arthropod: Definition from Answers.com

Arthropods are animals with exoskeletons (external skeletons), segmented bodies, and jointed legs. They are the largest group of animals on Earth and include insects, crustaceans, and arachnids. Insects include organisms such as beetles, grasshoppers, and butterflies. They are mostly terrestrial, small in size, and typically herbivorous. Many species of insects are used as food, and they are traditional food sources in many areas of the tropics. Crustaceans include lobsters, crabs, crayfish, and shrimp. They are mostly aquatic animals, and some, like lobsters and crabs, are relatively large animals. (Crustaceans are discussed below, and are covered in further detail in the article "Crustaceans and Shellfish.") Throughout history, the larger crustacean species have been highly prized food sources. Arachnids include spiders and scorpions, some forms of which are used as food.

The arthropod's exoskeleton is a tough cuticle made of chitin that protects the organism and provides anchor points for muscles. The exoskeleton in crustaceans is rich with calcium carbonate and is particularly hard and thick. The exoskeleton limits an organism's ability to grow in size and must be periodically shed (molted) as the organism grows. Most arthropods go through a series of molts and become more adultlike with each succeeding one.

Some insects, like flies, wasps, beetles, and butterflies, go through larval and pupal stages that are quite different from the adult stages of those species. As embryos, these organisms develop into a larva that is relatively immobile and specializes in eating and storing fat. The larva then transforms into a pupa (an intermediate stage between larva and adult), and finally into an adult that is highly mobile and specializes in reproduction. In insects that undergo such a metamorphosis, the larva is generally the largest form and the one that humans typically prefer as food. The advantage, for humans, of consuming larval insects is that during immature stages of development, insects are soft-bodied and typically high in fat; in addition, the larval stage is often the stage of the life cycle in which individual insects can be found in the greatest aggregations. For example, in the order Lepidoptera (butterflies and moths), insects are in their largest form and have the highest energy (caloric) value during the larval stage of the life cycle. In contrast, the adult forms of Lepidoptera have lower body mass, a hardened exoskeleton, and are more mobile and widely dispersed than larvae.

The crustaceans used as food are aquatic animals that are widespread geographically. Shrimp, lobsters, and crabs inhabit marine ecosystems, and crayfish inhabit freshwater ecosystems. Shrimp are the smallest crustacean and range in size from that of a small insect to over twenty centimeters (seven to eight inches). They tend to live close to the bottom, or in midwater, and feed on plants and small animals. They are food for predatory fish like cod, pollock, and flounder. Lobsters, crabs, and crayfish are larger than shrimp and are important benthic (bottom-dwelling) predators in local ecosystems. The American (Homarus americanus) and European (Homarus gammarus) species of lobster are found in the northern Atlantic Ocean. Adults feed on plant material, shellfish, sea urchins, and crabs. They are solitary animals that defend territory around their shelter (spaces under rocks or large crevices), and they are most active in foraging at night. Spiny (rock) lobsters are found in warm tropical and temperate seas. They feed on snails and clams and small crustaceans and are prey for sharks, octopus, and finfish. They lack the larger claws of the American and European lobsters and are gregarious animals that sometimes migrate long distances.

Crabs are the rounder bodied (compared to shrimp and lobsters) crustaceans that walk sideways; some even swim. The species of crab used as food vary in size from less than two pounds for the Dungeness crabs (Cancer magister) to up to twenty-five pounds for the Alaskan king crab (Paralithodes camtschaticus). Adult crabs are omnivores and dominant predators in local food webs. They feed on shellfish, finfish, and other crustaceans, as well as on detritus (debris). Crabs are widely distributed geographically: Species like the gazami crab (Portunus trituberculatus), the swimming crab (Portunus pelagicus), and the blue crab (Callinectus sapidus) are tropical or subtropical in distribution. The snow or queen crab (Cheonoecetes opilio) is found in the cold seas of the North Atlantic and Pacific Oceans and the Sea of Japan. The most spectacular crabs are the king crabs that live off the coast of Alaska. The red king crab (Paralithodes camtschaticus) is the largest: males of this species can grow to up to twenty-five pounds and have a leg span of five feet across. The blue and the golden king crabs (Paralithodes platypus and Lithodes aequispinus) are somewhat smaller than the red king crab, but they are still king-sized.

Crayfish (or crawfish) look somewhat like lobsters, but they inhabit freshwater ecosystems and are primarily temperate in distribution. North America contains the greatest species diversity of crayfish. They feed on aquatic and semiaquatic vegetation, invertebrates, and detritus. North American species range in size from two to three ounces (50 to 80 grams), but much larger species exist in Australia.

History of Consumption

European populations and European-derived populations in North America historically have placed taboos on entomophagous eating practices (the consumption of insects) and continue to do so. This is notwithstanding the repeated attempts by entomologists to make insects more appealing. One of the best-known attempts is Ronald Taylor's 1975 book Butterflies in My Stomach, and the accompanying recipe guide, Entertaining with Insects (1976).

Although entomophagous eating practices have ceased in Europe, insects were at one time frequently eaten throughout the continent. Rural inhabitants of Europe consumed Cockchafer grubs until the 1800s, and these grubs were an important source of protein in Ireland during the famine of 1688. The Greeks and Romans also held some insects in high esteem as a food source. Ancient Greeks considered grasshoppers a delicacy, and even Aristotle wrote of eating cicadas. He considered them tastiest just before the final instar (stage between two molts), but females laden with eggs were also considered to be very good. The Greeks and Romans also ate a large Melolonthid grub, possibly Lucanus cervus, which Pliny wrote was fattened before consumption.

For many other populations the consumption of insects has continued into the early twenty-first century, or not long before that time. In Mexico a well-known example of cuisine involving insects is ahauatle, a mixture of hemiptera eggs, that Francisco Hernandez first described in 1649. The eggs were also dried and used as a condiment in the preparation of a traditional Christmas Eve dish, revoltijo. In Colombia the giant queen ants of the genus Atta are considered a gastronomical delicacy. There the consumption of giant queen ants can be traced to precolonial times: Gonzalo Jimenez de Quesada, founder of the Colombian capital city Santa Fe de Bogotá, first described their use by local peoples in the highlands in 1555.

