Arthropoda

The phylum Arthropoda is the largest and most varied in the animal kingdom. It includes well over one million described species. This represents approximately three-quarters of all known biological organisms, living or extinct. Countless arthropods remain undescribed (not yet named and studied), and the actual number of living species could be as high as ten million or more. Some of the more well-known arthropods include insects, crustaceans, and spiders, as well as the fossil trilobites . Arthropods are found in virtually every known marine (ocean-based), freshwater, and terrestrial (land-based) ecosystem, and vary tremendously in their habitats, life histories, and dietary preferences.

Characteristics of Arthropods

Despite the remarkable variety of arthropod species, all share aspects of a single basic body plan. All arthropods possess a stiff exoskeleton (external skeleton) composed primarily of chitin . In some species, lipids, proteins, and calcium carbonate may also contribute to the exoskeleton. The external skeleton offers organisms protection as well as support for the body. Its walls provide anchors for the attachment of muscles. The exoskeleton is incapable of growth, and is molted (shed) repeatedly during the growth of the animal. This process is called ecdysis. Molting allows for rapid growth until the newly secreted exoskeleton hardens.

Arthropod bodies are divided into segments. However, a number of segments are sometimes fused to form integrated body parts known as tagmata. This process of fusion is called tagmosis. The head, thorax, and abdomen are examples of tagmata. Arthropods also have appendages with joints (the word "arthropod" means "jointed feet"). In early, primitive anthropods, each body segment was associated with a single pair of appendages (attachments). However, in most species some appendages have been modified to form other structures, such as mouthparts, antennae, or reproductive organs. Arthropod appendages may be either biramous (branched) or uniramous (unbranched).

Some arthropods have highly developed sense organs. Most species have paired compound eyes , and many also have a number of simpler eyes called ocelli. Arthropods have an open circulatory system (without blood vessels) that consists of a tube that is the heart and an open hemocoel , the coelom of the animal, in which blood pools. Arthropods also have a complete gut with two openings, the mouth and the anus.

Gas exchange in the phylum occurs in various ways. Some species have gills, while others employ tracheae, or book lungs. The tracheal respiratory system consists of external openings called spiracles that are linked to a system of branched tubules which allow respiratory gases to reach internal tissues. Arthropods are characterized by a brain as well as a nerve ring around the area of the pharynx, in the oral cavity. A double nerve cord extends backwards along the ventral surface of the body, and each body segment is associated with its own ganglion, or mass of nerve cells. In most arthropod species, the sexes are separate. Fertilization usually occurs internally, and most species are egg laying. While some species exhibit direct development, in which eggs hatch as miniature versions of adults, other species pass through an immature larval stage and undergo a dramatic metamorphosis before reaching adult form.

Major Groups of Arthropods

Arthropods are divided into four subphyla. These are the Chelicerata, the Crustacea, the Uniramia, and the Trilobita. The last consists exclusively of extinct forms.

Subphylum Chelicerata.

The chelicerates include the horseshoe crabs , scorpions, spiders, ticks, mites, sea spiders, and other related species. They are characterized by the presence of two tagmata (fused segments), a cephalothorax (fused head and thorax), and an abdomen. They possess six pairs of unbranched appendages. These include a pair of chelicerae , a pair of pedipals, and four pairs of legs.

The class Arachnida includes scorpions, spiders, ticks, and mites. There are over 100,000 described species in this class. The majority are land-based and most are found in fairly warm, dry habitats. Like other chelicerates, arachnids have six pairs of appendages. The first pair, the chelicerae, is typically adapted for killing and consuming prey. The second pair, pedipals, have a sensory function, and may include both receptors sensitive to touch and receptors sensitive to chemical changes. The final four pairs of appendages are walking legs. Arachnids have fairly simple eyes that register only changes in light levels. Of the arachnids, spiders (which make up the Order Araneae) are the most diverse. All spiders are able to spin webs using modified appendages called spinnerets. These are located in the rear abdomen. Webs are used for a variety of purposes in different species. In many, they are used to catch prey and to build nests. Spiderwebs can even be used for movement, as in those species that create parachutes to catch the air, enabling them to descend safely. Many spiders have toxic poisons to immobilize prey or to use in self-defense; perhaps the most famous of these is the black widow. Spiders prey primarily on insects, and are often ecologically important for this reason. Scorpions (order Scorpiones) are arachnids characterized by a pair of claws and a long, jointed tail with a poisonous sting at the end. Ticks and mites (order Acari) are ectoparasites. They embed themselves in the skin of vertebrate animals and feed on blood. Certain tick species carry diseases such as Lyme disease and Rocky Mountain spotted fever.

