Arachnids

Arachnids (pronounced uh-RACK-nidz; class Arachnida) form the second-largest group of land arthropods (phylum Arthropoda) after the class Insecta. There are over 70,000 species of arachnids that 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. The marine horseshoe crabs and sea spiders are near relatives.

Physical characteristics

Arachnids have paired, jointed appendages (parts that are attached to the main body), a hardened exoskeleton (exo means "outer"), a segmented (divided into parts) body, and a well-developed head. Their body consists of two main parts: a fused head and thorax, and an abdomen. There are six pairs of appendages on the body: the first pair are clawlike fangs near the mouth used for grasping and cutting; the second pair serve as general-purpose mouth parts that may be modified for special functions; and the last four pairs of appendages are the walking legs.

Most arachnids live on land and breathe by means of book lungs (socalled because their thin membranes are arranged like the pages of a book) or by tracheae (small tubes that distribute air from the outside throughout the body), or both. Most are flesh-eating predators. They feed by piercing the body of their prey and directly consuming its body fluids or by releasing digestive secretions that predigest the food before they eat it.

Scorpions

Scorpions have large, pincerlike second appendages and a segmented abdomen that is broad in front and narrows to become taillike, ending in a sharp pointed stinger. The stinger contains a pair of poison glands with openings at the tip. The venom is neurotoxic (poisonous to the nerves) but, except in a few species, is not potent enough to harm humans. Scorpions have book lungs for breathing. They engage in complex courtship behavior before mating, and newly hatched young are carried on the mother's back for one to two weeks. Scorpions are nocturnal (active at night) and feed mostly on insects. During the day they hide in crevices, under bark, or in other secluded places. They occur worldwide in tropical and subtropical regions.

Spiders

In spiders, the abdomen is separated from the joined head and thorax by a narrow waist. The large and powerful first appendages of some spiders contain poison glands at their base, while the tips serve as fangs that inject the poison into prey. The second appendages of spiders are long and leglike. In male spiders, these appendages each contain an organ used to transfer sperm to the female. Some species of spiders have only book lungs for breathing, while others have both book lungs and tracheae.

Spiders have organs called spinnerets at the tip of the abdomen that contain silk glands. The spinnerets draw secretions from the silk glands to produce fine threads of silk. These are used to build webs, ensnare prey, package sperm to be transferred to the female, and make egg sacs. Although all spiders produce silk, not all weave webs. Spiders have courtship patterns prior to mating that are quite varied.

Spiders are found worldwide and live in many different habitats—in burrows in the ground, in forests, in human habitations, and even under water. Spiders are predators, feeding mostly on insects. Despite their reputation as fearsome animals, they actually benefit humans by keeping some insect populations under control. The bite of only about 30 species are dangerous, but rarely fatal, to humans.

Mites and ticks

Most mites and ticks are very small, mites being microscopic and ticks measuring only 0.2 to 1 inch (5 to 25 millimeters) in length. The oval body of these arachnids consists of the fused head and thorax and the abdomen. The first two pairs of appendages are small and are used for feeding. Adult ticks and mites have four pairs of walking legs, but the larvae have only three pairs. Breathing occurs through tracheal tubes.

The ticks are mostly bloodsucking parasites (organisms that feed on others) that attach themselves to the outer body of mammals, such as dogs, deer, and humans. In addition to injecting poison into the host while sucking blood, ticks can transmit other disease-causing organisms resulting in Rocky Mountain spotted fever, Lyme disease, relapsing fever, typhus, and Texas cattle fever. In order to lay eggs, a female tick must suck the host's blood until her body is filled; this feeding process is known as engorgement and requires attachment to the host for several days, after which the

engorged female may be three or more times her original size. The larval and nymphal stages likewise feed before they molt (shed their skin) and progress to the next stage of development. Ticks have specialized sense organs that enable them to locate a host more than 25 feet (about 7.5 meters) away.

