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Spiders (arachnids) are breathing arthropods with eight legs and a tiger family with poisonous snakes and gases. They are the largest order of arachnids, ranking seventh in the total biodiversity of all other species. Spiders are found in every continent except Antarctica, and in almost every habitat except air and ocean colonization. As of November 2015, taxonomists have recorded at least 45700 species of spiders and 113 families. However, since 1900, more than 20 different classification methods have been put forward, and the scientific community has been divided on how to classify all these subjects.
Anatomically, spiders differ from other arthropods in that the usual body parts are fused into two targets, the head, chest and abdomen, and are connected by a small cylindrical pedicel. Unlike insects, spiders have no antennae. In all the most primitive groups (mesothelia), spiders have the most centralized nervous system of all arthropods, because all their ganglia are fused in the head and chest. Unlike most arthropods, the spider's limbs do not have extensors, but are stretched by hydraulic pressure.
Spiders are pincers, so they are arthropods. As arthropods, they have: limbs connected to each other, all covered with chitin and protein epidermis; a head composed of several parts fused during embryonic development. As pincers, their bodies are composed of two tags with similar functions. These two sets of fragments have similar functions: the most important part is called the head chest or precursor, which is the complete fusion of these fragments. In insects, two separate tags will be formed, namely the head and chest. The tagma at the rear is called an abdominal or eyelid tumor. In spiders, the head, chest and abdomen are connected by small cylindrical pedicels. The segment fusion pattern of arthropods is unique in arthropods, usually the first segment disappears in the early stage of development, so arthropods lack the antennae of most arthropods. In fact, the only appendage in front of the mouth of the crayfish is a pair of crayfish, which lack anything that can be used directly as "jaw". The first appendage behind the mouth is called pedipalps, which play different roles in different chelate groups.