The class Merostomata includes the horseshoe crabs. Horseshoe crabs are an extremely ancient marine lineage. Only five species have survived to the present. They are characterized by a long appendage called a telson that projects from the rear end of the body, which is used in flipping the animal over when it is lying on its carapace. They use book gills to breathe and generally feed on small invertebrates.

The class Pycnogonida consists of the sea spiders. There are 2,000 described species, all of which are marine. Most species are fairly small. Like spiders, they have small bodies with long legs. They use an extensible proboscis to suck nutrients from the bodies of soft invertebrates.

Subphylum Crustacea.

The subphylum Crustacea includes lobsters, crabs, shrimp, barnacles, and other related organisms. There are approximately 40,000 described species. The majority are marine, but there are freshwater and land-based representatives as well. Unlike other arthropods, the crustacean exoskeleton often includes calcium carbonate, which offers added rigidity. Crustaceans generally have three tagmata: a head, a thorax, and an abdomen. There are two pairs of antennae, complicated mouthparts consisting of two pairs of maxillae (upper jaws) and one pair of mandibles (lower jaws) used in food processing, and a series of branched appendages. These appendages are associated with the thorax. Some function as walking legs while others may be specialized for capturing prey. The abdomen is sometimes equipped with swimmerets (small swimming legs that are also used for other purposes, including as copulatory organs in males and for egg carrying in females) and a tail that is composed of modified appendages in addition to a telson. Some crustacean species have well-developed sensory systems, including highly sensitive compound eyes on stalks, ears, chemoreceptors for taste and/or smell, telson and hairs or bristles that function as touch receptors. Crustaceans have a wide variety of ways to capture food. Some are filter feeders , while others are scavengers or predators. In most species, the sexes are separate. Some species pass through what is called a nauplius larval stage prior to metamorphosing into adults, while others have direct development and bypass the larval stage. Crustaceans use gills to inhale and exhale air.

The class Branchiopoda include the brine shrimp, water fleas, and other related groups. Species in this class are generally small and tend to live in freshwater habitats or in salty lakes. Most species have a large number of segments with minimal fusing of segments, or tagmiosis. The majority are filter feeders.

The class Maxillopoda includes the barnacles and related groups. Maxillopods have a head, thorax, and abdomen along with a telson projecting from the back end of their bodies. Most species are small and feed using their maxillae. Barnacles, however, are sessile (immobile) filter feeders. They are often seen in large numbers, anchored to structures such as ship bottoms or piers.

The class Malacostraca has over 20,000 species and is the largest group within the Crustacea. Most species are marine, but others are freshwater or terrestrial. The largest order, Decapoda, includes shrimp, crabs, crayfish, and lobsters. Other well-known malacostracans include krill as well as a terrestrial group, the sowbugs. The malacostracans exhibit a variety of feeding strategies. The more primitive species tend to be filter feeders. Others are scavengers. Crabs and lobsters are active predators. They have a pair of chelipeds, also known as claws or pincers, which are used to capture and carry prey. Pincers have evolved to serve other functions as well, however, and in various species are used for digging, defense from predators, or in courtship rituals. Some malacostracan species are parasites. Many malacostracans, including many of the larval forms, are critical components of oceanic plankton, a critical component of oceanic food webs.

Subphylum Uniramia.

Uniramia is the largest subphylum within the arthropods. It includes the centipedes, the millipedes, and the insects, as well as a few smaller related groups. The name Uniramia comes from the unbranched appendages that characterize members of the group. Species generally have two or three tagmata. There are one pair of antennae and two pairs of maxillae. Respiration occurs via tracheae. Uniramians generally have separate sexes.

The class Chilopoda includes the centipedes, a diverse group of over 5,000 species. These terrestrial organisms are characterized by a very large number of segments, often well over 100. The largest centipedes reach lengths of up to 25 centimeters (10 inches). Each centipede body segment, aside from a few at the head and tail of the organism, is associated with a single pair of legs. All centipedes are carnivorous, and the appendages that are frontmost have been modified to form large poisonous fangs that are used to immobilize prey. Centipedes feed primarily on earthworms and insects. Species of centipedes are generally egg laying, and in some, the female remains to guard the eggs. Development is direct—there is no larval stage. In some species, juveniles hatch with the same number of segments as an adult, while in others, individuals add segments with each molt.

