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The external structure of arachnids

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In the head-chest there are two nerve nodes that form a lot of brain nerves, and extend from the brain to the legs, eyes and other parts of the body. The brain occupies 20-30% of the volume of the cephalothorax.

Spiders have several sensory organs to get an impression of the environment in which they live. The spider has no ears. Mammals are heard using hairs located in the ear, where a wave of air pressure is converted into electrical signals. These signals are sent to the brain and are interpreted as sound. The spider hears with the help of very tiny hairs (thrichobotria) located on his legs. Via Thrichobotria the spider is able to very accurately determine the place of sound emission, interpreting the movement of air produced by this sound.

The eyes of a spider are very different between families. Spiders that hunt without prey, like wolf spiders (Lycosidae) spider lynx (Oxyopidae) and spiders horses (Salticidae) have a well-developed vision. Horse spiders can see almost as well as humans. Experiments have shown that they even have the ability to distinguish colors. Cave spiders that live in the dark do not see at all or see very poorly. They are completely dependent on sounds and sensations.
Orb-web spiders, for example Araneus diadematus have very small eyes. They practically do not need eyesight to catch prey. They have a very well-developed sensory mechanism that facilitates the detection of movements in her network.

Spiders smell using special sensitive hair located on their legs. There is no taste sensation in the spider's mouth. The spider feels whether its prey is edible with the help of chemically sensitive hairs on its legs.

Distribution and habitat

Arachnids are one of the oldest and most diverse classes of animals. In the world there are about 70 thousand species, most of which live on land. There are species that live in fresh water. Among ticks there are ectoparasitic species - live on the body of a person or animal.

Arachnids are ubiquitous. There are units that live in the tropics and subtropics. Scorpions are found in the temperate zone, and some species of ticks and spiders can live in polar conditions.

External structure

The body of the animal consists of two departments:

There are two pairs of oral organs on the cephalothorax: tentacles and chelicera. The first pair of limbs are chelicerae; at the ends they have claws. It is on them that there are ducts of poisonous glands, with the help of which the animal defends itself and kills prey.

The second pair of limbs covered with bristles is the tentacle. They are also an organ of smell and touch.

The next 4 pairs are walking legs. At the ends they have claws and are also covered with bristles. As a result, we get 6 pairs of limbs.

The abdomen is covered with a soft shell. There are no limbs on it, and in some spiders they are modified into spider warts. On top of the warts, the ducts of the glands, forming a web, open. On the abdomen there are exits of the respiratory, digestive and reproductive organs.

Fig. 1. External structure

Most arachnids in the extremities lack muscles. They move due to the effect of hemolymph pressure. Some types of scorpions have a muscle that can flex a couple of joints at once.

The integument of the body is complexly organized and is formed by a single-layer epithelium that forms the chitinous membrane. To protect against damage, as well as excess water loss, chitin is covered with a wax-like film. Many species on the surface of the body have hairs that perform a protective function and are sensory organs.

Features of the nervous system

The arachnoid nervous system is also diverse in structure. Outwardly, it is a whole abdominal chain, but there are a number of features:

  • In the brain there is no department responsible for the work of antennas in crustaceans and insects,
  • The front and rear sections regulate the work of arachnid eyes, as well as chelicera,
  • Ganglia in most cases are concentrated, forming a ganglion mass.

Fig. 2. Nervous system (blue color)

Sensory organs

Touch for spiders is of great importance, the presence of hairs on the body is proof of this. Each individual hair is attached to the bottom of a special fossa, which connects it with sensitive cells.

Sensitive hairs are able to pick up the slightest fluctuations in air or cobwebs. Depending on the intensity of the vibrations, spiders distinguish between the nature of irritation.

The lyre-shaped organs located throughout the body are responsible for the chemical senses.

The organs of vision are eyes that have a simple structure. It is difficult to answer the question: “How many eyes do arachnids have?” Because it all depends on the species. In general, their number varies from 2 to 12. Despite the number of pairs of eyes, the vision in this class is weak and they see at an insignificant distance.

Fig. 3. The layout of the eyes in different species

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