The consumption of a wide variety of insects has been reported among Amerindian groups in South American rain forests, and insects have probably been part of that region's diet for a very long time. The insects that appear to be consumed most commonly are ants of the genus Atta, palm grubs, and caterpillars of various sorts. The naturalist Alfred Wallace first described the consumption of Atta queen ants in 1854:

They are eaten alive; the insect being held by the head as we hold a strawberry by its stalk, and the abdomen being bitten off, the body, wings and legs are thrown down to the floor, where they continue to crawl along apparently unaware of the loss of their posterior extremities.

Palm grubs, the large, fatty, legless larvae of wood-boring weevils (Rhynchophorus) found in the pith of felled palm trees, are a highly esteemed food among Amerindians. Bancroft, writing in the eighteenth century, claimed that palm grubs were equally highly esteemed by Europeans in Surinam, particularly by the French.

In Africa the use of insects as food is quite widespread and probably has deep historical roots. The mopane worm (Gonimbrasia belina), the so-called snack that crawls, is one of the best known edible caterpillars. Termites are also utilized as food, especially in the early rainy season when the reproductive forms swarm from the nest. At one time, termites were such an important addition to the diet that their mounds were often disputed as property. Locusts (grasshoppers that go into a swarming phase), in particular the desert locust (Schistocerca gregaria), also play a large role in the diet of Africans. In African history the locusts were so popular that people actually welcomed the arrival of swarms.

In the Middle East the desert locust was also a major source of food historically. Perhaps the most well-known incident involving locust eating was John the Baptist's ordeal in the desert during which he survived on locusts (St. John's bread) and honey. By using locusts as food he was observing the decree of Moses, "These ye may eat; the locust after his kind and the bald locust after his kind, and the cricket after his kind and the grasshopper after his kind" (Leviticus 9:22).

In Asia the consumption of insects as food was described from the Chung-Qiu dynasty (770–475 B.C.E.) and continues to the present day. The most commonly consumed food insects in that region are bee brood (larvae and pupae), beetles such as Dytiscid and Hydrophilid beetles, and the giant water beetle (Lethocerus indicus), the larvae of weevils like Rhynchophorus, and locusts of the genera Oxya and Locusta. Perhaps the most well-known insect eaten in the region is the pupa of the silkworm Bombyx mori.

In Australia the black honey ant (Camponotus inflatus) is a highly sought-after food of Aboriginal Australians and is even considered a totem animal by some clans. It is similar to the honey ant found throughout North and Central America: a modified worker ant with an enlarged body the size of a grape that is full of nectar. Digging up these ants is still considered an important traditional practice and is still taught to children. Witchetty grubs were also an important food of Australian Aborigines. The name witchetty grub refers to any number of root-boring larvae and probably includes Cossid moth larvae (Xyleutes leuchomochla), giant ghost moth larvae (Hepialidae), and longicorn beetle larvae (Cerambycidae). One of the most unique and well-documented examples of entomophagous eating habits in Australia was the annual feast of bugong moths (Agrotis infusa), which occurred until the 1890s. These moths migrate from the plains to aestivate (the summer equivalent of hibernation) in the rock crevices of the Bugong Mountains. Aboriginal Australians from many different tribes traditionally gathered to feast on them. Evidence of these feasts has been carbon-dated as early as 1000 C.E.

Procurement and Capture

The harvesting of insects varies greatly by species because it is tailored to the ecological and behavioral characteristics of different species, as well as the stage of the life cycle sought. Harvesting is typically done for subsistence or to satisfy the demands of a local market.

The harvesting of larval forms like grubs and caterpillars is relatively easy as long as the food source is known. Caterpillars like mopane worms can be picked from their host trees (mopane trees), or for species like the Pandora moth (Colorado Pandora lindseyi), gathered as they descend from their host trees to pupate in the soil. The larva of wood-boring weevils like Rhynchophorus can be harvested by splitting open the palm trees they inhabit, and the larva of root-boring grubs like wichetty grubs can be harvested from the roots of their host plant.

Harvesting mobile adults is more of a challenge. One strategy is to harvest at a point of high aggregation. The giant queen ants of the genus Atta can be collected as they swarm from the nest on nuptial flights early in the rainy season. Some termites, like Macrotermes, can be harvested in the same way. The bogong moths are smoked out of the rock crevices where they gather to aestivate. Social insects that live in large colonies, like ants and termites, can be dug out or lured out by intruding smoke or by inserting a probe, which the soldiers defending the colony will attack. At least one arachnid, the tarantula, can also be attracted out of its burrow using a probe.

Another strategy is to create an aggregation. For grasshoppers and crickets this is done by surrounding them by hunters carrying sticks and driving them into holes or trenches. They can also be captured by dragging bags or nets along the ground and collecting them. A third strategy is to attract the insects to a flame or a light. One species of giant queen ants, as well as some termites and dragonflies, can be attracted to a flame that conveniently singes their wings and makes them very easy to collect. At lease one species of beetle can be attracted to a black light.

Preparation and Consumption

In areas where insects are a traditional part of the diet, they are typically consumed raw or are prepared like other foods, especially other animal food. For example, in Japan grasshoppers, silkworm pupae, and bee pupae are cooked in soy sauce and sugar and served as appetizers. In other parts of Asia, larvae of various sorts, beetles, scorpions, and tarantulas are served fried or stir-fried with vegetables and typical seasonings. In Africa, mopani worms are eaten raw, fried, or cooked in a typical stew after they have been squeezed to remove gut contents.

In general, soft-bodied forms like larvae and pupae are typically fried, grilled, or stewed with local vegetables and seasonings. Larger, hard-bodied forms (such as adults with exoskeletons) like grasshoppers and locusts are typically soaked or cooked in salted water and then sun-dried, or even grilled like shrimp. The legs and wings are typically removed before they are consumed. The exoskeleton of these organisms is retained and provides a certain crunchiness. Smaller organisms with exoskeletons, like ants and termites, are often roasted or fried. In the past, Native North Americans roasted both grasshoppers and crickets and pounded them together with seeds and berries to make a cake called a "desert fruitcake," which could be sun-dried and stored.