The class Diplopoda consists of the millipedes, a group that includes over 8,000 described species. Like centipedes, millipedes have a large number of segments. However, they differ from centipedes in that each segment has two pairs of legs rather than just one. Millipedes do not have fangs, and in fact, most species are either herbivorous or scavengers. Many millipedes do, however, exude (ooze) poisonous or noxious substances as a defense against potential predators. Millipedes are often found in decaying organic matter or in moist soils. They are effective burrowers. Like some species of centipedes, they lay eggs in nests that are attended by the female. Millipedes add body segments as they grow and molt.

The class Insecta is the largest class in the animal kingdom. There are nearly one million described species, and no doubt countless others that have yet to be named. Insects are found in a wide variety of terrestrial and freshwater habitats, and there are even a few marine forms.

Insects have three tagmata, or fused segments: a head, a thorax, and an abdomen. They have a pair of antennae; a series of complex, highly variable mouthparts, which vary greatly from species to species; and three pairs of legs. Both the antennae and mouthparts are evolved from modified appendages (walking legs, most likely). Most insect species also have two pairs of wings, although these are absent in a few very primitive species and have been reduced in others, becoming nonfunctional or adapted for a different purpose. Insect legs and wings are associated with the thorax, not the abdomen, which does not usually carry appendages except for appendages that are evolved into reproductive organs. A theory of the origin of insect flight maintains that wings evolved from external gills that were present in certain primitive groups. Aside from their breathing function, these gills served as flaps that assisted insects in leaping and jumping, and were advantageous because they made escape from predators more likely. Gradual increases in wing size allowed for gliding movement, and ultimately for flapping flight.

Insects have highly elaborated sense organs. For example, they may possess a pair of compound eyes as well as several cranial ocelli, or simple eyes. The compound eye is made up of hundreds of individual facets, or parts. Each facet points in a different direction. An individual facet provides information regarding the color and intensity of light but does not provide a complete image. Together, however, the numerous facets create a combined, mosaic image of the world. Compound eyes are particularly effective for seeing nearby objects; distance vision is not as good. The greatest advantage of compound eyes is that they are able to register changes in the visual field much more quickly than eyes with lenses. This is particularly important for detecting motion, as well as for the rapid maneuvering required during flight. Many insects also have well-developed ears. Some species also have an extraordinary ability to detect chemicals. This is especially true in species that use chemical signals called pheromones for detection of a sexual partner. The pheromones are emitted by receptive females and picked up by males, which use them to locate potential mates.

Insects breathe through the tracheal system, described earlier. Because of limits on the spread of gas in the trachea, insects are restricted to a comparatively small size. The excretory system of insects consists of structures known as Malpighian tubules. The sexes are separate in insects, and fertilization occurs internally in most species.

The variety in patterns of insect development is exceptionally high. Most insects pass through several stages before reaching the final adult form. Insects may be described as either hemimetabolous or holometabolous. In hemimetabolous forms, the hatched young resemble adults reasonably closely, although they may be sexually immature and may lack wings. In holometabolous insects, on the other hand, there is a distinct larval stage that is dramatically different from the adult stage in almost all ways: morphology (form and structure), diet, and habitat. In holometabolous insects, there are usually several different larval stages separated by molts. After a period in which the larva grows, it then enters a sessile pupal phase during which a dramatic metamorphosis occurs, and the insect emerges from the pupa with its adult form.

Certain insect groups are highly social. Termites and many species of Hymenoptera (ants, wasps, and bees) are eusocial , meaning that their colonies include a caste (a segment of the population) that reproduces as well as a large number of individuals that do not. The evolution of nonreproductive species seems to pose a problem because it appears to defy natural selection, which emphasizes the production of offspring. However, direct reproduction is not the only way for an individual to pass on its genes. For example, because an individual's siblings share some of its genes, contribution to the production of a large number of siblings will also result in an individual's genes being represented in the population. This is what occurs in the eusocial insects. In addition, unusual behaviors in termites (repeated cycles of inbreeding) and unusual genetic systems in hymenopterans (haplodiploidy, in which males of the species are haploid while females are diploid) increase the proportion of genes shared by siblings.

Insects play many vital roles in maintaining ecological systems. Many insects act as pollinators to higher plants. Others are important in decomposition. Many species are agricultural pests or parasites, and have a dramatic impact on humans. The fruit fly Drosophila melanogaster is one of the most well-studied biological organisms and serves as a model species for studies of genetics , development, and evolution.

