Arthropod

Arthropoda Temporal range: 538.8 –0  Ma PreꞒ Ꞓ O S D C P T J K Pg N Earliest Cambrian (Fortunian)–Recent
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	ParaHoxozoa
Clade:	Bilateria
Clade:	Nephrozoa
(unranked):	Protostomia
Superphylum:	Ecdysozoa
(unranked):	Panarthropoda
(unranked):	Tactopoda
Phylum:	Arthropoda Gravenhorst, 1843[1][2]
Subphyla, unplaced genera, and classes
" paraphyletic, sometimes treated as lobopodians ) Kerygmachelidae † Pambdelurion † (possible lobopodian) opabiniid ) Parvibellus † (possible "Siberiid Lobopodian") Opabiniidae † Radiodonta † Cucumericrus † (possible radiodont) Caryosyntrips † (possible radiodont) Bradoriida † Deuteropoda Artiopoda † Trilobita † Agnostida (possibly trilobites)[3] † Nektaspida † Aglaspidida Cheloniellida † Bushizheia † Erratus[4] † Fengzhengia[5] † Fuxianhuiida † Isoxyida[6] † Kiisortoqia † Kylinxia[6] † Megacheira † (possibly paraphyletic) Chelicerata Habeliida † Pycnogonida Megacheira? † Dibasterium † Offacolidae † Prosomapoda "Synziphosurina" (paraphyletic) Xiphosura Dekatriata Chasmataspidida † Houia † Winneshiekia † Eurypterida † Arachnida Phosphatocopina (possible stem mandibulate)[7] † Mandibulata Hymenocarina † Euthycarcinoidea † Thylacocephala?[8] † Myriapoda Symphyla Pauropoda Diplopoda Chilopoda Pancrustacea Oligostraca Ostracoda Mystacocarida Branchiura Pentastomida Multicrustacea Cyclida[9] † Thecostraca Copepoda Malacostraca Allotriocarida Cephalocarida Branchiopoda Remipedia Hexapoda Collembola Protura Diplura Insecta Incertae sedis Aaveqaspis[10] † Cambropachycope[11] † Camptophyllia[12] † Chuandianella[13] † Goticaris[14] † Marrellomorpha † Parioscorpio[15] † Sarotrocercus[16] † Strabopida[17] † Wingertshellicus[18] †
Diversity
around 1,170,000 species.
Synonyms
Condylipoda Latreille, 1802

Arthropods (

Arthropoda is the largest animal phylum with the estimates of the number of arthropod species varying from 1,170,000 to 5 to 10 million and accounting for over 80 per cent of all known living animal species.^[29][30] One arthropod sub-group, the insects, includes more described species than any other taxonomic class.^[31] The total number of species remains difficult to determine. This is due to the census modeling assumptions projected onto other regions in order to scale up from counts at specific locations applied to the whole world. A study in 1992 estimated that there were 500,000 species of animals and plants in Costa Rica alone, of which 365,000 were arthropods.^[31]

They are important members of marine, freshwater, land and air ecosystems and 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.^[32] Both the smallest and largest arthropods are crustaceans. The smallest belong to the class Tantulocarida, some of which are less than 100 micrometres (0.0039 in) long.^[33] The largest are species in the class Malacostraca, with the legs of the Japanese spider crab potentially spanning up to 4 metres (13 ft)^[34] and the American lobster reaching weights over 20 kg (44 lbs).

Segmentation

Head

_______________________

Thorax

_______________________

Abdomen

_______________________

Segments and tagmata of an arthropod^[32]

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.^[32]

The three-part appearance of many insect bodies and the two-part appearance of spiders is a result of this grouping.^[36] There are no external signs of segmentation in mites.^[32] Arthropods also have two body elements that are not part of this serially repeated pattern of segments, an ocular somite at the front, where the mouth and eyes originated,^[32][37] and a telson at the rear, behind the anus.