Relations to Human Biology

Arthropods are animals and are therefore generally comparable to other animal foods in terms of their nutritional composition. Insects have protein content similar to that of meats like beef and pork. The quality of the protein, however, appears to vary greatly among species; in most cases it is better in terms of amino acid composition than that of plant foods like grains and legumes. The larval stages of arthropods like palm grubs and wichetty grubs are quite high in fat and are similar in that regard to U.S.-style hot dogs. Caterpillars tend to be more muscular and, hence, higher in protein. In terms of micronutrients, insects generally have reasonable quantities of iron, calcium, and B vitamins. As mentioned earlier, the crunchy exoskeleton of insects like grasshoppers is partially composed of chitin, a substance not digested by humans. Little is known about the potential toxic or anti-nutritional factors of insects, although in areas where pesticides are used, toxicity may be of serious concern for all species.

Contemporary Issues

There is a worldwide general trend towards the reduction of entomophagous eating practices. This may be due to the increased use of pesticides to control insects in agricultural zones or the trend toward the adoption of westernized diets (in other words, diets like those of North Americans and Europeans) in which insects have extremely low status as food or are taboo. Despite the general reduction in the consumption of insects as food, there have been efforts to commercialize some food insects. Entrepreneurs in Australia have introduced some local delicacies like black honey ants, witchetty grubs, bardi grubs (the larvae of a Cerambycid beetle), and Trigona bees to the commercial food market, and some Australian restaurants include insects on their menus. Entrepreneurs in South Africa market mopani worms, and the appearance of caterpillars as ingredients has been a general trend on menus in Africa. Some Asian countries also export food insects as specialty items: Thailand exports frozen steamed ant larvae and pupae, Korea exports pupa of the silkworm Bombyx mori, and Japan exports bee pupae in soy to the United States.

There has also been research and development into the rearing of insects as "mini-livestock" in order to meet the subsistence needs, especially the protein needs, of impoverished rural populations. The idea of purposefully raising insects for food is not as far-fetched as it might seem: for example, many societies have been raising bees for a long time.

Bibliography

Caddy, John F., ed. Marine Invertebrate Fisheries: Their Assessment and Management. New York: John Wiley and Sons, 1989.

Chaffin, Yule. Alaska's Southwest: Koniag to King Crab. Anchorage: Chaffin, 1967.

DeFoliart, Gene R. "Insects as Food: Why the Western Attitude Is Important." Annual Review of Entymology 44 (1999): 21–50.

Goddard, J. S. "Food and Feeding." In Freshwater Crayfish: Biology, Management and Exploitation, edited by D. M. Holdich and R. S. Lowery. London and North Ryde: Croom Helm, 1988. Portland, Ore.: Timber Press, 1988.

Paoletti, Maurizio, and Sandra G. F. Bukkens, eds. "Minilive-stock." Special issue of Ecology of Food and Nutrition 36, no. 2–4 (1997).

Phillips, B. F., and J. Kittaka, eds. Spiny Lobsters: Fisheries and Culture. 2d ed. Malden, Mass.: Fishing News Books, 2000.

Pitre, Glen. The Crawfish Book: The Story of Man and Mudbugs Starting in 25,000 B.C and Ending With the Batch Just Put on to Boil. Jackson: University Press of Mississippi, 1993.

Tannahill, Reay. Food in History. New York: Stein and Day, 1973.

Taylor, Ronald L. Butterflies in My Stomach. Santa Barbara, Calif.: Woodbridge Press, 1975.

Taylor, Ronald L., and Barbara J. Carter. Entertaining with Insects. Santa Barbara, Calif.: Woodbridge Press, 1976.

Toussaint-Samat, Maguelonne. A History of Food, translated by Anthea Bell. Paris: Bordas, 1987. New York: Barnes and Noble, 1998.

—Darna L. Dufour

Arthropods Fossil range: 540–0 Ma PreЄ Є O S D C P T J K Pg N Cambrian – Recent

Extinct and modern arthropods
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Superphylum:	Ecdysozoa
Phylum:	Arthropoda Latreille, 1829
Subphyla and Classes
Subphylum Trilobitomorpha Trilobita – trilobites (extinct) Subphylum Chelicerata Arachnida – spiders, scorpions, etc. Xiphosura – horseshoe crabs, etc. Pycnogonida – sea spiders Eurypterida – sea scorpions (extinct) Subphylum Myriapoda Chilopoda – centipedes Diplopoda – millipedes Pauropoda Symphyla Subphylum Hexapoda Insecta – insects Entognatha Subphylum Crustacea Branchiopoda – brine shrimp etc. Remipedia Cephalocarida – horseshoe shrimp Maxillopoda – barnacles, fish lice, etc. Ostracoda – seed shrimp Malacostraca – lobsters, crabs, shrimp, etc.

An arthropod is an invertebrate animal having an exoskeleton (external skeleton), a segmented body, and jointed appendages. Arthropods are members of the Phylum Arthropoda (from Greek ἄρθρον arthron, "joint", and ποδός podos "foot", which together mean "jointed feet"), and include the insects, arachnids, crustaceans, and others. Arthropods are characterized by their jointed limbs and cuticles, which are mainly made of α-chitin; the cuticles of crustaceans are also biomineralized with calcium carbonate. The rigid cuticle inhibits growth, so arthropods replace it periodically by molting. The arthropod body plan consists of repeated segments, each with a pair of appendages. It is so versatile that they have been compared to Swiss Army knives, and it has enabled them to become the most species-rich members of all ecological guilds in most environments. They have over a million described species, making up more than 80% of all described living species, and are one of only two animal groups that are really successful in dry environments – the other being the amniotes. They range in size from microscopic plankton up to forms a few meters long.

Arthropods' primary internal cavity is a hemocoel, which accommodates their internal organs and through which their blood circulates; they have open circulatory systems. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and their brains are formed by fusion of the ganglia of these segments and encircle the esophagus. The respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong.

Their vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many setae (bristles) that project through their cuticles.

Arthropods' methods of reproduction and development are diverse; all terrestrial species use internal fertilization, but this is often by indirect transfer of the sperm via an appendage or the ground, rather than by direct injection. Aquatic species use either internal or external fertilization. Almost all arthropods lay eggs, except for scorpions, who give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and eventually undergo a total metamorphosis to produce the adult form. The level of maternal care for hatchlings varies from zero to the prolonged care provided by scorpions.

