Arachnids are arthropods, comprising some 70,000 named species, and include spiders, scorpions, harvestmen, ticks and mites. Nearly all are terrestrial.
There is no separate head and the body is organized into two parts called the prosoma, covered by a carapace-like shield, and the opisthosoma, or abdomen, which may be segmented. There are 6 pair of appendages. The first pair, the chelicerae, serve in feeding and defense. The next pair, the pedipalps have been adapted for feeding, locomotion, and reproductive functions. The other 4 pairs serve as limbs.
The Acari or Acarina is the taxonomic group that includes ticks and mites. This group is the most diverse and abundant of the arachnids, and its fossil history goes back to the Devonian era. There are three major lineages, the Opilioacariformes, Acariformes and Parasitiformes, with more than 45,000 known species worldwide.
The Acariformes are the most diverse of the three orders of mites. There are over 32,000 described species in 351 families.
Many species of Parasitiformes, including the ticks, are parasitic, but not all; for example, about half of the 10,000 known species in the suborder Mesostigmata are predatory and cryptozoan, living in the soil-litter, rotting wood, dung, carrion, nests or house dust.
Order of arachnids, the tailless whip scorpions, comprising about 17 genera and 136 species that are found in tropical and subtropical regions worldwide. Amblypygids are small, their bodies are broad and highly flattened and the first pair of legs are extremely long and modified to act as sensory organs. They have no silk glands or venomous fangs, but can have prominent pincer-like pedipalps.
There are about 35,000 species of spider worldwide.
They are characterized by four pairs of legs and an unsegmented furry or polished "abdomen". They all produce a poison in their fangs that acts as a neurotoxin. They also produce silk from their spinnerets though not all build webs. They are predators, catch their prey in various ways, immobilize it with their venom and then suck out the body fluids.
Order of spider-like animals commonly known as harvestmen or daddy-long-legs.
The body is not divided into two sections as in true spiders, but is rounded and very small compared to the long legs. They can be predators, grasping small invertebrates with their pedipalps and chewing with their chelicerae, but they also scavenge on dead animals and plants.
Order of false scorpions with about 2000 species.
They are very small, live in such places as under stones, in soil or under bark. Although anatomically distinct, superficially they resemble scorpions, having formidable pedipalps with which they attack their prey. They then immobilise it, before ripping it up with their dagger-like chelicerae and sucking out the fluids. They do not have a sting.
Scorpions are characterized by the great pincers, the pedipalps, with which they grasp their prey. They have four pairs of legs and a segmented tail ending with a sting characteristically arched up over the back. They prey mainly on insects, or other scorpions, but the large ones also attack lizards and snakes. The venom is very toxic.
Most scorpions are nocturnal and live in deserts or rainforests, with some being arboreal. There are about 1200 species and they are slow-growing and long-lived.
Order of spider-like animals commonly known as the sun spiders or wind spiders because they can run so fast. They are anatomically different from true spiders, have no poison, spin no web, and rip their prey apart with large, dagger-like chelicerae. There are about 900 species.
Order of invertebrates in the class Arachnida.
Commonly known as whip scorpions, they are superficially similar to scorpions but the first pair of legs is modified to serve as antennae-like sensory organs. They have a whip-like flagellum at the end of their abdomen but are not venomous and many species also have large scorpion-like pedipalps (pincers). Whip scorpions are carnivorous, nocturnal hunters.