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An arthropod (, (gen. ποδός)) is an
invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebra ...
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
having an
exoskeleton An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular ...

exoskeleton
, a segmented body, and paired jointed
appendage An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism's body. In invertebrate biology, an appendage refers to any of the homology (biology), homologous body parts that may extend from a ...
s. Arthropods form the
phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of a noun typically denotes a q ...
Euarthropoda,Reference showing that Euarthropoda is a phylum: which includes
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
s,
arachnid Arachnida () is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytica ...

arachnid
s,
myriapod Myriapoda (Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycen ...
s, and
crustacean Crustaceans (Crustacea ) form a large, diverse arthropod taxon which includes such animals as crabs, lobsters, crayfish, Caridea, shrimp, krill, Dendrobranchiata, prawns, woodlice, barnacles, copepods, amphipoda, amphipods and mantis shrimp. The ...
s. The term Arthropoda () as originally proposed refers to a proposed grouping of Euarthropods and the phylum
Onychophora Onychophora (from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following period ...

Onychophora
. They are distinguished by their jointed limbs and
cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non-homology (biology), homologous, differing in the ...
made of
chitin units that repeat to form long chains in β-(1→4)-linkage. of the chitin molecule. Chitin ( carbon, C8H13O5N)n ( ) is a long-chain polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical su ...

chitin
, often mineralised with
calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together ...

calcium carbonate
. The arthropod
body plan A body plan, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to many members of a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ) ...
consists of segments, each with a pair of appendages. In order to keep growing, they have to go through
Moulting In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
, which sheds their skin. Arthropods are bilaterally symmetrical and their body possesses an
external skeleton An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular ...

external skeleton
. Some species have wings. They are an extremely diverse group, with up to 10 million species. The
haemocoel The circulatory system, also called the cardiovascular system or the vascular system, is an organ system that permits blood Blood is a body fluid in humans and other animals that delivers necessary substances such as nutrient A nutrient ...
, an arthropod's internal cavity, accommodates its interior
organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma Parenchyma () is the bulk of functional ...
, and through which its
haemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood Blood is a body fluid in humans and other animals that delivers necessary substances such as nutrient A nutrient is a substance used by an organism to survive, grow, and reprod ...
– analogue of
blood Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers t ...

blood
– circulates; it has an
open circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an Biological system, organ system that permits blood to circulate and transport nutrients (such as amino acids and electrolytes), oxygen, carbon dioxide, ...

open circulatory system
. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their
nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

nervous system
is "ladder-like", with paired
ventral Standard anatomical terms of location deal unambiguously with the of s, including s. Terms used generally derive from or roots and used to describe something in its . This position provides a definition of what is at the front ("anterior"), be ...
nerve cords running through all segments and forming paired
ganglia A ganglion is a group of neuron cell bodies in the peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In Biology, biology, the nervous system is a Complex system, high ...

ganglia
in each segment. Their heads are formed by fusion of varying numbers of segments, and their
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tiss ...

brain
s are formed by fusion of the ganglia of these segments and encircle the
esophagus The esophagus (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American E ...

esophagus
. The
respiratory The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex biological network, network which connects several biologically relevant entities. Biological organization spans sever ...

respiratory
and excretory systems of arthropods vary, depending as much on their environment as on the
subphylum In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Convention (norm), convention in zoology that rules the formal scientific name, scientific naming of organisms treated as animals. It is als ...
to which they belong. Arthropods use combinations of
compound eye as imaged by an electron microscope A compound eye is a Eye, visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidium, ommatidia, which are tiny independent photoreception units that consist of ...

compound eye
s and pigment-pit
ocelli A simple eye (sometimes called a pigment pit) refers to a form of eye Eyes are organs of the visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photorecept ...
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
spider Spiders (order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, and the ...

spider
s 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
seta In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...

seta
e that project through their cuticles. Similarly, their reproduction and development are varied; all terrestrial species use
internal fertilization Internal fertilization is the union of an egg and sperm cell during sexual reproduction Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organism ...
, 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 External fertilization is a mode of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Greek ...
. 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 Among animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aero ...
, such as
aphid Aphids are small sap-sucking insects and members of the Taxonomic rank, superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white Erio ...

aphid
s. Arthropod hatchlings vary from miniature adults to grubs and
caterpillar Caterpillars ( ) are the larva, larval stage of members of the order Lepidoptera (the insect order comprising butterfly, butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawfly ...