The versatility of the arthropod modular body plan has made it difficult for zoologists and paleontologists to classify them and work out their evolutionary ancestry, which dates back to the Cambrian period. From the late 1950s to late 1970s, it was thought that arthropods were polyphyletic, that is, there was no single arthropod ancestor. Now they are generally regarded as monophyletic. Traditionally the closest evolutionary relatives of arthropods were considered to be annelid worms, as both groups have segmented bodies. It is now generally accepted that arthropods belong to the superphylum Ecdysozoa ("animals that molt"), while annelids belong to another superphylum, Lophotrochozoa. The relationships between various arthropod groups are still actively debated.

Although arthropods contribute to human food supply both directly as food and more importantly as pollinators of crops, they also spread some of the most severe diseases and do considerable damage to livestock and crops.

1 Description
2 Reproduction and development
3 Evolution
4 Classification of arthropods
5 Interaction with humans
6 References
7 External links

Description

Arthropods are invertebrates with segmented bodies and jointed limbs.^[1] The limbs form part of an exoskeleton, which is mainly made of α-chitin, a derivative of glucose.^[2] One other group of animals, the tetrapods, has jointed limbs, but tetrapods are vertebrates and therefore have endoskeletons.^[3]

Diversity

Estimating the total number of living species is extremely difficult because it often depends on a series of assumptions in order to scale up from counts at specific locations to estimates for the whole world. A study in 1992 estimated that there were 500,000 species in Costa Rica alone, of which 365,000 were arthropods.^[4] Another estimate indicates that arthropods have 1,170,000 described species, and account for over 80% of all known living species.^[5] They are important members of marine, freshwater, land and air ecosystems, and are one of only two major animal groups that have adapted to life in dry environments; the other is amniotes, whose living members are reptiles, birds and mammals.^[6] One arthropod sub-group, insects, is the most species-rich member of all ecological guilds (ways of making a living) in land and fresh-water environments.^[4] The lightest insects weigh less than 25 micrograms (millionths of a gram),^[7] while the heaviest weigh over 70 grams (2.5 oz).^[8] Some living crustaceans are much larger, for example the legs of the Japanese spider crab may span up to 4 metres (13 ft).^[7]

Segmentation

Head

_______________________

Thorax

_______________________

Abdomen

_______________________

Segments and tagmata of an arthropod^[6]

= Body

= Coxa (base)

= Gill branch

// = Gill
filaments

= Leg
branch

Structure of a biramous appendage^[9]

The embryos of all arthropods are segmented, built from a series of repeated modules. The last common ancestor of living arthropods probably consisted of a series of undifferentiated segments, each with a pair of appendages that functioned as limbs. However all known living and fossil arthropods have grouped segments into tagmata in which segments and their limbs are specialized in various ways;^[6] The three-part appearance of many insect bodies and the two-part appearance of spiders is a result of this grouping;^[9] in fact there are no external signs of segmentation in mites.^[6] Arthropods also have two body elements that are not part of this serially repeated pattern of segments, an acron at the front, ahead of the mouth, and a telson at the rear, behind the anus. The eyes are mounted on the acron.^[6]

The original structure of arthropod appendages was probably biramous, with the upper branch acting as a gill while the lower branch was used for walking. In some segments of all known arthropods the appendages have been modified, for example to form gills, mouth-parts, antennae for collecting information,^[9] or claws for grasping;^[10] arthropods are "like Swiss Army knives, each equipped with a unique set of specialized tools."^[6] In many arthropods, appendages have vanished from some regions of the body, and it is particularly common for abdominal appendages to have disappeared or be highly modified.^[6]

Mnd

Mnd

= acron

= segments included in head

= body segments

x = lost during development

= eyes

= nephridia

O = nephridia lost during development

A = antenna

L = Leg

C = Chelicera

P = Pedipalp

Ci = Chilarium

Mnd = Mandible

Mx = Maxilla

The Arthropod head problem^[11]

The most conspicuous specialization of segments is in the head. The four major groups of arthropods – Chelicerata (includes spiders and scorpions), Crustacea (shrimps, lobsters, crabs, etc.), Tracheata (arthropods that breathe via channels into their bodies; includes insects and myriapods), and the extinct trilobites – have heads formed of various combinations of segments, with appendages that are missing or specialized in different ways.^[6] In addition some extinct arthropods, such as Marrella, belong to none of these groups, as their heads are formed by their own particular combinations of segments and specialized appendages.^[12] Working out the evolutionary stages by which all these different combinations could have appeared is so difficult that it has long been known as "the Arthropod head problem".^[13] In 1960 R.E. Snodgrass even hoped it would not be solved, as trying to work out solutions was so much fun.^[14]

Exoskeleton

Main article: Arthropod exoskeleton

Seta (bristle)

= Biomineralization, only in crustaceans

= Trichogen cell, produces seta

= Gland cell, secretes epicuticle

Structure of arthropod cuticle^[15]

Arthropod exoskeletons are made of cuticle, a non-cellular material secreted by the epidermis.^[6] Their cuticles vary in the details of their structure, but generally consist of three main layers: the epicuticle, a thin outer waxy coat that moisture-proofs the other layers and gives them some protection; the exocuticle, which consists of chitin and chemically hardened proteins; and the endocuticle, which consists of chitin and unhardened proteins. The exocuticle and endocuticle together are known as the procuticle.^[16] Each body segment and limb section is encased in hardened cuticle. The joints between body segments and between limb sections are covered by flexible cuticle.^[6]

The exoskeletons of most aquatic crustaceans are biomineralized with calcium carbonate extracted from the water. Some terrestrial crustaceans have developed means of storing the mineral, since on land they cannot rely on a steady supply of dissolved calcium carbonate.^[17] Biomineralization generally affects the exocuticle and the outer part of the endocuticle.^[16] Two recent hypotheses about the evolution of biomineralization in arthropods and other groups of animals propose that it provides tougher defensive armor,^[18] and that it allows animals to grow larger and stronger by providing more rigid skeletons;^[19] and in either case a mineral-organic composite exoskeleton is cheaper to build than an all-organic one of comparable strength.^[19]^[20]