Some well-known insect groups include the Thysanura (silverfish), Ephemeroptera (mayflies), Odonata (dragonflies), Orthoptera (grasshoppers, crickets, katydids), Blattaria (cockroaches), Isoptera (termites), Heteroptera (true bugs), Homoptera (cicadas and aphids), Coleoptera (beetles), Siphonaptera (fleas), Diptera (flies), Lepidoptera (butterflies and moths), and Hymenoptera (ants, bees, and wasps).

Subphylum Trilobita.

The subphylum Trilobita includes only extinct species found in fossil form. The trilobites were a primitive group of marine species that was particularly abundant during the Cambrian (570 million years ago) and Ordovician (505 million years ago) periods. The group became extinct at the end of the Permian (286 million years ago). Trilobites had flattened, oval-shaped bodies. Most were a few inches long, although one species is known to have attained a length of 0.6 meters (2 feet).

see also Phylogenetic Relationships of Major Groups.

Jennifer Yeh

Bibliography

Blaney, Walter M. How Insects Live. London: Elsevier-Phaidon, 1976.

Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 1990.

Chapman, Reginald Frederick. The Insects: Structure and Function. New York: Cambridge University Press, 1998.

Corti, Walter Robert. Butterflies and Moths. New York: Odyssey Press, 1964.

Dunca, Winifred. Webs in the Wind: The Habits of Web-Weaving Spiders. New York: Ronald Press Company, 1949.

Evans, Arthur V. An Inordinate Fondness for Beetles. New York: Henry Holt and Company, 1996.

Foelix, Rainer F. Biology of Spiders. Cambridge, MA: Harvard University Press, 1982.

Fortey, Richard A. Trilobite!: An Eyewitness of Evolution. New York: Alfred Knopf, 2000.

Friedlander, Cecil Paul. The Biology of Insects. New York: Pica Press, 1977.

Gauld, Ian David, and Barry Bolton, eds. The Hymenoptera. New York: Oxford University Press, 1988.

Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W. W. Norton and Co., 1996.

Hickman, Cleveland P., Larry S. Roberts, and Allan Larson. Animal Diversity. Dubuque, IA: Wm. C. Brown, 1994.

Holldobler, Bert, and Edward O. Wilson. The Ants. Cambridge, MA: Harvard University Press, 1990.

Wade, Nicholas, ed. The Science Times Book of Insects. New York: Lyons Press, 1998.

Arthropoda [Gr.,=jointed feet], largest and most diverse animal phylum. The arthropods include crustaceans , insects , centipedes , millipedes , symphylans, pauropodans, and the extinct trilobites . Arthropods are characterized by a segmented body covered by a jointed external skeleton (exoskeleton), with paired jointed appendages on each segment; a complex nervous system with a dorsal brain, connective nerves passing around the anterior end of the digestive tract, and a ventral nerve cord with a ganglion in each body segment; an open circulatory system with a dorsal heart into which blood flows through paired openings (ostia); and a greatly reduced body cavity (coelom). Because the jointed exoskeleton blocks growth of the organism, it must be shed periodically. This phenomenon, called molting, or ecdysis, is a characteristic feature of the phylum; it permits rapid growth in size and significant change in body form until the new exoskeleton, secreted by the animal, has hardened. Arthropods are mainly terrestrial, but aquatic representatives are well known. There are three subphyla, comprising nine classes.

Subphylum Trilobita

The trilobites comprise a wholly extinct group of primitive marine arthropods. They were extremely abundant in the Cambrian and Ordovician geologic periods, becoming extinct in the Permian. The flattened, oval body was composed of a head covered by a dorsal shield, a trunk (thorax), and a terminal segment (pygidium). Most of the 3,900 species ranged in length from 1 to 4 in. (2.5-10 cm); some planktonic forms were smaller, and some species were as long as 2 1/2 ft (76 cm). Triarthrus eatoni was a fossil trilobite common in the Ordovician seas.

Subphylum Mandibulata

The mandibulates constitute the largest and most varied arthropod group and are characterized by the presence of modified appendages (mandibles) flanking the mouth and used as jaws. There are six classes, all characterized by various aspects of body form.

Class Crustacea

Members of class Crustacea are characterized by two pairs of antennae and two pairs of modified appendages (maxillae) used for food handling. There are over 40,000 species of crustaceans, including lobsters, shrimps, crayfish, crabs, copepods, barnacles, and a large number of minute planktonic forms. Crustaceans are the only arthropods that are mainly aquatic, and most of them are marine. Some have spread to humid areas near water. They use gills for respiration. The thoracic region typically bears walking legs (pereiopods), also used for capturing prey. The abdominal region often is equipped with swimmerets (pleopods) and a tail fan made up of a pair of appendages (uropods) and the telson. Their excretory organs are modified nephridia, as a rule producing a dilute urine that contains a great deal of ammonia.

