The class Arachnids includes mainly terrestrial species (more than 60 thousand species). These include scorpions, hay makers, ticks, spiders, and other members of the class. Among ticks and spiders, there are secondary aquatic forms (for example, the silver spider).
In Arachnids, the body is divided into two sections – the cephalothorax and abdomen, there are no antennae.
On the cephalothorax there are four pairs of walking limbs and two pairs of modified limbs (mouth organs – chelicerae and leg tentacles), which serve to capture and grind food.
The spider grabs its prey with its hook-shaped chelicerae. Inside the chelicera there is a channel through which digestive juice from the poisonous glands located at the base of the chelicera enters the victim’s body. Near the chelicerae there are short, sensory organs of touch, covered with sensitive hairs – the leg tentacles.
At the lower end of the abdomen, there are three pairs of arachnoid warts that produce cobwebs – these are modified abdominal limbs.
The liquid released from spider warts instantly hardens in the air and turns into a strong spider thread.
Different parts of the spider web warts give off different types of cobwebs. Spider web threads vary in thickness, strength, stickiness. The spider uses various types of web to build a trapping net: at its base there are stronger and not sticky threads, and concentric threads are thinner and more sticky. Spiders use webs to strengthen the walls of their hideouts and to make cocoons for their eggs. Young spiders use long cobweb threads to move through space, which facilitates their dispersal. With the help of spider webs, spiders can descend from tree branches and other supports to the ground and rise.
In ponds and rivers with slowly flowing water, a silver water spider lives, which builds its nest in the water from cobwebs and fills it with air.
Arachnids have simple eyes.
The relatively weak development of the organs of vision is compensated by the well-developed organs of touch, which play a leading role in the orientation of arachnids in the environment. They also have organs that react to chemical stimuli, as well as organs of smell and taste.
The respiratory organs in spiders are the lungs (pulmonary sacs) and the trachea.
Scorpions have only lungs.
In ticks, gas exchange is carried out through the skin, but there are no special respiratory organs.
The circulatory system is not closed. Colorless blood.
The spider’s digestive system consists of the mouth, pharynx, esophagus, stomach, intestine, and anus.
Arachnids are predators. To attack other animals, they have various devices: from poisonous glands to spider warts for making trapping nets. Spiders inject digestive juice into the victim’s body, which dissolves its tissues. This is how extraintestinal digestion occurs. The spider then sucks in (with the sucking stomach) the liquefied food. In the midgut, long blind outgrowths increase its volume and absorption surface. The remnants of undigested food are removed through the anus.
The nervous system consists of a well-developed epopharyngeal ganglion and an abdominal chain. Arachnids have a complex instinctive behavior.
Arachnids are dioecious animals. Fertilization in representatives of some species is external, in others – internal. Parthenogenesis occurs in them – reproduction without fertilization, when the female lays unfertilized eggs, from which only females develop.
Usually arachnids lay eggs, but there are also viviparous ones.
Development without transformation, small individuals, similar to adults, emerge from the eggs. In many species, care for the offspring is observed: females guard the cocoon with eggs.
Distribution and significance
Scorpions live in countries with warm or hot climates, sometimes found in the mountains. Scorpions hunt at night. By releasing poison, scorpions immobilize or kill prey. They are fed by various arachnids, lizards or murine rodents. Scorpios can be dangerous to humans.
Ticks are a group of arachnids in which the body is not subdivided into sections. There are many pests and parasites among ticks, some are dangerous to humans, as they can transmit pathogens of various diseases, for example, encephalitis, a serious disease of the nervous system. Dangerous for humans and the scabies mite – the causative agent of scabies. It damages human skin, gnaws a course in it and lays eggs there. This usually occurs at the folds of the body where the skin is softer, such as on the hands. Scabies mites are easily transmitted from person to person. Herbivorous mites damage grain, flour and bread stores. Some feed on plant sap.
Spiders represent the largest group of arachnids. Spiders feed on various insects, some eat earthworms, snails, ants, lizards, frogs, mice, birds, tadpoles, small fish.