Many mites are parasites of birds and mammals. They can be ectoparasites, feeding on the outer skin, or endoparasites, feeding on the underlying tissues. The ectoparasites live on the host's body surface, while the endoparasites dig tunnels under the host's skin in which they live and reproduce. While some parasitic mites transmit disease organisms, many cause diseases themselves. These include scabies and mange—contagious skin diseases characterized by inflammation, irritation, and intense itching.

Many more mites are free living, that is, not parasitic. Some, such as the chigger, are parasitic as larvae but free living in the nymph and adult stages. Free-living house dust mites cause allergies in many people. Others can cause the destruction of stored grain and other products.

Harvestmen (daddy longlegs)

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

[See also Arthropods ]

Arachnida (Amblypygi, harvestmen, mites, Palpigradi, pseudoscorpions, scorpions, spiders, Uropygi, whipscorpions; phylum Arthropoda, subphylum Chelicerata) Class of chelicerates which have book lungs or tracheae derived from gills, indicating their aquatic derivation. They have invaded most terrestrial habitats and have secondarily invaded aquatic habitats, although to a very much smaller extent (there is only one species of aquatic spider, and only one species of mite in ten is aquatic). Except for many plant and animal parasites found among the mites, and some scavenging harvestmen (order Opiliones or Phalangida), most arachnids are predatory. Scorpions have been recorded from the Silurian Period and a Silurian scorpion, Palaeophonus nuncius, was perhaps the first terrestrial animal. The first fossil spiders are known from the Devonian. The class is extremely diverse, but apart from the mites the body is in two portions, the prosoma (anterior portion) bearing the four pairs of legs, the eyes, the pedipalps, and the chelicerae; and the opisthosoma (posterior portion) containing most of the internal organs and glands. The two portions may be broadly jointed, or connected by a pedicel. The prosoma has a dorsal shield (carapace), and the opisthosoma is segmented in most orders, but not in spiders and mites and only very weakly in harvestmen. The number of eyes varies up to 12 in some scorpions, but generally vision is poor, many species being nocturnal and equipped with sensory hairs to detect prey. Pedipalps function as hands, and the chelicerae as jaws or teeth. In all arachnids the mouth is small, and food is generally predigested by enzymes from the mid-gut. Reproductive organs are placed on the ventral surface of the abdomen, and courtship may be complex and prolonged, with parental care of the young common to all. All arachnids are dioecious. The production of silk and poison is characteristic of some orders, but the methods of production and their origins are varied. Silk is produced from abdominal glands in spiders, from the mouth region in mites, and from the chelicerae in pseudoscorpions. Poison is produced from the chelicerae of spiders, the tails of scorpions, and the pedipalps of pseudoscorpions. There are 11 orders, with more than 60 000 species. Members of five orders occur in northern Europe, the remainder being tropical in distribution.

Arachnid

Spiders, mites, ticks, and scorpions make up the class Arachnida. Arachnids are members of the phylum Arthropoda, which also includes crustaceans (such as crabs and shrimp), insects, and other animals with an exoskeleton and jointed legs. Although arachnids vary in form and behavior, they share certain characteristics. All arachnids have two body segments, eight legs, and no antennae or wings. Unlike many insects, arachnids do not go through metamorphosis but hatch from eggs as miniature adults. Most arachnids are carnivores, often delivering digestive enzymes to their victims externally (by squirting it onto or injecting it into the dead or paralyzed prey), and then sucking in the liquefied food. Most arachnids have poor vision and rely mostly on sensing chemicals and vibrations. The jumping spiders, an exception, have excellent vision.

The most common arachnids are mites (order Acari) and spiders (order Araneae). Although mites outdo spiders in sheer numbers, and likely also in numbers of species, mites are all very small (often microscopic) and hard to observe. They usually parasitize plants or animals, and are very abundant in most terrestrial environments. Spiders, although less widely distributed, are found on all continents except Antarctica, and in almost all habitats except the ocean. (Sea spiders are neither true spiders nor arachnids.) Because of their greater physical size, spiders have been studied more, and have played more of a role in human society throughout history.