caterpillar
s that lack jointed limbs and eventually undergo a total
metamorphosis Metamorphosis is a biological process Biological processes are those processes that are vital for an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are ...
to produce the adult form. The level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by
social insects Eusociality (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 mil ...
. The evolutionary ancestry of arthropods dates back to the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These peri ...
period. The group is generally regarded as
monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...

monophyletic
, and many analyses support the placement of arthropods with
cycloneuralia Cycloneuralia is a clade of ecdysozoan animals including the Scalidophora (Kinorhyncha Kinorhyncha ( grc, κινέω, kīnéō, I move, ' "snout") is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing ab ...
ns (or their constituent clades) in a superphylum
Ecdysozoa Ecdysozoa () is a group of protostome animals, including Arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the ...

Ecdysozoa
. Overall, however, the
basal Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''. Science * Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure * Basal (medicine), a minimal level that is neces ...
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 A pollinator is an animal that moves pollen File:Pollen Tube.svg, Pollen Tube Diagram Pollen is a powdery substance consisting of pollen grains which are Sporophyte, microsporophytes of spermatophyta, seed plants, which produce male gametes ...

pollinators
of crops. Some species are known to spread severe disease to humans,
livestock Livestock are the domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the reproduction and care of another group to secure a more predictabl ...
, and crops.


Etymology

The word ''arthropod'' comes from the
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
''árthron'', "
joint A joint or articulation (or articular surface) is the connection made between bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the skeleton in most vertebrate animals. Bones protect the various organs of th ...

joint
", and ''pous'' ( gen. ''podos (ποδός)''), i.e. "foot" or "
leg A leg is a weight-bearingIn orthopedics Orthopedic surgery or orthopedics, is the branch of surgery concerned with conditions involving the musculoskeletal system. Orthopedic surgeons use both surgical and nonsurgical means to treat muscul ...

leg
", which together mean "jointed leg". The designation "Arthropoda" was coined in 1848 by the German physiologist and zoologist Karl Theodor Ernst von Siebold (1804–1885).


Description

Arthropods are
invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebra ...
s with segmented bodies and jointed limbs. The
exoskeleton An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular ...

exoskeleton
or
cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non-homology (biology), homologous, differing in the ...
s consists of
chitin units that repeat to form long chains in β-(1→4)-linkage. of the chitin molecule. Chitin ( carbon, C8H13O5N)n ( ) is a long-chain polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical su ...

chitin
, a polymer of
glucosamine Glucosamine (C6H13NO5) is an amino sugar and a prominent precursor in the biochemical Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organism In biology, an organism (from Ancien ...

glucosamine
. The cuticle of many crustaceans, beetle mites, and millipedes (except for bristly millipedes) is also biomineralized with
calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together ...

calcium carbonate
. Calcification of the endosternite, an internal structure used for muscle attachments, also occur in some
opiliones The Opiliones (formerly Phalangida) are an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, ...

opiliones
.


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. 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. They are important members of marine, freshwater, land and air
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...

ecosystem
s, and are one of only two major animal groups that have adapted to life in dry environments; the other is
amniote Amniotes (from Greek ἀμνίον ''amnion'', "membrane surrounding the fetus", earlier "bowl in which the blood of sacrificed animals was caught", from ἀμνός ''amnos'', "lamb") are a clade A clade (; from grc, , ''klados'', "bran ...
s, whose living members are reptiles, birds and mammals. Ruppert, Fox & Barnes (2004), pp. 518–522 One arthropod sub-group,
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
s, is the most species-rich member of all ecological guilds in land and freshwater environments. The lightest insects weigh less than 25 micrograms (millionths of a gram), while the heaviest weigh over . Some living crustaceans are much larger; for example, the legs of the
Japanese spider crab The Japanese spider crab (''Macrocheira kaempferi'') is a species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is ofte ...

Japanese spider crab
may span up to , with the heaviest of all living arthropods being the
American lobster The American lobster (''Homarus americanus'') is a species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defin ...
, topping out at over 20 kg (44 lbs).