The cuticle can have setae (bristles) growing from special cells in the epidermis. Setae are as varied in form and function as appendages. For example, they are often used as sensors to detect air or water currents, or contact with objects; aquatic arthropods use feather-like setae to increase the surface area of swimming appendages and to filter food particles out of water; aquatic insects, which are air-breathers, use thick felt-like coats of setae to trap air, extending the time they can spend under water; heavy, rigid setae serve as defensive spines.^[6]

Although all arthropods use muscles attached to the inside of the exoskeleton to flex their limbs, some still use hydraulic pressure to extend them, a system inherited from their pre-arthropod ancestors;^[21] for example, all spiders extend their legs hydraulically and can generate pressures up to eight times their resting level.^[22]

Molting

The exoskeleton cannot stretch and thus restricts growth. Arthropods therefore replace their exoskeletons by molting, or shedding the old exoskeleton after growing a new one that is not yet hardened. Molting cycles run nearly continuously until an arthropod reaches full size.^[23]

In the initial phase of molting, the animal stops feeding and its epidermis releases molting fluid, a mixture of enzymes that digests the endocuticle and thus detaches the old cuticle. This phase begins when the epidermis has secreted a new epicuticle to protect it from the enzymes, and the epidermis secretes the new exocuticle while the old cuticle is detaching. When this stage is complete, the animal makes its body swell by taking in a large quantity of water or air, and this makes the old cuticle split along predefined weaknesses where the old exocuticle was thinnest. It commonly takes several minutes for the animal to struggle out of the old cuticle. At this point the new one is wrinkled and so soft that the animal cannot support itself and finds it very difficult to move, and the new endocuticle has not yet formed. The animal continues to pump itself up to stretch the new cuticle as much as possible, then hardens the new exocuticle and eliminates the excess air or water. By the end of this phase the new endocuticle has formed. Many arthropods then eat the discarded cuticle to reclaim its materials.^[23]

Because arthropods are unprotected and nearly immobilized until the new cuticle has hardened, they are in danger both of being trapped in the old cuticle and of being attacked by predators. Molting may be responsible for 80 to 90% of all arthropod deaths.^[23]

Internal organs

= heart

= gut

= brain, nerve cord, ganglia

O = compound eye

Basic arthropod body structure^[24]

Arthropod bodies are also segmented internally, and the nervous, muscular, circulatory and excretory systems have repeated components.^[6] Arthopods come from a lineage of animals that have a coelom, a membrane-lined cavity between the gut and the body wall that accommodates the internal organs. The strong, segmented limbs of arthropods eliminate the need for one of the coelom's main ancestral functions, as a hydrostatic skeleton, which muscles compress in order to change the animal's shape and thus enable it to move. Hence the coelom of the arthropod is reduced to small areas around the reproductive and excretory systems. Its place is largely taken by a hemocoel, a cavity that runs most of the length of the body and through which blood flows.^[25]

Arthropods have open circulatory systems, although most have a few short, open-ended arteries. In chelicerates and crustaceans, the blood carries oxygen to the tissues, while hexapods use a separate system of tracheae. Many crustaceans, but few chelicerates and tracheates, use respiratory pigments to assist oxygen transport. The most common respiratory pigment in arthropods is copper-based hemocyanin; this is used by many crustaceans and a few centipedes. A few crustaceans and insects use iron-based hemoglobin, the respiratory pigment used by vertebrates. As with other invertebrates and unlike among vertebrates, the respiratory pigments of those arthropods that have them are generally dissolved in the blood and rarely enclosed in corpuscles.^[25]

The heart is typically a muscular tube that runs just under the back and for most of the length of the hemocoel. It contracts in ripples that run from rear to front, pushing blood forwards. Elastic ligaments, or small muscles, connect the heart to the body wall and expand sections that are not being squeezed by the heart muscle. Along the heart run a series of paired ostia, non-return valves that allow blood to enter the heart but prevent it from leaving before it reaches the front.^[25]

Arthropods have a wide variety of respiratory systems. Small species often do not have any, since their high ratio of surface area to volume enables simple diffusion through the body surface to supply enough oxygen. Crustacea usually have gills that are modified appendages. Many arachnids have book lungs. Tracheae, systems of branching tunnels that run from the openings in the body walls, deliver oxygen directly to individual cells in many insects, myriapods and arachnids.^[26]

Living arthropods have paired main nerve cords running along their bodies below the gut, and in each segment the cords form a pair of ganglia from which sensory and motor nerves run to other parts of the segment. Although the pairs of ganglia in each segment often appear physically fused, they are connected by commissures (relatively large bundles of nerves), which give arthropod nervous systems a characteristic "ladder-like" appearance. The brain is in the head, encircling and mainly above the esophagus. It consists of the fused ganglia of the acron and one or two of the foremost segments that form the head – a total of three pairs of ganglia in most arthropods, but only two in chelicerates, which do not have antennae or the ganglion connected to them. The ganglia of other head segments are often close to the brain and function as part of it. In insects these other head ganglia combine into a pair of subesophageal ganglia, under and behind the esophagus. Spiders take this process a step further, as all the segmental ganglia are incorporated into the subesophageal ganglia, which occupy most of the space in the cephalothorax (front "super-segment").^[27]

There are two different types of arthropod excretory systems. In aquatic arthropods, the end-product of biochemical reactions that metabolise nitrogen is ammonia, which is so toxic that it needs to be diluted as much as possible with water. The ammonia is then eliminated via any permeable membrane, mainly through the gills. All crustaceans use this system, and its high consumption of water may be responsible for the relative lack of success of crustaceans as land animals.^[28] Various groups of terrestrial arthropods have independently developed a different system: the end-product of nitrogen metabolism is uric acid, which can be excreted as dry material; Malpighian tubules filter the uric acid and other nitrogenous waste out of the blood in the hemocoel, and dump these materials into the hindgut, from which they are expelled as feces.^[28] Most aquatic arthropods and some terrestrial ones also have organs called nephridia ("little kidneys"), which extract other wastes for excretion as urine.^[28]