The most interesting, distinctive, and useful adaptation of the spiders is their silk. Spiders secrete silk (a kind of protein ) using organs on their abdomens called spinnerets. Spiders put the silk to a multitude of uses: building webs, covering egg sacs, lining their burrows, constructing safety tethers, even making "parachutes" for the dispersal of young spiders on a windy day. Some jumping spiders have even been observed attaching a thread to a wall like a bungee cord and then jumping into the air to catch an insect in flight.

see also Arthropod; Crustacean; Insect

Robbie Hart

Bibliography

Levi, Herbert W., and Lorna R. Levi. Spiders and Their Kin. New York: St. Martin's Press, 2001.

Mason, Adrienne. The World of the Spider. San Francisco: Sierra Club Books, 1999.

Arachnida(harvestmen, mites, palpigrades, pseudoscorpions, scorpions, spiders, whip scorpions, etc.) A class of Arthropoda (subclass Chelicerata) which have book lungs or tracheae derived from gills, indicating their aquatic derivation. They have invaded most terrestrial habitats and have secondarily invaded aquatic habitats, although to a very much smaller extent (there is only one species of aquatic spider, and only one species of mite in ten is aquatic). Except for many plant and animal parasites found among the mites, and some scavenging harvestmen (order Opiliones or Phalangida), most arachnids are predatory. Scorpions have been recorded from the Silurian Period and a Silurian scorpion, Palaeophonus nuncius, was perhaps the first terrestrial animal. The first fossil spiders are known from the Devonian. The class is extremely diverse, but apart from the mites the body is in two portions. The number of eyes varies up to 12 in some scorpions, but generally vision is poor, many species being nocturnal and equipped with sensory hairs to detect prey. Pedipalps function as hands, and the chelicerae as jaws or teeth. In all arachnids the mouth is small, and food is generally predigested by enzymes from the mid-gut. Reproductive organs are placed on the ventral surface of the abdomen, and courtship may be complex and prolonged, with parental care of the young common to all. All arachnids are dioecious. The production of silk and poison is characteristic of some orders, but the methods of production and their origins are varied. Silk is produced from abdominal glands in spiders, from the mouth region in mites, and from the chelicerae in pseudoscorpions. Poison is produced from the chelicerae of spiders, the tails of scorpions, and the pedipalps of pseudoscorpions. There are 11 orders, with 60 000 species. Members of 5 orders occur in northern Europe, the remainder being tropical in distribution.

Arachnida (arachnids: mites, scorpions, spiders, etc.) Class of terrestrial chelicerates (Chelicerata) which have book lungs or tracheae derived from gills, indicating their aquatic derivation. Of recent terrestrial animals, the arachnids are probably the oldest known class, scorpions having been recorded from the Silurian Period. A Silurian scorpion, Palaeophonus nuncius, was perhaps the first terrestrial animal. The first fossil spiders are known from the Devonian. The class is extremely diverse, but except in the mites the body is in two portions: the prosoma (anterior portion) which bears the four pairs of legs, the eyes, the pedipalps (second pair of appendages), and the chelicerae (first pair of appendages, usually pincer-like); and the opisthosoma (posterior portion) which contains most of the internal organs and glands. The two portions may be broadly jointed, or connected by a stalk or pedicel. The prosoma has a dorsal shield or carapace, and the opisthosoma is segmented in most orders, but not in spiders and mites and only very weakly in harvestmen. The number of eyes varies, and can be as many as 12 in some scorpions.

