Directional movement
A taxis is a directional movement response to a stimulus. The organism moves in a particular direction relative to the source of that stimulus.
Positive taxis
The organism moves towards the stimulus.
For example, mobile bacteria can show positive chemotaxis by moving towards an area containing a higher concentration of glucose.
Negative taxis
The organism moves away from the stimulus.
A mosquito moving away from an aerosol spray is an example of negative chemotaxis.
Taxes can occur in response to different environmental stimuli. For example, movement in relation to light is a phototaxis.
Euglena can show positive phototaxis by swimming towards a light source using its flagellum. This movement can place it where light is available for photosynthesis. Moths moving towards light are another example of positive phototaxis.
Because a taxis directs movement relative to the stimulus, it can help an organism reach favourable conditions or move away from unfavourable ones.