Crustaceans are herbivores, carnivores, or scavengers and are often vital elements of the food chain. Some, such as lobsters, shrimp, and crayfish, are important economically as edible shellfish. Barnacles are notorious as fouling organisms of ship bottoms and harbor installations. Some crustaceans are significant parasites of other aquatic organisms. As a rule they pass through a complex set of molts during development, involving a series of larval stages. The characteristic larva is called a nauplius, with three pairs of appendages. More appendages are added as the organism passes through its developmental molts. The cuticle of crustaceans, unlike that of other arthropods, contains calcium deposits. The most familiar subclasses are the Branchiopoda—which includes the orders Notostraca (tadpole shrimps), Diplostraca (clam shrimps and water fleas), Ostracoda (ostracods), and Copepoda (copepods) and the Cirripedia (barnacles), a subclass—and the Malacostraca, which includes the orders Stomatopoda (mantis shrimps), Mysidacea (opossum shrimps), Isopoda (isopods), Amphipoda (amphipods), and Decapoda (crayfish, lobsters, shrimps, and crabs).

Class Chilopoda

Class Chilopoda includes the 5,000 species of centipedes, all of which are terrestrial. Centipedes are carnivorous and predacious, immobilizing their prey, usually consisting of smaller arthropods, with the aid of their fangs. The body is composed of a head region bearing a pair of antennae, a pair of mandibles, and two pairs of maxillae, and a trunk region with one pair of legs on each segment. The anterior pair of trunk appendages (prehensors) is equipped with poison glands. Juveniles may have fewer appendages than adults or may hatch with adult segmentation; new segments are added during developmental molts. Chilopods are found throughout the globe in tropical as well as temperate climates.

Class Diplopoda

There are about 8,000 species belonging to class Diplopoda, which comprises the millipedes and is found worldwide. The head region has a pair of antennae, a pair of mandibles, and two pairs of maxillae that are usually fused into a single mouthpart, the chilognatharium. Millipedes possess a tracheal system for respiration. They are herbivores or scavengers on dead plant material. Many are protected by glands that produce toxic or unpleasant compounds.

Class Pauropoda

There are about 500 known species belonging to class Pauropoda. Pauropods are soft-bodied, small (0.5-2.0 mm long), soil-inhabiting arthropods that are distributed worldwide. They are elongated and have 9-11 pairs of legs, but they have no trachea and no heart.

Class Symphyla

Members of class Symphyla are rapid runners that range in length from 1 to 4 in. (2.5-10 cm). The class includes some 160 species. They are mainly scavengers on decayed vegetation, but one species, Scutigerella immaculata, is a serious pest of certain crops. Symphylans have twelve pairs of legs and resemble the centipedes.

Class Insecta

Class Insecta is the largest of the arthropod classes, containing hundreds of thousands of species. Except for a few primitive or highly modified forms, insects are characterized by having one or two pairs of wings attached to the thorax. The head region bears a pair of antennae, a pair of mandibles, and two pairs of modified maxillae forming the mouthparts. The abdomen is well set off from the thorax and has no appendages except reduced ones that are modified as reproductive organs. The typical insect head bears compound eyes and one or more simple eyes and is covered by a continuous exoskeletal armor. The thorax is made up of three segments, each bearing a pair of legs. The last two segments usually bear a pair of wings. Insects are predominantly terrestrial and have tracheae for gas exchange. Insects are also characterized by unique excretory organs, known as Malpighian tubules, which are useful in conserving water.

Members of the class are extremely varied. They have adapted to many different kinds of feeding and play a variety of important roles in their ecological communities. Mouthparts may be adapted to chewing either plant or animal food, for sucking plant sap or blood, or for lapping or swabbing moisture such as fruit juices or animal body fluids. Some burrow and feed in soil or plant tissue, some are runners or jumpers that feed at or near the ground level, and others feed on the wing.