Arachnida A class of terrestrial arthropods of the phylum Chelicerata, comprising about 65 000 species and including spiders, scorpions, harvestmen, ticks, and mites. An arachnid's body is divided into an anterior prosoma and a posterior opisthosoma. The prosoma bears chelicerae, pedipalps, and four pairs of walking legs. The opisthosoma may bear various sensory or silk-spinning appendages (see spinneret). Arachnids are generally carnivorous, feeding on the body fluids of their prey or secreting enzymes to digest prey externally. Spiders immobilize their prey with poison injected by the fanglike chelicerae, while scorpions grasp their prey in large clawed pedipalps and may poison it using the posterior stinging organ. Ticks and some mites are parasitic but most arachnids are free-living. They breathe either via tracheae (like insects) or by means of thin highly folded regions of the body wall called lung books. See also Acarina.

arachnid (ərăk´nĬd), mainly terrestrial arthropod of the class Arachnida, including the spider, scorpion, mite, tick, harvestman (daddy longlegs), and a few minor groups. The body is divided into a cephalothorax with six pairs of appendages, and an abdomen. The first two pairs of appendages are used to kill and crush prey (most arachnids being carnivorous); the remaining four pairs are walking legs. Arachnids have simple eyes and no antennae but are equipped with sensory bristles. Some respire with air tubes, but most possess primitive respiratory organs called book lungs. Arachnids are classified in the phylum Arthropoda, subphylum Chelicerata, class Arachnida.

arachnid Arthropod of the class Arachnida, which includes the spider, tick, mite, scorpion, and harvestman. Arachnids have four pairs of jointed legs, two distinct body segments (cephalothorax and abdomen), and chelicerate jaws (consisting of clawed pincers). They lack antennae and wings.

a·rach·nid / əˈraknid/ • n. any arthropod of the class Arachnida, having four pairs of walking legs and simple eyes, including spiders, scorpions, mites, and ticks. • adj. of or relating to arachnids.

arachnid XIX. — F. arachnide or modL. arachnida (n. pl.), f. Gr. arákhnē spider; see -ID2.

arachnid See ARACHNIDA.

arachnid •carangid • alleged • aged •frigid, rigid •turgid • naked • wicked • whizz-kid •orchid • crooked •pallid, valid •gelid • skidlid • eyelid •solid, squalid, stolid •Euclid • unsullied • annelid •chrysalid • Ozalid • desmid • timid •Fatimid •humid, tumid •pyramid • MacDiarmid • crannied •arachnid • Enid • hominid • honied •Leonid, Oceanid •salmonid • Achaemenid •unaccompanied • Sassanid • learned •winged •rapid, sapid, vapid •intrepid, tepid •insipid, lipid •limpid • poppied • torpid •Cupid, stupid •canopied

Arachnid ★½ 2001 (R)

Plane carrying a rescue crew on a mission to find a downed pilot crashes on a tropical island that contains a gigantic, carnivorous alien spider. Nothing that hasn't been seen before. 95m/C VHS, DVD . Chris Potter, Neus Asensi, Jose Sancho, Alex Reid; D: Jack Sholder; W: Mark Sevi; C: Carlos Gonzalez; M: Francesc Gener.

Arachnid

An arachnid is an invertebrate (an animal without a backbone) that has four pairs of jointed walking legs. Most arachnids, like spiders, ticks, scorpions, and mites, live on land and have two main body parts. Many arachnids prey on other invertebrates, while some are parasites. Unlike insects, however, arachnids have no jaws, antennae, wings, or compound eyes

(many tiny, individual eyes that altogether make a single image that resembles a mosaic). Arachnids are probably the most unpopular type of arthropod (an invertebrate with jointed appendages and a segmented body).

An arachnid is a member of the phylum (or primary division of the animal kingdom) Arthropoda since it has a jointed body case, an exoskeleton (a hard, outer support structure), and no backbone. The class name Arachnida comes from a story in Greek mythology in which a young girl named Arachne spun a silken cloth more perfectly than did the goddess Athena, who became so angry that she turned the girl into a spider. Arachnids are sometimes thought to be insects, but what defines an arachnid and prevents it from being considered an insect is the number of legs it has. An arachnid has eight legs. An insect has six legs. A crustacean has ten legs. Thus a spider and a tick are arachnids, while a centipede and an ant are not. Among other characteristics of arachnids is their highly developed sense of sight. They also have a rigid yet versatile exoskeleton that, combined with muscles and jointed limbs, allows them to have great flexibility and mobility. Its skeleton armor also keeps them waterproof and prevents them from losing body fluids by evaporation.

