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Arachnids (class Arachnida) form the second largest group of terrestrial arthropods (phylum Arthropoda) with the class Insecta being the most numerous. There are over 70,000 species of arachnids, which include such familiar creatures as scorpions, spiders, harvestmen or daddy longlegs, and ticks and mites, as well as the less common whip scorpions, pseudoscorpions, and sun spiders. Arachnids are members of the subphylum Chelicerata, which also includes the phylogenetically ancient horseshoe crabs.

Like other arthropods, arachnids have paired, jointed appendages, a hardened exoskeleton, a segmented body, and a well-developed head. They differ from other arthropods by the organization of their body into two main parts, the prosoma (equivalent to the head and thorax of insects) and the opisthosoma (or the abdomen). There are six pairs of appendages associated with the prosoma. The first pair are stabbing appendages near the mouth called chelicerae, used for grasping and cutting, and the second pair are called pedipalps or general purpose mouthparts. The last four pairs of appendages are the walking legs. Most arachnids are terrestrial and respire by means of book lungs, or by tracheae (air tubes from the outside to the tissues), or both. Most arachnids are terrestrial carnivorous predators. They feed by piercing the body of their prey, and then either directly ingesting its body fluids, or by releasing digestive secretions onto the outside of the prey to predigest the food before ingestion.

Scorpions (order Scorpiones) are distinguished by their large, pincer-like pedipalps, and a segmented abdomen consisting of a broad anterior part and a narrow posterior part ("tail"), which ends in a sharp pointed stinger. The latter contains a pair of poison glands whose ducts open at its tip. The venom is neurotoxic (attacking nerve functions) but, except in a handful of species, not potent enough to harm humans. Scorpions breathe by means of book lungs. Mating in scorpions is preceded by a complex courtship behavior. Newly-hatched scorpions are carried around by the mother on her back for one to two weeks. Scorpions are nocturnal, and feed mostly on insects. During the day they hide in crevices, under bark, and in other secluded places. They are distributed worldwide in tropical and subtropical regions.

In spiders (order Araneae) the abdomen is not segmented and it is separated from the cephalothorax by a narrow waist. The large and powerful chelicera of some spiders contains a poison gland at the base, while the tips serve as fangs to inject the poison into the prey. The pedipalps of spiders are long and leg-like. In male spiders, the pedipalps each contain a palpal organ, used to transfer sperm to the female. Some species of spider have only book lungs for respiration, while others have both book lungs and tracheae. Spiders possess silk producing glands whose secretion is drawn into fine threads by structures called spinnerets located on the lower side A Cosmetid harvestman. JLM Visuals. Reproduced by permission.

of the abdomen. Different types of silk are produced and used for a variety of purposes, including orb-weaving, ensnaring prey, packaging sperm to be transferred to the female, and making egg sacs. Although all spiders produce silk, not all weave orbs. The courtship patterns of spiders are quite varied.

Spiders have a worldwide distribution and are ubiquitous, living in and around human habitations, in burrows in the ground, in forests, and even underwater. Spiders are predators, feeding mostly on insects. Despite their reputation as fearsome animals, spiders actually benefit humans by keeping some insect populations under control. The bite of the very few potentially harmful species of spiders is rarely fatal to humans.

Most mites and ticks (order Acari) are small, mites being microscopic, and ticks measuring only 0.2–1 in (5–25 mm) in length. The oval body of acarines consists of the fused cephalothorax and abdomen. The chelicerae and pedipalps are small, and form part of the feeding apparatus. Adult ticks and mites have four pairs of walking legs, but the larvae have only three pairs. Respiration in acarines is by tracheae. The ticks are mostly bloodsucking ectoparasites of mammals. In addition to sucking blood, and injecting poison into the host in the process, ticks transmit the agents of diseases such as Rocky Mountain spotted fever, Lyme disease, relapsing fever, typhus, and Texas cattle fever. A female tick needs to engorge by feeding on her host's blood before she can lay eggs. An engorged female is three or more times her original size. The feeding requires attachment to the host for days. The larval and nymphal stages likewise feed before they molt and progress to the next stage. Ticks have specialized sense organs which enable them to locate a host more than 25 ft (7.5 m) away.

Many mites are either ecto- or endoparasites of birds and mammals, feeding on the skin and underlying tissues. Many more mites are free living. Some, such as the chigger, are parasitic as larvae but free living in the nymph and adult stages. The ectoparasites live on the host's body surface, while the endoparasites excavate tunnels under the host's skin in which they live and reproduce. While some parasitic mites transmit disease organisms, many produce diseases such as scabies, mange, or cause an intense itch. Ticks and parasitic mites are clearly of great economic importance. Some free-living mites are also of considerable importance because they cause destruction of stored grain and other products. House dust mites cause allergies in many people.

Harvestmen (order Opiliones) look superficially like spiders but differ in many respects. Harvestmen lack a waist separating the abdomen from the cephalothorax and their abdomen is segmented. They can ingest solid food as well as fluids. They do not produce silk and are non-venomous. Harvestmen feed on insects and contribute to insect control, although they are less important in this respect than spiders.

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