Most primitive insects are wingless and have a relatively weak exoskeleton. These are forced to seek humid, protected habitats. Juveniles of primitive insects closely resemble the parents and undergo little change other than growth after hatching. This is called ametaboly. Many of the winged insects undergo paurometabolous development, hatching as nymphs that resemble the parent in many ways but that have small buds instead of wings. With each molt these juveniles change somewhat, and the wings increase in size as the young gradually assume the form of the adult. Some insects have adapted to an aquatic life to a certain extent, and in their juvenile stages they are found in ponds and streams. Some of these are hemimetabolous; the juveniles are naiads, i.e., they resemble the nymphs of paurometabolous insects, but their wings do not grow during the juvenile molts, even though other body changes occur. Instead, the last molt before the adult stage is reached involves full development of the wings, after which the insect takes up a terrestrial existence. The least primitive of the insects are termed holometabolous. In holometaboly, the eggs hatch to release the usually wormlike larvae, which are often equipped with false legs in the abdominal region to aid in locomotion. Wing buds are entirely lacking. Although the larvae grow at each molt, they do not begin to resemble the adult until later. During the larval stage the young insect enters into a quiescent pupal stage. At the end of this stage a major metamorphosis occurs, and the insect emerges with all the adult organs.

Insects often cause great losses in agriculture, attack stored products, parasitize humans and domesticated animals and plants, and serve as important carriers of disease organisms. They are also beneficial, producing honey and silk and pollinating the flowers of the majority of flowering plants.

Bibliography

See H. B. Boudreaux, Arthropod Phylogeny (1979, repr. 1987); G. Eisenbeis and W. Wichard, Atlas on the Biology of Soil Arthropods (1987); J. L. Thompson-Cloudsley, Evolution and Adaptation of Terrestrial Arthropods (1988).

Arthropods

Arthropods are invertebrate (without a backbone) animals of the phylum Arthropoda that have a segmented body, jointed legs, and a tough outer covering or exoskeleton. They include insects, crustaceans (lobsters, crabs, shrimp, crayfish), millipedes, centipedes, horseshoe crabs, arachnids (spiders, ticks, and mites) and sea spiders. Together, arthropods comprise the largest and most varied group of invertebrates on Earth.

Characteristics

The bodies of arthropods are divided into different segments, each having a specialized role. The segments have numerous paired, jointed appendages (legs, antennae, claws, and external mouth parts) that serve many varied functions. The exoskeleton acts as a protective covering to the underlying segmented body. It also provides an attachment for muscles and a barrier to water loss for animals living on land. It is made mostly of chitin (pronounced KIE-tuhn), a rigid, complex carbohydrate, and is usually covered by a hardened, waxy cuticle. The cuticle acts as a hinge between segments, allowing the body to bend and move to the right or left. Periodically, the rigid exoskeleton is shed in a process called molting. The temporarily soft animal then swells in size, and its new, larger exoskeleton hardens.

Arthropods are divided into chelicerates (pronounced kih-LIH-suhruhts), meaning "claw-horned ones," and mandibulates, meaning "jawed ones." The bodies of chelicerates are divided into two parts: a fused head and thorax, and an abdomen. They have no antennae, and most have four pairs of jointed legs. They are named for their first pair of appendages, which are modified as clawlike fangs used for feeding. The chelicerates include the arachnids, the marine horseshoe crabs, and the sea spiders.

The mandibulates have one or two pairs of appendages that function as antennae on their head, with the next pair modified as jaws for feeding. Included in this group are the crustaceans (crabs, lobsters, crayfish), the millipedes and centipedes, and the insects. The body of insects

is divided into three regions: a head, a thorax, and a clearly segmented abdomen. The thorax usually has three pairs of legs and two pairs of wings attached to it. Centipedes and millipedes have a head and a narrow, segmented trunk, the former having one pair of legs per segment and the latter having two. Crustaceans have many different body shapes. In most, the head and thorax are fused and separate from the abdomen. Their segmented bodies are often hidden by their hard outer shell.

Arthropods usually have more than one pair of eyes, which may include both simple and compound pairs. (A compound eye is made up of many separate units for receiving light, each with its own lens.) Breathing in land-arthropods is usually accomplished through air tubes called tracheae. Oxygen enters the air tubes from the outside through small openings in the body and is distributed to all the tissues. Arachnids, such as spiders, also breathe through book lungs, thin flaps of tissue arranged like the pages of a book. Arthropods that live in water generally breathe through gills.

Life cycle

Arthropods begin as eggs and can follow several different life cycles, depending on the group. Some insects hatch as miniature adults, while others hatch as nymphs and develop by stages into adults. Still others hatch as larvae and enter a resting stage as pupae, during which they may be enclosed in a cocoon and go through internal changes before emerging as adults. During their various developmental stages, known as metamorphosis, arthropods may shed their outer covering several times (molt).

Ecological importance

Arthropods are of ecological importance because of their sheer numbers and extreme diversity. More than 874,000 living species of arthropods have been identified, making up more than 80 percent of all named species of animals. However, it is estimated that many more thousands of arthropods exist that have not yet been named. Most of these unnamed species are small beetles and other insects, and most of these occur in old-growth tropical rain forests—areas that have not yet been well explored.