Senses

Main article: Arthropod eye

The stiff cuticles of arthropods would block out information about the outside world, except that they are penetrated by many sensors or connections from sensors to the nervous system. In fact, arthropods have modified their cuticles into elaborate arrays of sensors. Various touch sensors, mostly setae, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of taste and smell, often by means of setae. Pressure sensors often take the form of membranes that function as eardrums, but are connected directly to nerves rather than to auditory ossicles. The antennae of most hexapods include sensor packages that monitor humidity, moisture and temperature.^[29]

Head of a wasp with three ocelli (centre), and compound eyes at the left and right

Most arthropods have sophisticated visual systems that include one or more usually both of compound eyes and pigment-cup ocelli ("little eyes"). In most cases ocelli are only capable of detecting the direction from which light is coming, using the shadow cast by the walls of the cup. However the main eyes of spiders are pigment-cup ocelli that are capable of forming images,^[29] and those of jumping spiders can rotate to track prey.^[30]

Compound eyes consist of fifteen to several thousand independent ommatidia, columns that are usually hexagonal in cross section. Each ommatidium is an independent sensor, with its own light-sensitive cells and often with its own lens and cornea.^[29] Compound eyes have a wide field of view, and can detect fast movement and, in some cases, the polarization of light.^[31] On the other hand the relatively large size of ommatidia makes the images rather coarse, and compound eyes are shorter-sighted than those of birds and mammals – although this is not a severe disadvantage, as objects and events within 20 centimetres (7.9 in) are most important to most arthropods.^[29] Several arthropods have color vision, and that of some insects has been studied in detail; for example, the ommatidia of bees contain receptors for both green and ultra-violet.^[29]

Most arthropods lack balance and acceleration sensors, and rely on their eyes to tell them which way is up. The self-righting behavior of cockroaches is triggered when pressure sensors on the underside of the feet report no pressure. However many malacostracan crustaceans have statocysts, which provide the same sort of information as the balance and motion sensors of the vertebrate inner ear.^[29]

The proprioceptors of arthropods, sensors that report the force exerted by muscles and the degree of bending in the body and joints, are well understood. However, little is known about what other internal sensors Arthropods may have.^[29]

Reproduction and development

Compsobuthus werneri female with young (white)

A few arthropods, such as barnacles, are hermaphroditic, that is, each can have the organs of both sexes. However, individuals of most species remain of one sex all their lives.^[32] A few species of insects and crustaceans can reproduce by parthenogenesis, for example, without mating, especially if conditions favor a "population explosion". However most arthropods rely on sexual reproduction, and parthenogenetic species often revert to sexual reproduction when conditions become less favorable.^[33] Aquatic arthropods may breed by external fertilization, as for example frogs also do, or by internal fertilization, where the ova remain in the female's body and the sperm must somehow be inserted. All known terrestrial arthropods use internal fertilization, as unprotected sperm and ova would not survive long in these environments. In a few cases the sperm transfer is direct from the male's penis to the female's oviduct, but it is more often indirect. Some crustaceans and spiders use modified appendages to transfer the sperm to the female. On the other hand, many male terrestrial arthropods produce spermatophores, waterproof packets of sperm, which the females take into their bodies. A few such species rely on females to find spermatophores that have already been deposited on the ground, but in most cases males only deposit spermatophores when complex courtship rituals look likely to be successful.^[32]

The nauplius larva of a prawn

Most arthropods lay eggs,^[32] but scorpions are viviparous: they produce live young after the eggs have hatched inside the mother, and are noted for prolonged maternal care.^[34] Newly-born arthropods have diverse forms, and insects alone cover the range of extremes. Some hatch as apparently miniature adults (direct development), and in some cases, such as silverfish, the hatchlings do not feed and may be helpless until after their first molt. Many insects hatch as grubs or caterpillars, which do not have segmented limbs or hardened cuticles, and metamorphose into adult forms by entering an inactive phase in which the larval tissues are broken down and re-used to build the adult body.^[35] Dragonfly larvae have the typical cuticles and jointed limbs of arthropods but are flightless water-breathers with extendable jaws.^[36] Crustaceans commonly hatch as tiny nauplius larvae that have only three segments and pairs of appendages.^[32]

Evolution

Last common ancestor

The last common ancestor of all arthropods is reconstructed as a modular organism with each module covered by its own sclerite (armor plate) and bearing a pair of biramous limbs.^[37] Whether the ancestral limb was uniramous or biramous is far from a settled debate, though. This Ur-arthropod had a ventral mouth, pre-oral antennae and dorsal eyes at the front of the body. It was a non-discriminatory sediment feeder, processing whatever sediment came its way for food.^[37]

Fossil record

Marrella, one of the puzzling arthropods from the Burgess Shale

It has been proposed that the Ediacaran animals Parvancorina and Spriggina, from around 555 Mya, were arthropods.^[38]^[39]^[40] Small arthropods with bivalve-like shells have been found in Early Cambrian fossil beds dating 541 to 539 million years ago in China.^[41]^[42] The earliest Cambrian trilobite fossils are about 530 million years old, but the class was already quite diverse and worldwide, suggesting that they had been around for quite some time.^[43] Re-examination in the 1970s of the Burgess Shale fossils from about 505 million years ago identified many arthropods, some of which could not be assigned to any of the well-known groups, and thus intensified the debate about the Cambrian explosion.^[44]^[45]^[46] A fossil of Marrella from the Burgess Shale has provided the earliest clear evidence of molting.^[47]

The earliest fossil crustaceans date from about 513 million years ago in the Cambrian,^[48] and fossil shrimp from about 500 million years ago apparently formed a tight-knit procession across the seabed.^[49] Crustacean fossils are common from the Ordovician period onwards.^[50] They have remained almost entirely aquatic, possibly because they never developed excretory systems that conserve water.^[28]

Arthropods provide the earliest identifiable fossils of land animals, from about 419 million years ago in the Late Silurian, and terrestrial tracks from about 450 million years ago appear to have been made by arthropods.^[51] Arthropods were well pre-adapted to colonize land, because their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water.^[52] Around the same time the aquatic, scorpion-like eurypterids became the largest ever arthropods, some as long as 2.5 metres (8.2 ft).^[53]