Originally it seems that each appendage-bearing segment had two separate pairs of appendages: an upper, unsegmented

The most conspicuous specialization of segments is in the head. The four major groups of arthropods –

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".^[45] In 1960, R. E. Snodgrass even hoped it would not be solved, as he found trying to work out solutions to be fun.^{[Note 2]}

Exoskeleton

Arthropod exoskeletons are made of

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.^[48] Biomineralization generally affects the exocuticle and the outer part of the endocuticle.^[47] Two recent hypotheses about the evolution of biomineralization in arthropods and other groups of animals propose that it provides tougher defensive armor,^[49] and that it allows animals to grow larger and stronger by providing more rigid skeletons;^[50] and in either case a mineral-organic composite exoskeleton is cheaper to build than an all-organic one of comparable strength.^[50][51]

The cuticle may have

The exoskeleton cannot stretch and thus restricts growth. Arthropods, therefore, replace their exoskeletons by undergoing ecdysis (moulting), or shedding the old exoskeleton, the exuviae, after growing a new one that is not yet hardened. Moulting cycles run nearly continuously until an arthropod reaches full size. The developmental stages between each moult (ecdysis) until sexual maturity is reached is called an instar. Differences between instars can often be seen in altered body proportions, colors, patterns, changes in the number of body segments or head width. After moulting, i.e. shedding their exoskeleton, the juvenile arthropods continue in their life cycle until they either pupate or moult again.^[54]

In the initial phase of moulting, the animal stops feeding and its epidermis releases moulting fluid, a mixture of

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. Moulting may be responsible for 80 to 90% of all arthropod deaths.^[54]

Internal organs

  = heart

  = gut

  = brain /

ganglia

 O = eye

Basic arthropod body structure

Arthropod bodies are also segmented internally, and the nervous, muscular, circulatory, and excretory systems have repeated components.

Arthropods have open

The heart is 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. Sections not being squeezed by the heart muscle are expanded either by elastic ligaments or by small muscles, in either case connecting the heart to the body wall. 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.^[55]

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.^[56] 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.^[57]

Nervous system

Living arthropods have paired main nerve cords running along their bodies below the gut, and in each segment the cords form a pair of

The stiff

Most arthropods have sophisticated visual systems that include one or more usually both of

Reproduction and development

Aphid giving birth to live young from an unfertilized egg

Harvestmen mating

A few arthropods, such as

It has been proposed that the Ediacaran animals Parvancorina and Spriggina, from around 555 million years ago, were arthropods,^[73][74][75] but later study shows that their affinities of being origin of arthropods are not reliable.^[76] Small arthropods with bivalve-like shells have been found in Early Cambrian fossil beds dating 541 to 539 million years ago in China and Australia.^{[77][78][79][80]} The earliest Cambrian trilobite fossils are about 520 million years old, but the class was already quite diverse and worldwide, suggesting that they had been around for quite some time.^[81] In the Maotianshan shales, which date back to 518 million years ago, arthropods such as Kylinxia and Erratus have been found that seem to represent transitional fossils between stem (e.g. Radiodonta such as Anomalocaris) and true arthropods.^[82][6][39] 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.^[83][84][85] A fossil of Marrella from the Burgess Shale has provided the earliest clear evidence of moulting.^[86]

The earliest fossil of likely

Arthropods provide the earliest identifiable fossils of land animals, from about 419 million years ago in the Late Silurian,^[56] and terrestrial tracks from about 450 million years ago appear to have been made by arthropods.^[92] Arthropods possessed attributes that were easy coopted for life on land; their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water.^[93] Around the same time the aquatic, scorpion-like eurypterids became the largest ever arthropods, some as long as 2.5 m (8 ft 2 in).^[94]

The oldest known

Further analysis and discoveries in the 1990s reversed this view, and led to acceptance that arthropods are

Protostomes	Chaetognatha molluscs, brachiopods, etc.) Ecdysozoa Nematoida (nematodes and close relatives) priapulids and Kinorhyncha, and Loricifera) Panarthropoda Onychophorans Tactopoda Tardigrades Euarthropoda Chelicerates Mandibulata † Euthycarcinoids Myriapods Pancrustacea Crustaceans Hexapods
Relationships of Ecdysozoa to each other and to annelids, etc.,[115][failed verification] including euthycarcinoids[116]

