Arachnids Temporal range: Early Silurian – present
Left to right: Phidippus mystaceus (Araneae), Pseudoscorpion (Pseudoscorpiones), Hottentotta tamulus (Scorpiones), Ixodes ricinus (Ixodida), Heterophrynus (Amblypygi), Aceria anthocoptes (Trombidiformes), Harvestman (Opiliones), Galeodes caspius (Solifugae), and a Whip scorpion (Thelyphonidae).
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Class:	Arachnida Lamarck, 1801
Orders
Ricinulei Opiliones – harvestmen Solifugae – camel spiders Acariformes mites Parasitiformes mites and ticks Pseudoscorpiones – pseudoscorpions Scorpiones – scorpions †Phalangiotarbi (extinct) †Trigonotarbida (extinct) †Uraraneida (extinct) Araneae – spiders †Haptopoda (extinct) Amblypygi – whip spiders Schizomida – short-tailed whipscorpions Palpigradi – micro-whipscorpions Thelyphonida – vinegaroons ?Xiphosura – horseshoe crabs

Basic characteristics of arachnids include four pairs of legs (1) and a body divided into two segments: the cephalothorax (2) and the abdomen (3).

The arachnids are a class of eight-legged arthropods. They are a highly successful group of mainly terrestrial invertebrates: spiders,scorpions, harvestmen, ticks, and mites, and a number of smaller groups.

In 2019, a molecular phylogeny study put horseshoe crabs in the Arachnida.