The oldest known arachnid is the trigonotarbid Palaeotarbus jerami, from about 420 million years ago in the Silurian period.^[54] Attercopus fimbriunguis, from 386 million years ago in the Devonian period, bears the earliest known silk-producing spigots, but its lack of spinnerets means it was not one of the true spiders,^[55] which first appear in the Late Carboniferous over 299 million years ago.^[56] The Jurassic and Cretaceous periods provide a large number of fossil spiders, including representatives of many modern families.^[57] Fossils of aquatic scorpions with gills appear in the Silurian and Devonian periods, and the earliest fossil of an air-breathing scorpion with book lungs dates from the Early Carboniferous period.^[58]

The oldest definitive insect fossil is the Devonian Rhyniognatha hirsti, dated at 396 to 407 million years ago, but its mandibles are of a type found only in winged insects, which suggests that the earliest insects appeared in the Silurian period.^[59] The Mazon Creek lagerstätten from the Late Carboniferous, about 300 million years ago, include about 200 species, some gigantic by modern standards, and indicate that insects had occupied their main modern ecological niches as herbivores, detritivores and insectivores. Social termites and ants first appear in the Early Cretaceous, and advanced social bees have been found in Late Cretaceous rocks but did not become abundant until the Mid Cenozoic.^[60]

Evolutionary family tree

The velvet worm (Onychophora) is closely related to Arthropods^[61]

From the late 1950s to the late 1970s, Sidnie Manton and others argued that arthropods are polyphyletic, in other words, they do not share a common ancestor that was itself an arthropod. Instead, they proposed that three separate groups of "arthropods" evolved separately from common worm-like ancestors: the chelicerates, including spiders and scorpions; the crustaceans; and the uniramia, consisting of onychophorans, myriapods and hexapods. These arguments usually bypassed trilobites, as the evolutionary relationships of this class were unclear. Proponents of polyphyly argued the following: that the similarities between these groups are the results of convergent evolution, as natural consequences of having rigid, segmented exoskeletons; that the three groups use different chemical means of hardening the cuticle; that there were significant differences in the construction of their compound eyes; that it is hard to see how such different configurations of segments and appendages in the head could have evolved from the same ancestor; and that crustaceans have biramous limbs with separate gill and leg branches, while the other two groups have uniramous limbs in which the single branch serves as a leg.^[62]

onychophorans,
including Aysheaia and Peripatus

armored lobopods,
including Hallucigenia and Microdictyon

anomalocarid-like taxa,
including modern tardigrades as
well as extinct animals like
Kerygmachela and Opabinia

	Anomalocaris

	arthropods, including living groups and extinct forms such as trilobites

Simplified summary of Budd's "broad-scale" cladogram (1996)^[61]

Further analysis and discoveries in the 1990s reversed this view, and led to acceptance that arthropods are monophyletic, in other words they do share a common ancestor that was itself an arthropod.^[63]^[64] For example Graham Budd's analyses of Kerygmachela in 1993 and of Opabinia in 1996 convinced him that these animals were similar to onychophorans and to various Early Cambrian "lobopods", and he presented an "evolutionary family tree" that showed these as "aunts" and "cousins" of all arthropods.^[61]^[65] These changes made the scope of the term "arthropod" unclear, and Claus Nielsen proposed that the wider group should be labelled "Panarthropoda" ("all the arthropods") while the animals with jointed limbs and hardened cuticles should be called "Euarthropoda" ("true arthropods").^[66]

A contrary view was presented in 2003, when Jan Bergström and Xian-Guang Hou argued that, if arthropods were a "sister-group" to any of the anomalocarids, they must have lost and then re-evolved features that were well-developed in the anomalocarids. The earliest known arthropods ate mud in order to extract food particles from it, and possessed variable numbers of segments with unspecialized appendages that functioned as both gills and legs. Anomalocarids were, by the standards of the time, huge and sophisticated predators with specialized mouths and grasping appendages, fixed numbers of segments some of which were specialized, tail fins, and gills that were very different from those of arthropods. This reasoning implies that Parapeytoia, which has legs and a backward-pointing mouth like that of the earliest arthropods, is a more credible closest relative of arthropods than is Anomalocaris.^[67] In 2006, they suggested that arthropods were more closely related to lobopods and tardigrades than to anomalocarids.^[68]

Protostomes

Lophotrochozoa (annelids, molluscs, brachiopods, etc.

Ecdysozoa

Nematoida (nematodes and close relatives

Loricifera

Scalidophora (priapulids and Kinorhyncha)

	Crustaceans

	Hexapods

Relationships of Ecdysozoa to each other and to annelids, etc.,^[69] including Euthycarcinoids^[70]

Higher up the "family tree", the Annelida have traditionally been considered the closest relatives of the Panarthropoda, since both groups have segmented bodies, and the combination of these groups was labelled Articulata. There had been competing proposals that arthropods were closely related to other groups such as nematodes, priapulids and tardigrades, but these remained minority views because it was difficult to specify in detail the relationships between these groups.

In the 1990s, molecular phylogenetics analyses that compared sequences of RNA and DNA produced a coherent scheme showing arthropods as members of a superphylum labelled Ecdysozoa ("animals that molt"), which contained nematodes, priapulids and tardigrades but excluded annelids. This was backed up by studies of the anatomy and development of these animals, which showed that many of the features that supported the Articulata hypothesis showed significant differences between annelids and the earliest Panarthropods in their details, and some were hardly present all in arthropods. This hypothesis groups annelids with molluscs and brachiopods in another superphylum, Lophotrochozoa.

If the Ecdysozoa hypothesis is correct, then segmentation of arthropods and annelids either has evolved convergently or has been inherited from a much older ancestor, and has been subsequently lost in several other lineages, such as the non-arthropod members of the Ecdysozoa.^[71]^[69]

Classification of arthropods

Euarthropoda

Paradoxopoda

	Myriapoda

	Chelicerata

Pancrustacea

Cirripedia

	Remipedia

	Collembola

	Branchiopoda

	Cephalocarida

	Malacostraca

Insecta

Phylogenetic relationships of the major extant arthropod groups, derived from mitochondrial DNA sequences.^[72] Highlighted taxa are parts of the subphylum Crustacea.