Arthropods occupy an enormous variety of Earth's habitats. Most species of crustaceans live in water (that is, are aquatic), although a few such as wood lice and land crabs occur in moist habitats on land. The spiders, mites, scorpions, and other arachnids are almost entirely land animals, as are the extremely diverse insects.

Arthropods are both harmful and helpful to humans. A few species are transmitters of bacteria or viruses that cause diseases such as malaria, yellow fever, encephalitis, and Lyme disease. Scorpions, some spiders, and bees and wasps have poison glands and can hurt or even (though rarely) kill people by injecting poison through stingers. Some arthropods are a nutritious source of food in many parts of the world, and insects play an important role in pollination (a process necessary for production in many plants).

[See also Arachnids; Crustaceans; Insects ]

Arthropod

Arthropods are a phylum within the animal kingdom. They include four classes: Chelicerates (such as spiders, mites, ticks, scorpions, and horseshoe crabs), the extinct Trilobites, Crustaceans (such as lobsters, crabs, and shrimp), and Uniramians (millipedes, centipedes, and the most numerous group of all, the insects). The defining features of arthropods are their exoskeletons (hard outer coverings), segmented bodies, and jointed appendages , from which they derive their name (arthro means "joint," pod means "foot").

The exoskeleton, secreted by the outer tissue layer, is composed of protein and a nitrogenous carbohydrate called chitin , which in crustaceans is fortified with calcium carbonate crystals. To grow, most arthropods either shed (molt) the exoskeleton periodically or grow as soft-bodied larvae before undergoing metamorphosis into the adult, hard-bodied form. Some arthropods (such as millipedes) have legs on nearly every segment. However, most arthropods have evolved reduced numbers of legs, with many other appendages taking on highly specialized roles. Examples include the antennae and hardened mouth parts on head segments, and egg-clasping ovipositors on rear segments.

Arthropods are the most numerous of all animal phyla, both in numbers of species and numbers of individuals, primarily due to insect diversity and numbers. There are at least one million recorded species of arthropods, with the actual number probably ten or even twenty times that amount.

see also Arachnid; Crustacean; Insect

Richard Robinson

Bibliography

Daly, H. V., J. T. Doyen, and A. H. Purcell. Introduction to Insect Biology and Diversity. New York: Oxford University Press, 1998.

arthropods The arthropods are a very large phylum of organisms that are characterized by the presence of a segmented body with a hard exoskeleton and many jointed legs (the name ‘Arthropoda’ means ‘jointed legs’). More species of arthropods have been described than all other kinds of animals put together: almost three-quarters of the million or so known animal species are arthropods. The major groups of modern arthropods are the crustaceans (lobsters, crabs, barnacles, and ostracods) the centipedes, the millipedes, the arachnids (spiders and scorpions), and the insects, which are by far the largest class of all, totalling almost 500 000 species. Important fossil groups are the trilobites, a large group of Palaeozoic marine arthropods of uncertain affinity, and the eurypterids, Palaeozoic marine forms related to the arachnids. It has been suggested that the arthropod body plan might be polyphyletic (i.e. have developed several times), and four phyla are thus sometimes designated: Trilobita, Crustacea, Chelicerata (eurypterids and arachnids), and Uniramia (insects, millipedes, and centipedes).

Because of the adaptability of their body plan, the arthropods have been important and successful organisms since their first appearance in the Early Cambrian. Although they originated in the ocean they have since moved into fresh water, and they were also the first animals to move on to the land and into the air. Despite this, the only groups that are used extensively in geology are the trilobites, which are important biostratigraphic indicators in the Palaeozoic, and the ostracods, which are used throughout the Phanerozoic.

David K. Elliott

arthropod Any invertebrate animal that, characteristically, possesses an outer body layer – the cuticle – that functions as a rigid protective exoskeleton; growth is thus possible only by periodic moults (see ecdysis). There are over one million species of arthropods, inhabiting marine, freshwater, and terrestrial habitats worldwide. The arthropod body is composed of segments (see metameric segmentation) usually forming distinct specialized body regions, e.g. head, thorax, and abdomen. These segments may possess hardened jointed appendages, modified variously as mouthparts, limbs, wings, reproductive organs, or sense organs. The main body cavity, containing the internal organs, is a blood-filled haemocoel, within which lies the heart. Although in older classifications arthropods are placed in a single phylum, Arthropoda, the origins and relationships of the various groups of arthropods remain uncertain, and they are now usually assigned to three separate phyla according to the basic structure of their appendages: Crustacea (shrimps, barnacles, crabs, etc.); Uniramia (or Mandibulata), including the classes Hexapoda (insects), Chilopoda (centipedes), and Diplopoda (millipedes); and Chelicerata, including the Arachnida (spiders, scorpions, mites, and ticks).

