards a source or away from it is called phototaxis. E.g. Euglena
- Euglena moves towards the source of light (positive phototaxis) while Planarians, cockroaches, silver fish, earthworms, slugs etc. move away from the source of light showing negative phototaxis.
- Amoeba and Paramecium avoid both direct sunlight and total darkness. They respond positively to normal or weak light.
- Housefly and moths show positive response to light while mosquito exhibits the negative response.
- Thermotaxis:
- Response of animals to temperature is known as thermotaxis.
- Animals thrive to remain in an optimum temperature range. E.g Paramecium (24-280C), Amoeba (20-250C) which is a positive thermotaxis.
- An avoiding reaction (negative thermotaxis) is shown to temperature higher or lower than this.
- Chemotaxis:
- Animals showing response to the chemical substances is called chemotaxis.
- It is negative in most of the cases. E.g. Amoeba shows negative chemotaxis to strong solutions of alkalis and to sugars.
- Paramecium shows a positive response to a drop of weak acid solution however, it shows negative chemotaxis to salt solution.
- Mosquitoes show negative chemotaxis to chemicals like king mat or
- Geotaxis:
- Animal’s response to gravity is called geotaxis.
- Some of the animals show negative responses to gravity while other show positive response. E.g Amoeba exhibits mostly positive response at it drops to the bottom of the container filled with water whereas Paramecium shows negative geotaxis.
- Housefly shows negative response to gravity whereas the fruit fly when placed in a vertical glass tube, moves upwards showing negative geotaxis.
- Rheotaxis:
- Response to current of air or water is called rheotaxis.
- Amoeba shows negative response, while Paramecium shows positive response, orienting themselves with their anterior ends upstream and swimming against the current.
- Some of the fish are also positively rheotactic. Eg. Trouts living in stream, not swept away by the water current.
- In winds, birds and insects mostly fly upward although they may go in any direction.
- Thigmotaxis:
- Response to contact or touch is called thigmotaxis.
- Paramecium when moves slowly and comes in contact with any object like algae or a plant stem, becomes quiet. However, it shows an avoiding reaction when its anterior end is strongly touched with a solid object.
- Galvanotaxis or electrotaxis:
- Movement of an organism or any of its parts in a particular direction in response to an electric current is called galvanotaxis or electrotaxis.
- In positive galvanotaxis, animals move towards the negative pole or cathode when a weak electric current is supplied. E.g. Paramecium, when exposed to weak electric current moves towards cathode.