Higher up the "family tree", the

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 subsequently lost in several other lineages, such as the non-arthropod members of the Ecdysozoa.^[117][115]

Evolution of fossil arthropods

Aside from the four major living groups (

• The "Giant" or "Siberiid Lobopodians", such as Jianshanopodia, Siberion and Megadictyon, are the most basal grade in the total-group Arthropoda.
• The "Gilled Lobopodians", such as Kerygmachela, Pambdelurion and Opabinia, are the second most basal grade.
• The Radiodonta, which traditionally known as anomalocaridids come in third position, and are thought to be monophyletic.
• A possible "upper stem-group" assemblage of more uncertain position^[118] but contained within Deuteropoda:^[119] the Fuxianhuiida, Megacheira, and multiple "bivalved forms" including Isoxyida and Hymenocarina.

The

Controversies remain about the positions of various extinct arthropod groups. Some studies recover Megacheira as closely related to chelicerates, while others recover them as outside the group containing Chelicerate and Mandibulata as stem-group euarthropods.[120] The placement of the Artiopoda (which contains the extinct trilobites and similar forms) is also a frequent subject of dispute.^[121] The main hypotheses position them in the clade Arachnomorpha with the Chelicerates. However, one of the newer hypotheses is that the chelicerae have originated from the same pair of appendages that evolved into antennae in the ancestors of Mandibulata, which would place trilobites, which had antennae, closer to Mandibulata than Chelicerata, in the clade Antennulata.^[120][122] The fuxianhuiids, usually suggested to be stem-group arthropods, have been suggested to be Mandibulates in some recent studies.^[120] The Hymenocarina, a group of bivalved arthropods, previously thought to have been stem-group members of the group, have been demonstrated to be mandibulates based on the presence of mandibles.^[118]

• Radiodonts, Opabiniids, Gilled Lobopodians and the more traditional Lobopodians are all examples of basal stem-group arthropod lineages from the Cambrian
• 
  Anomalocaris
  (Radiodonta)
• 
  Opabinia and Utaurora
  (Opabiniidae)
• 
  Mobulavermis
  (Kerygmachelidae)
• 
  Facivermis
  (Luolishaniidae)

• Marrellomorphs, megacherians, funxianhuiids and phosphatocopines are some examples of Cambrian arthropods whose classification remains difficult
• 
  Marrella
  (Marrellomorpha)
• 
  Leanchoilia
  (Megacheira)
• 
  Fuxianhuia
  (Fuxianhuiida)
• 
  Dabashanella
  (Phosphatocopina)

• Other examples of now extinct arthropod groups include
• 
  Acutiramus
  (Eurypterida)
• 
  Apankura
  (Euthycarcinoidea)
• 
  Trimerus
  (Artiopoda)
• 
  Concavicaris
  (Thylacocephala)

Evolution and classification of living arthropods

The phylum Arthropoda is typically

1. trilobites
   .
2. harvestmen, spiders, scorpions and related organisms characterized by the presence of chelicerae, appendages just above/in front of the mouthparts. Chelicerae appear in scorpions and horseshoe crabs as tiny claws that they use in feeding, but those of spiders have developed as fangs that inject venom
   .
3. body segments
   each of which bearing one or two pairs of legs (or in a few cases being legless). All members are exclusively terrestrial.
4. springtails, and proturans, with six thoracic
   legs.