Arthropoda A highly diverse phylum of jointed-limbed animals, which includes the crustaceans, arachnids, and insects as the major components, as well as the classes Symphyla, Pauropoda, Chilopoda (centipedes), Diplopoda (millipedes), and the extinct trilobites and eurypterids (Merostomata). They appeared first in the Cambrian, already well diversified with such forms as the trilobites, trilobitoids, ostracodes, and crabs present, implying an earlier, hidden history, reaching back into the Precambrian. The Arthropoda comprise more than 75% of all animal species that have been described. Embryological evidence shows that they are derived either from primitive polychaete worms, or from ancestors common to both. Arthropods share with annelid worms a metamerically segmented body (metameric segmentation), at least in the embryo, a dorsal heart, a dorsal anterior brain, and a ventral nerve cord that has segmental, ganglionic swellings. The limbs of all arthropods are paired, jointed, and segmental, and the body has a chitinous exoskeleton. Primitively, the limbs and cuticular plates correspond to the metameric segmentation of the body, but in many groups there is considerable loss and/or fusion of segments.

Arthropoda A highly diverse phylum of jointed-limbed animals, which includes the crustaceans, arachnids, and insects as the major components, as well as the classes Symphyla, Pauropoda, Chilopoda (centipedes), Diplopoda (millipedes), and the extinct Trilobita and eurypterids (see CHELICERATA; and MEROSTOMATA). Arthropods first appeared in the Cambrian, already well diversified with such forms as the trilobites, trilobitoids, Ostracoda, and crabs present, implying an earlier, hidden history, reaching back into the Precambrian. Embryological evidence shows that they are derived either from primitive Polychaeta, Annelida, or from ancestors common to both. Arthropods share with annelid worms a metamerically segmented body (see METAMERIC SEGMENTATION), at least in the embryo, a dorsal heart, a dorsal anterior brain, and a ventral nerve cord that has segmental, ganglionic swellings. The limbs of all arthropods are paired, jointed, and segmental, and the body has a chitinous exoskeleton. Primitively, the limbs and cuticular plates correspond to the metameric segmentation of the body, but in many groups there is considerable loss and/or fusion of segments.

Arthropoda A highly diverse phylum of jointed-limbed animals, which includes the crustaceans, arachnids, and insects as the major components, as well as the classes Symphyla, Pauropoda, Chilopoda (centipedes), Diplopoda (millipedes), and the extinct trilobites and eurypterids. Arthropods comprise some 80 per cent of all animal species that have been described. They appeared first in the Cambrian, already well diversified, implying an earlier, hidden history reaching back into the Precambrian. Embryological evidence shows that they are a monophyletic group derived either from primitive polychaete worms or from ancestors common to both. Arthropods share several important features with annelid worms. The limbs of all arthropods are paired, jointed, and segmental, and the body has a chitinous exoskeleton. Primitively, the limbs and cuticular plates correspond to the segmentation of the body, but in many groups there is considerable loss and/or fusion of segments.

arthropod Member of the largest animal phylum, Arthropoda. Living forms include crustacean, arachnid, centipede, millipede, and insect. The species (numbering well over 1 million) are thought to have evolved from annelids. All have a hard outer skin of chitin that is attached to the muscular system on the inside. The body is divided into segments, modified among different groups, with each segment originally carrying a pair of jointed legs. In some animals, legs have evolved into jaws, sucking organs or weapons. Arthropods have well-developed digestive, circulatory and nervous systems. Land forms use tracheae for respiration.

ar·thro·pod / ˈär[unvoicedth]rəˌpäd/ • n. any invertebrate of the phylum Arthropoda, with a segmented body, an external skeleton, and jointed limbs, including insects, spiders, and crustaceans.

arthropods See ARTHROPODA.

arthropod •bod, clod, cod, god, hod, mod, nod, od, odd, plod, pod, prod, quad, quod, rod, scrod, shod, sod, squad, tod, Todd, trod, wad •demigod • amphipod • unipod •tripod • isopod • myriapod • decapod •cephalopod • monopod • macropod •gastropod • arthropod • sauropod •ramrod • Nimrod • hotrod • pushrod •goldenrod • Novgorod • slipshod •roughshod • eisteddfod • tightwad