Taxis/Tactic Response

It is a locomotory response to a unidirectional stimulus.

The stimulus is unidirectional, i.e. a stimulus from one direction (specific direction).

The responses are grouped according to the stimuli which cause them.

1. Phototaxis (light)
2. Aerotaxis (O₂ concentration₎
3. Osmotaxis (osmotic pressure)
4. Rheotaxis (wind/water currents)
5. Chemotaxis (chemicals)
6. Thermotaxis (temperature)
7. Hydrotaxis (water)

Phototaxis – locomotory response to light intensity and direction.

Example; euglena, spirogyra and fruit flies move towards light. Wood lice, maggots and termites usually move away from light.

Aerotaxis – locomotory response to variations in oxygen concentration.

Example; Amoeba move from an area of low oxygen to high oxygen concentration.

Osmotaxis – locomotory response to variation in osmotic pressure.

Example; marine crabs burrow in the sand to avoid dilution of the body fluids.

Rheotaxis – locomotory response to variation in direction of water or air currents.

Example; fishes and planarians move against water currents. Butterflies and moths fly into wind currents in order to detect the scent of flowers.

Chemotaxis – locomotory response to variation of chemical changes in the environment of an organism.

Example; movement of the male gamete (spermatozoa) towards the female gamete (ovum). Mosquitoes fly away from insecticide repellants.

Thermotaxis – locomotory response to variation in temperature.

Example; paramecium move from low temperature (15⁰C) area to moderate temperature (25⁰C) area.

Survival Value of Tactic Responses

1. Enable organisms escape from harmful stimuli e.g. excessive heat, predators
2. Enable organisms to seek favourable habitats and acquire resources e.g. nutrients, and mates.
3. Chemotaxis enables fertilisation to take place.

Question

The diagram below represents a set-up during an experiment.

(a) (i) What was the experiment investigating?

• To investigate how ants respond to moisture/water/hydrotaxis

(ii) State the likely identity for substance K

• Anhydrous Calcium chloride/drying agent

(iii) Explain your answer in (ii) above.

• All moisture was absorbed hence cobalt (II) chloride remained blue

(iv) Account for the observations made in flask 2.

• More ants moved into the flask 2; due to presence of moisture/water vapour;