It is called tactism to a form of innate response of lower animals to environmental stimuli. It is also known as taxi or taxi. This type of response is present mainly in invertebrates.
It is equivalent to the tropism of plants. It consists of the movement of the animals towards or away from the stimulus. The type of response is genetically encoded, that is, it is an inherited response that does not require learning.
The main characteristic of tactism is its directionality. Depending on the direction of displacement in relation to the source of the stimulus, tactisms can be classified as positive or negative. In positive tactism the organism moves closer to the stimulus. In negative tactism, on the contrary, it moves away from this.
Tactisms are associated with the attraction or repulsion of a stimulus by mobile organisms or cells. A receptor capable of capturing the stimulus is always present.
The most prominent characteristic of tactism is directionality. Movement occurs in direct response to the source of stimulation. The cell or organism moves in different ways towards the stimulus.
Tactisms have evolved in all living beings. In prokaryotes they are of great importance for food. In this group the receptors tend to be quite simple.
In eukaryotes the receptors tend to be a bit more complex, depending on the group. Within protists and plants, tactisms are mainly associated with the movement of reproductive cells.
In animals, the most complex receptors are present, generally associated with the nervous system. They are of great importance for processes of sexual reproduction and feeding. Likewise, tactisms are involved in protecting against predators..
Human beings develop some tactisms. For example, sperm are moved by chemical and temperature stimuli. There are also tactisms that may be involved in the development of agoraphobia.
Depending on the way the organisms move as well as the number of receptors, different mechanisms are present. Among these we have:
Orientation occurs by alternating lateral movements. It occurs in organisms with a single receptor. Apparently, the body compares the intensity of the stimulus between one position and another.
This mechanism is presented in Euglena, earthworms and the larvae of some Diptera. On Euglena, the receiver compares the intensity of the light and generates lateral movements.
In Diptera larvae, there is a photoreceptor on the head that differentiates between different intensities of light. The larva moves its head from side to side and moves in the opposite direction to the stimulus of light.
It occurs in organisms that have paired intensity receptors. In this case, the orientation is direct and the organism turns for or against the stimulus.
When the organism is stimulated by two sources, the orientation is towards an intermediate point. This is determined by the relative intensity of both sources..
If one of the two receptors is covered, the movement is in circles. This mechanism occurs in various arthropods, mainly insects..
In this case, when two sources of stimulus are presented, the animal chooses one of them and directs its movement for or against it. However, the orientation changes from one source to another following a zigzag course..
This type of movement has been observed in bees (Apis) and hermit crabs.
These tactism mechanisms are associated with the direction of orientation of the movement. Two types are known:
The movement maintains a constant angle relative to the source of the stimulus. Night butterflies fly keeping the light at right angles to their body. In this way they move parallel to the ground.
For their part, bees fly from the hive to the flowers at a constant angle to the sun. The ants also move following a fixed angle with respect to the sun, to return to their nest.
Movement orientation is based on memory. In some wasps, the movement is in circles around the nest.
Apparently they have a mind map that helps them orient themselves and get back to it. In this map, the distance and topography of the area where the nest is located are important..
Depending on the source of stimulation of the movement, the following types occur:
The organism moves stimulated by the direction of the wind. In animals, they place their body parallel to the direction of the air stream..
It has been observed in moths as a mechanism for locating pheromones. Also, in earthworms to orient themselves towards a particular odor.
The stimulus for movement are changes in atmospheric pressure. In some Diptera a slight decrease in barometric pressure increases flight activity.
It has been observed in some bacteria. Changes in energy levels from electron transport mechanisms can act as a stimulus.
Cells can move in response to electron donor or acceptor gradients. It affects the location of species that are arranged in different strata. May influence the structure of microbial communities in the rhizosphere.
It is the positive or negative movement associated with a light gradient. It is one of the most common tactics. It occurs in both prokaryotes and eukaryotes and is associated with the presence of photoreceptors that receive the stimulus
In filamentous cyanobacteria, cells move toward the light. Eukaryotes are able to differentiate the direction of light, to move for or against it.
The response is associated with electrical stimuli. It occurs in various types of cells such as bacteria, amoebae, and molds. It is also common in protist species, where hair cells show strong negative galvanotactism..
The stimulus is the force of gravity. It can be positive or negative. Positive geotacticism occurs in rabbit sperm.
In the case of some groups of Protistas like Euglena Y Paramecium, the movement is against gravity. Similarly, negative geotacticism has been observed in newborn rats..
Various organisms have the ability to perceive water. Some are sensitive to changes in humidity in the environment.
Water stimulus receptor neurons have been found in insects, reptiles, amphibians, and mammals.
Various organisms use the earth's magnetic field to move. In animals that have large migratory movements such as birds and sea turtles, it is quite common.
Neurons in the nervous system of these animals have been shown to be magnetosensitive. Allows orientation both vertically and horizontally.
Cells migrate against or in favor of a chemical gradient. It is one of the most common taxias. It is of great importance in the metabolism of bacteria, since it allows them to move towards food sources.
Chemotaxis is associated with the presence of chemoreceptors that can perceive the stimulus for or against the substances present in the environment..
Organisms respond to the direction of water currents. It is common in fish, although it has been observed in worm species (Biomphalaria).
Sensors are presented that perceive the stimulus. In some fish, such as salmon, rheotaxis can be positive in one stage of development and negative in another..
Cells move for or against a temperature gradient. It occurs in both unicellular and multicellular organisms.
Sperm from various mammals have been shown to have positive thermotaxis. They are able to detect small changes in temperature that guide them towards the female gamete.
It is observed in some animals. They prefer to stay in contact with the surfaces of inanimate objects and not be exposed to open spaces.
It is considered that this behavior can contribute to orientation as well as not being exposed to possible predators. In humans, the occurrence of exaggerated thigmotacticism has been associated with the development of agoraphobia.