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
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	ParaHoxozoa
Clade:	Bilateria
Clade:	Nephrozoa
(unranked):	Protostomia
Superphylum:	Ecdysozoa
(unranked):	Panarthropoda
(unranked):	Tactopoda
Phylum:	Arthropoda; von Siebold, 1848
Subphyla, unplaced genera, and classes
	†Dinocaridida †Kerygmachela (Sometimes treated as lobopodian); †Pambdelurion (Sometimes treated as lobopodian); †Opabiniidae; †Radiodonta (e.g. Anomalocaris); ; †Isoxys (Isoxyida); †Erratus; †Fengzhengia; †Kylinxia; Class †Megacheira (possibly paraphyletic); †Kiisortoqia; †Fuxianhuiida; †Bradoriida; Class †Artiopoda † Trilobita – trilobites; †Agnostida (uncertain if they are trilobites); †Nektaspida; †Aglaspidida; †Cheloniellida; ; Subphylum Chelicerata Pycnogonida – sea spiders; † sea scorpions; Xiphosura – horseshoe crabs; Arachnida – mites, scorpions, spiders, etc.; ; †Phosphatocopina (stem mandibulate?); Clade Mandibulata †Hymenocarina; †Euthycarcinoidea; †Thylacocephala?; Subphylum Myriapoda pseudocentipedes; pauropods; Diplopoda – millipedes; Chilopoda – centipedes; ; Subphylum Pancrustacea Superclass Oligostraca seed shrimp; Mystacocarida; fish lice; tongue worms; ; Superclass Multicrustacea Thecostraca – barnacles; Copepoda – copepods; amphipods, decapods, krill, etc.; ; Clade Allotriocarida horseshoe shrimp; water fleas, etc.; remipedes; Hexapoda – insects, etc.; ; ; ; Incertae sedis †Camptophyllia; †Chuandianella; †Parioscorpio; †Sarotrocercus; †Wingertshellicus; †Marrellomorpha; ;
Diversity
	around 1,170,000 species.
Synonyms
	Condylipoda Latreille, 1802

Arthropods (

mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. Arthropods are bilaterally symmetrical and their body possesses an external skeleton. In order to keep growing, they must go through stages of moulting, a process by which they shed their exoskeleton to reveal a new one. Some species have wings

. They are an extremely diverse group, with up to 10 million species.

The

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.

Arthropods use combinations of

ocelli for vision. 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 bristles known as setae that project through their cuticles. Similarly, their reproduction and development are varied; all terrestrial species use internal fertilization, but this is sometimes 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, but many species give birth to live young after the eggs have hatched inside the mother, and a few are genuinely viviparous, such as aphids. 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 nonexistent to the prolonged care provided by social insects

.

The evolutionary ancestry of arthropods dates back to the

basal relationships of animals are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated. Today, Arthropods contribute to the human food supply both directly as food, and more importantly, indirectly as pollinators of crops. Some species are known to spread severe disease to humans, livestock, and crops

.

Etymology

The word arthropod comes from the

gen. podos (ποδός)), i.e. "foot" or "leg", which together mean "jointed leg".^[13] The designation "Arthropoda" was coined in 1848 by the German physiologist and zoologist Karl Theodor Ernst von Siebold (1804–1885).^[14]^[15]

In common parlance, terrestrial arthropods are often called bugs.

true bugs", insects of the order Hemiptera.^[18]

Description

Arthropods are invertebrates with segmented bodies and jointed limbs.^[19] The exoskeleton or cuticles consists of chitin, a polymer of N-Acetylglucosamine.^[20] The cuticle of many crustaceans, beetle mites, the clades Penetini and Archaeoglenini inside the beetle subfamily Phrenapatinae,^[21] and millipedes (except for bristly millipedes) is also biomineralized with calcium carbonate. Calcification of the endosternite, an internal structure used for muscle attachments, also occur in some opiliones,^[22] and the pupal cuticle of the fly Bactrocera dorsalis contains calcium phosphate.^[23]

Diversity

Estimates of the number of arthropod species vary between 1,170,000 and 5 to 10 million and account for over 80 percent of all known living animal species.^[24]^[25] The 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.^[26]

They are important members of marine, freshwater, land and air

malacostracans are much larger; for example, the legs of the Japanese spider crab may span up to 4 metres (13 ft),^[28] with the heaviest of all living arthropods being the American lobster

, topping out at over 20 kg (44 lbs).