The phylogeny of the major extant arthropod groups has been an area of considerable interest and dispute.^[124] Recent studies strongly suggest that Crustacea, as traditionally defined, is paraphyletic, with Hexapoda having evolved from within it,^[125][126] so that Crustacea and Hexapoda form a clade, Pancrustacea. The position of Myriapoda, Chelicerata and Pancrustacea remains unclear as of April 2012^[update]. In some studies, Myriapoda is grouped with Chelicerata (forming Myriochelata);^[127][128] in other studies, Myriapoda is grouped with Pancrustacea (forming Mandibulata),^[125] or Myriapoda may be sister to Chelicerata plus Pancrustacea.^[126]

The following cladogram shows the internal relationships between all the living classes of arthropods as of the late 2010s,^[129][130] as well as the estimated timing for some of the clades:^[131]

Arthropoda	Chelicerata Pycnogonida Euchelicerata Xiphosura Arachnida Mandibulata Myriapoda Chilopoda Progoneata Edafopoda Symphyla Pauropoda Diplopoda Pancrustacea Oligostraca Ostracoda Mystacocarida Ichthyostraca Branchiura Pentastomida Altocrustacea Multicrustacea Hexanauplia Copepoda Tantulocarida Thecostraca Malacostraca Allotriocarida Cephalocarida Athalassocarida Branchiopoda Labiocarida Remipedia Hexapoda Elliplura Collembola Protura Cercophora Diplura Insecta 440 mya 470 mya 493 mya	Entognaths

Subphyla	Classes	Members	Example species
Chelicerata	Arachnida	solifuges, mites, scorpions, spiders, ticks etc.	Araneae )
Myriapoda	Chilopoda	pill millipedes, flat-backed millipedes, etc. scutigeromorphs, lithobiomorphs, Scolopendromorphs , etc.	Diplopoda, Spirostreptida )
Crustacea	Copepoda Malacostraca Cephalocarida Branchiopoda Remipedia	water fleas, clam shrimp remipedes	Ocypode ceratophthalma (Malacostraca, Decapoda )
Hexapoda	Insecta Entognatha	springtails , etc.	Insecta, Lepidoptera )

Interaction with humans

Main article: Arthropods in culture

See also: Insects as food

butterfly conservatories

, educational exhibits, schools, research facilities, and cultural events.

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 per cent of the value of world agricultural production used for human food in 2005.^[144] Besides pollinating, bees produce honey, which is the basis of a rapidly growing industry and international trade.^[145]

The red dye

spinal meningitis and some cancers.^[149] Forensic entomology uses evidence provided by arthropods to establish the time and sometimes the place of death of a human, and in some cases the cause.^[150] Recently insects have also gained attention as potential sources of drugs and other medicinal substances.^[151]

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

biomimetic robots to move normally even with damaged or lost appendages.^[152][153]

Diseases transmitted by insects

Disease[154]	Insect	Cases per year	Deaths per year
Malaria	Anopheles mosquito	267 M	1 to 2 M
Dengue fever	Aedes mosquito	?	?
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 like

eczema.^[157]

Many species of arthropods, principally insects but also mites, are agricultural and forest pests.

Predatory mites may be useful in controlling some mite pests.^[162][163]

See also

• Dorsal lobe
• Invertebrate paleontology
• Minibeasts

Notes

1. pillbugs),^[22] but argues that "including legless creatures such as worms, slugs, and snails among the bugs stretches the word too much".^[23]
2. ^ "It would be too bad if the question of head segmentation ever should be finally settled; it has been for so long such fertile ground for theorizing that arthropodists would miss it as a field for mental exercise."^[46]
3. ^ The fossil was originally named Eotarbus but was renamed when it was realized that a Carboniferous arachnid had already been named Eotarbus.^[96]
4. ^ For a mention of insect contamination in an international food quality standard, see sections 3.1.2 and 3.1.3 of Codex 152 of 1985 of the Codex Alimentarius^[140]
5. ^ For examples of quantified acceptable insect contamination levels in food see the last entry (on "Wheat Flour") and the definition of "Extraneous material" in Codex Alimentarius,^[141] and the standards published by the FDA.^[142]

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Bibliography

• ISBN 978-0-09-174271-3
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• Ruppert, E. E.; R. S. Fox; R. D. Barnes (2004). Invertebrate Zoology (7th ed.).
  ISBN 978-0-03-025982-1
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External links

• "Arthropod" at the Encyclopedia of Life
• 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