Euarthropods are typically classified into five subphyla, of which one is extinct:^[73]

Trilobites are a group of formerly numerous marine animals that disappeared in the Permian-Triassic extinction event, though they were in decline prior to this killing blow, having been reduced to one order in the Late Devonian extinction.
Chelicerates include spiders, mites, scorpions and related organisms. They are characterised by the presence of chelicerae, appendages just above / in front of the mouth. Chelicerae appear in scorpions as tiny claws that they use in feeding, but those of spiders have developed as fangs that inject venom.
Myriapods comprise millipedes and centipedes and their relatives and have many body segments, each bearing one or two pairs of legs. They are sometimes grouped with the hexapods.
Hexapods comprise insects and three small orders of insect-like animals with six thoracic legs. They are sometimes grouped with the myriapods, in a group called Uniramia, though genetic evidence tends to support a closer relationship between hexapods and crustaceans.
Crustaceans are primarily aquatic (a notable exception being woodlice) and are characterised by having biramous appendages. They include lobsters, crabs, barnacles, crayfish, shrimp and many others.

Aside from these major groups, there are also a number of fossil forms, mostly from the Early Cambrian, which are difficult to place, either from lack of obvious affinity to any of the main groups or from clear affinity to several of them. Marrella was the first one to be recognized as significantly different from the well-known groups.^[12]

The phylogeny of the major extant arthropod groups has been an area of considerable interest and dispute. The validity of many of the arthropod groups suggested by earlier authors is being questioned by recent studies; these include Mandibulata, Uniramia and Atelocerata. The most recent studies tend to suggest a paraphyletic Crustacea with different hexapod groups nested within it. The remaining clade of Myriapoda and Chelicerata is referred to as Paradoxopoda or Myriochelata.^[72]^[74] However these results are derived from analyzing only living arthropods, and including extinct ones such as trilobites sometimes causes a swing back to the "traditional" view, placing trilobites as the sister-group of the Tracheata (hexapods plus myriapods), and chelicerates as least closely related to the other groups.^[75]

Since the International Code of Zoological Nomenclature recognises no priority above the rank of family, many of the higher-level groups can be referred to by a variety of different names.^[76]

Interaction with humans

Crustaceans such as crabs, lobsters, crayfish, shrimps and prawns have long been part of human cuisine, and are now farmed on a large commercial scale.^[77] Insects and their grubs are at least as nutritious as meat, and are eaten both raw and cooked in many non-European cultures.^[78] Cooked tarantulas are considered a delicacy in Cambodia,^[79] and by the Piaroa Indians of southern Venezuela, after the highly irritant hairs – the spider's main defense system – are removed.^[80] However, the greatest contribution of arthropods to human food supply is by pollination: a 2008 study examined the 100 crops that FAO lists as grown for food, and estimated pollination's economic value as €153 billion, or 9.5% of the value of world agricultural production used for human food in 2005.^[81] Besides pollinating, bees produce honey, which is the basis of a rapidly-growing industry and international trade.^[82]

The red dye cochineal, produced from a Central American species of insect, was economically important to the Aztecs and Mayans,^[83] and while the region was under Spanish control, becoming Mexico's second most-lucrative export;^[84] and it is now regaining some of the ground it lost to synthetic competitors.^[85] The blood of horseshoe crabs contains a clotting agent Limulus Amebocyte Lysate which is now used to test that antibiotics and kidney machines are free of dangerous bacteria, and to detect spinal meningitis and some cancers.^[86] Forensic entomology uses evidence provided by arthropods to establish the time and sometimes the place of death of a human, and in somes cases the cause.^[87]

The relative simplicity of the arthropods' body plan, allowing them to move on a variety of surfaces both on land and in water, have made them useful as models for robotics. The redundancy provided by segments allows arthropods and biomimetic robots to move normally even with damaged or lost appendages.^[88]^[89]

Disease^[90]	Insect	Cases per year	Deaths per year
Malaria	Anopheles mosquito	267 M	1 to 2 M
Yellow fever	Aedes mosquito	4,432	1,177
Filariasis	Culex mosquito	250 M	unknown

Although arthropods are the most numerous phylum on Earth, and thousands of arthropod species are venomous, they inflict relatively few serious bites and stings on humans. Far more serious are the effects on humans of diseases carried by blood-sucking insects. Other blood-sucking insects infect livestock with diseases that kill many animals and greatly reduce the usefulness of others.^[90]

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External links

Arthropods portal

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Arthropoda

Venomous Arthropods chapter in United States Environmental Protection Agency‎ and University of Florida/Institute of Food and Agricultural Sciences National Public Health Pesticide Applicator Training Manual
Arthropods - Arthropoda Insect Life Forms

Opisthokonta: Extant phyla of kingdom Animalia by subkingdom

Parazoa

Porifera (Calcarea, Demospongiae, Hexactinellida) · Placozoa (Trichoplax)

Mesozoa

Orthonectida · Rhombozoa

Eumetazoa

Radiata

Ctenophora · Cnidaria (Anthozoa, Hydrozoa, Scyphozoa, Cubozoa, Staurozoa, Myxozoa)

Bilateria

Protostomia

Ecdysozoa

Cycloneuralia: Scalidophora (Kinorhyncha, Loricifera, Priapulida) · Nematoida (Nematoda, Nematomorpha)

Panarthropoda: Onychophora · Tardigrada · Arthropoda

Spiralia

Deuterostomia

Ambulacraria	Hemichordata · Echinodermata · Xenoturbellida

Chordata	Craniata (Vertebrata, Hyperotreti) · Cephalochordata · Tunicata

Basal/disputed

Acoelomorpha (Acoela, Nemertodermatida) · Chaetognatha

v • d • e Extant arthropod classes by subphylum

Kingdom Animalia · Subkingdom Eumetazoa · (unranked) Bilateria · (unranked) Protostomia · Superphylum Ecdysozoa

Chelicerata	Arachnida · Xiphosura · Pycnogonida

Myriapoda	Chilopoda · Diplopoda · Pauropoda · Symphyla

Hexapoda	Insecta · Entognatha

Crustacea	Branchiopoda · Remipedia · Cephalocarida · Maxillopoda · Ostracoda · Malacostraca

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arthropod

Contents

Description

Diversity

Segmentation

Exoskeleton

Molting

Internal organs

Senses

Reproduction and development

Evolution

Last common ancestor

Fossil record

Evolutionary family tree

Classification of arthropods

Interaction with humans

References

External links