July 5, 2022

Arachnids, or arachnids (Arachnida), is a class of arthropods that includes spiders, mites, scorpions, grasshoppers, solpugs, and some smaller groups. They differ from other arthropods by dividing the body into two tagmas: cephalothorax (prosoma) and abdomen (opisthosoma). Usually representatives of the class have 8 walking limbs, pedipalps and chelicerae. Development is basically straight forward.

Body structure

The body of arachnids consists of a cephalothorax and an abdomen. The cephalothorax is covered with a dense cuticle and includes six segments. The ends of the first segment — chelicerae — are the organs of capturing and grinding food. In spiders, ducts of poisonous glands open at the tips of the chelicerae. The extremities of the second segment — the pedipalps — perform the function of holding prey, participate in the transfer of sperm into the female's genital tract, and serve as organs of touch. The extremities of the next four segments — the legs — are involved in movement, weaving webs (spiders), digging kidneys, keeping an egg cocoon, prey. They have olfactory and touch receptors. Unlike crustaceans, both antennae in spiders are reduced, faceted eyes are absent. The abdomen is connected to the cephalothorax by means of the seventh segment, which does not bear limbs. The ends of the abdomen, which carry the "gills", turn into "lungs". The ends of the tenth and eleventh segments form cobweb warts in spiders. A characteristic feature of arachnids is the tendency to fuse body parts, with the formation of a cephalothorax and abdomen. In the more primitive (phalanges), both departments still retain segmentation. Scorpions have segmentation only on the abdomen, the abdomen of spiders is no longer segmented, and ticks have even lost the division of the body into cephalothorax and abdomen.

Internal structure

Nervous system

The nervous system is characterized by the concentration of nerve nodes. Spiders have a single cluster of neurons in the front part of the cephalothorax - the cephalothoracic nerve mass. From the senses, touch (sensitive hairs on the body and limbs) and smell are developed. Vision is very weak, there are several pairs of simple points.

Digestive system

Most arachnids are predators adapted to feeding on semi-liquid food. Digestion in many arachnids occurs in two e