The spider is a good representative of the arachnid class. Besides their eight walking legs, they have a modified pair they use for handling food and another modified pair of appendages they use as poison fangs or claws.

Most spiders breathe by means of "book lungs." These are special organs located on the abdomen; they include chambers filled with hollow plates connected to tubes that extend to the outside of the skeleton. Air enters via these tubes and passes over plates that are richly supplied with blood. The oxygen diffuses into the blood and carbon dioxide is released. Arachnids, like spiders, are usually hunters, so they have sense detectors in their front end as well as six or eight pairs of eyes. Web-weaving in order to trap prey is probably the most distinctive characteristic of a spider, although not all spiders spin sticky webs. Some literally jump on top of their prey. Weaving a web is instinctive behavior, and webs are made of liquid silk (really a protein) produced from glands in the abdomen and shot out through "spinnerets" at the rear of spiders' bodies. Sticky threads are added later to catch and hold any prey that gets tangled. Spiders wait in hiding and stay connected to their web by a single thread that acts like a fishing bobber and signals that something is caught. The spider's feet are coated with an oily film so that it will not be caught in its own web. The spider bites its prey, immobilizing or killing it, and wraps it in silk for a later meal. Other arachnids, like scorpions, simply paralyze their prey with a sting. Similar to a spider, scorpions consume only the prey's bodily fluids.

Unlike spiders and scorpions, ticks are parasites. They burrow their heads into the skin of a mammal and feed on its blood. A tick is able to take in 200 times its own weight in blood at a single feeding. Although they do not always kill their victims, ticks may sometimes carry infectious diseases, such as lyme disease, which may be fatal if left untreated.

Arachnids reproduce sexually (through the union of male sperm and female eggs) and therefore must mate to produce offspring. For spiders however, mating can be a dangerous act. Since spiders not only eat other spiders but also those of their own species, it is not unusual for the female to kill and eat the smaller male immediately after mating with him. To avoid being eaten, some male spiders lock fangs with the female while they mate. Others tie the female in silk before mating, and still others present her with a meal already wrapped and ready to be eaten. Females lay their eggs in silken cocoons and sometimes keep watch over them until they hatch.

[See alsoArthropods ]

Arachnids

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, 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-related (related by evolution) ancient horseshoe crabs.

Like other arthropods, arachnids have paired, jointed appendages, a hardened exoskeleton, segmented body, and well developed head. Unlike other arthropods, their body has two main parts, the prosoma (equivalent to an insect’s head and thorax) and the opisthosoma (or 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 a neurotoxin that attacks nerve functions, but, except for a handful of species, is not potent enough to harm humans. Scorpions breathe by means of book lungs. Mating is preceded by 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 contain a poison gland at the base, while the tips serve as fangs to inject the poison into the prey. The pedipalps 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 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 fearsome reputation, spiders actually benefit humans by keeping some insect populations under control. Only a few species have a bite that can be fatal to humans.

Most mites and ticks (order Acari) are small, mites being microscopic and ticks measuring only 0.2-1 inch (5–25 mm) in length. The oval acarine body 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. Respiration is by tracheae. Ticks are mostly bloodsucking ectoparasites that prey on 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, scrub 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 that enable them to locate a host more than 25 feet (7.5 m) away.

Many mites are either ecto- or endoparasites of birds and mammals, feeding on the skin and underlying tissues. Many others are free living. Some, such as the chigger, are parasitic as larvae but free living in the nymph and adult stages. Ectoparasites live on the host’s body surface, while endoparasites excavate tunnels under the 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 detriment to humans. House dust mites cause allergies in many people. Some free-living mites are also of considerable importance because they cause destruction of stored grain and other products.

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 not venomous. Harvestmen feed on insects and contribute to insect control, although they are less important in this respect than spiders.

Arachnids

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

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.