Segmentation

Head

_______________________

Thorax

_______________________

Abdomen

_______________________

Segments and tagmata of an arthropod^[27]

biramous appendage.^[30]

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.^[27]

The three-part appearance of many insect bodies and the two-part appearance of spiders is a result of this grouping.^[31] There are no external signs of segmentation in mites.^[27] 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,^[27]^[32] 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

trilobites have another segmented branch known as exopods, but whether these structures have a single origin remain controversial.^[35]^[36]^[30] In some segments of all known arthropods the appendages have been modified, for example to form gills, mouth-parts, antennae for collecting information,^[31] or claws for grasping;^[37] arthropods are "like Swiss Army knives, each equipped with a unique set of specialized tools."^[27] In many arthropods, appendages have vanished from some regions of the body; it is particularly common for abdominal appendages to have disappeared or be highly modified.^[27]

Alignment of anterior body segments and appendages across various arthropod taxa, based on the observations until mid 2010s. Head regions in black.^[32]^[38]

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

Trilobita – have heads formed of various combinations of segments, with appendages that are missing or specialized in different ways.^[27] Despite myriapods and hexapods both having similar head combinations, hexapods are deeply nested within crustacea while myriapods are not, so these traits are believed to have evolved separately. 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.^[39]

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".^[40] 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

Illustration of an idealized arthropod exoskeleton.

Arthropod exoskeletons are made of

procuticle.^[42] 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.^[27]

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.^[43] Biomineralization generally affects the exocuticle and the outer part of the endocuticle.^[42] Two recent hypotheses about the evolution of biomineralization in arthropods and other groups of animals propose that it provides tougher defensive armor,^[44] and that it allows animals to grow larger and stronger by providing more rigid skeletons;^[45] and in either case a mineral-organic composite exoskeleton is cheaper to build than an all-organic one of comparable strength.^[45]^[46]

The cuticle may have

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.^[27]

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;^[47] for example, all spiders extend their legs hydraulically and can generate pressures up to eight times their resting level.^[48]

Moulting

Cicada

climbing out of its exoskeleton while attached to tree

The exoskeleton cannot stretch and thus restricts growth. Arthropods, therefore, replace their exoskeletons by undergoing ecdysis (moulting), or shedding the old exoskeleton after growing a new one that is not yet hardened. Moulting cycles run nearly continuously until an arthropod reaches full size.^[49]

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.

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

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.^[49]

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.[49]

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.

hemocoel, a cavity that runs most of the length of the body and through which blood flows.^[50]

Respiration and circulation

Arthropods have open

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, the respiratory pigments of those arthropods that have them are generally dissolved in the blood and rarely enclosed in corpuscles as they are in vertebrates.^[50]

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. Sections not being squeezed by the heart muscle are expanded either by elastic

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.^[50]

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.^[51] 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.^[52]

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

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").^[53]

Excretory system

There are two different types of arthropod excretory systems. In aquatic arthropods, the end-product of biochemical reactions that

nephridia ("little kidneys"), which extract other wastes for excretion as urine.^[54]

Senses

The stiff

auditory ossicles. The antennae of most hexapods include sensor packages that monitor humidity, moisture and temperature.^[55]

Most arthropods lack balance and

malacostracan crustaceans have statocysts, which provide the same sort of information as the balance and motion sensors of the vertebrate inner ear.^[55]

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.^[55]

Optical

Arthropod eyes

Head of a wasp

with three ocelli (center), and compound eyes at the left and right

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

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,^[55] and those of jumping spiders can rotate to track prey.^[56]

Compound eyes consist of fifteen to several thousand independent

ultra-violet.^[55]

Olfaction

Reproduction and development

Compsobuthus werneri

female with young (white)

A few arthropods, such as

hermaphroditic, that is, each can have the organs of both sexes. However, individuals of most species remain of one sex their entire lives.^[58] A few species of insects and crustaceans can reproduce by parthenogenesis, 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.^[59] The ability to undergo meiosis is widespread among arthropods including both those that reproduce sexually and those that reproduce parthenogenetically.^[60] Although meiosis is a major characteristic of arthropods, understanding of its fundamental adaptive benefit has long been regarded as an unresolved problem,^[61]

that appears to have remained unsettled.

ova remain in the female's body and the sperm must somehow be inserted. All known terrestrial arthropods use internal fertilization. Opiliones (harvestmen), millipedes, and some crustaceans use modified appendages such as gonopods or penises to transfer the sperm directly to the female. However, most 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.^[58]

penaeid shrimp

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[28]

[27]

[30]

[31]

[32]

[35]

[36]

[37]

[38]

[39]

[40]

[Note 2]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

[58]

[59]

[60]

[61]

Subphyla	Classes	Members	Example species
Chelicerata	Arachnida	Solifuges, Mites, Scorpions, Spiders, Ticks etc.	Araneae )
Myriapoda	Chilopoda	Scolopendromorphs , etc.	Diplopoda, Spirostreptida )
Crustacea	Copepoda Malacostraca Cephalocarida Branchiopoda Remipedia	Clam Shrimp Remipedes	Ocypode ceratophthalma (Malacostraca, Decapoda )
Hexapoda	Insecta Entognatha	Springtails , etc.	Insecta, Lepidoptera )

Disease^[146]	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

Arthropod

Etymology

Description

Diversity

Segmentation

Exoskeleton

Moulting

Internal organs

Respiration and circulation

Nervous system

Excretory system

Senses

Optical

Olfaction

Reproduction and development

Evolutionary history

Last common ancestor

Fossil record

Evolutionary family tree

Phylogeny of stem-group arthropods

Phylogeny of living arthropods

Classification

Interaction with humans

As predators

See also

Notes

References

Bibliography

External links