Segmentation

The
embryo An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...

embryo
s of all arthropods are segmented, built from a series of repeated modules. The
last common ancestor In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
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. The three-part appearance of many
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
bodies and the two-part appearance of
spider Spiders (order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, and the ...

spider
s is a result of this grouping; Gould (1990), pp. 102–106 in fact there are no external signs of segmentation in
mite Mites are small arachnid Arachnida () is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Cl ...
s. Arthropods also have two body elements that are not part of this serially repeated pattern of segments, an
acron Acron ( grc-gre, Ἄκρων), son of Xenon, was a Medicine in ancient Greece, Greek physician born at Agrigentum (Gk. Acragas). Life The exact dates of Acron is not known; but, as he is mentioned as being contemporary with Empedocles, who died a ...
at the front, ahead of the mouth, and a
telson Image:Penaeus diagram telson.png, upright=1.4, Diagram highlighting the telson of the prawn ''Litopenaeus setiferus'' The telson is the posterior-most division of the body of an arthropod. Depending on the definition, the telson is either consider ...
at the rear, behind the
anus The anus (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

anus
. The eyes are mounted on the acron. Originally it seems that each appendage-bearing segment had two separate pairs of appendages: an upper and a lower pair. These would later fuse into a single pair of
biramous The arthropod leg is a form of jointed of s, usually used for . Many of the terms used for arthropod leg segments (called podomeres) are of origin, and may be confused with terms for bones: ''coxa'' (meaning , plural ''coxae''), ''trochanter'', '' ...
appendages, with the upper branch acting as a
gill A gill () is a respiratory organ that many aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent ...
while the lower branch was used for locomotion. In some segments of all known arthropods the appendages have been modified, for example to form gills, mouth-parts,
antennae Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two coll ...
for collecting information, or claws for grasping; arthropods are "like , each equipped with a unique set of specialized tools." 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. The most conspicuous specialization of segments is in the head. The four major groups of arthropods –
Chelicerata The subphylum Chelicerata (from New Latin New Latin (also called Neo-Latin or modern Latin) is the revival of Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. ...
(includes
spider Spiders (order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, and the ...

spider
s and
scorpion Scorpions are predatory Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including hum ...

scorpion
s),
Crustacea Crustaceans (Crustacea ) form a large, diverse arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Eua ...
(
shrimp Shrimp are Decapoda, decapod crustaceans with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata. More narrow definitions may be restricted to Caridea, to smaller species of either group ...

shrimp
s,
lobster Lobsters are a family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and of society ...

lobster
s,
crab Crabs are decapod crustacean Crustaceans (Crustacea ) form a large, diverse arthropod taxon which includes such animals as crabs, lobsters, crayfish, Caridea, shrimp, krill, Dendrobranchiata, prawns, woodlice, barnacles, copepods, amphipoda ...

crab
s, etc.), Tracheata (arthropods that breathe via channels into their bodies; includes
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
s and
myriapod Myriapoda (Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycen ...
s), and the extinct
trilobite Trilobites (; meaning "three lobes") are a group of marine s that form the Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the of the ( ...

trilobite
s – have heads formed of various combinations of segments, with appendages that are missing or specialized in different ways. In addition, some extinct arthropods, such as ''
Marrella ''Marrella'' is an extinct genus of arthropod known from the middle Cambrian Burgess Shale of British Columbia. It is the most common animal represented in the Burgess Shale. History ''Marrella'' was the first fossil collected by Charles Doolit ...

Marrella
'', belong to none of these groups, as their heads are formed by their own particular combinations of segments and specialized appendages. Summarised in Gould (1990), pp. 107–121. 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 The (pan)arthropod head problem is a long-standing zoological Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, though it is not uncommon. is the branch of biology Biology is the natural science that studie ...

arthropod head problem
". In 1960, R. E. Snodgrass even hoped it would not be solved, as he found trying to work out solutions to be fun.


Exoskeleton

Arthropod exoskeletons are made of
cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non-homology (biology), homologous, differing in the ...
, a non-cellular material secreted by the
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
. Their cuticles vary in the details of their structure, but generally consist of three main layers: the
epicuticle The cuticle forms the major part of the integument In biology, integument is the natural covering of an organism or an organ, such as its skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, ...
, a thin outer
wax Waxes are a diverse class of organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, ...
y coat that moisture-proofs the other layers and gives them some protection; the
exocuticle The cuticle forms the major part of the integument In biology, integument is the natural covering of an organism or an organ, such as its skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, wi ...
, which consists of
chitin units that repeat to form long chains in β-(1→4)-linkage. of the chitin molecule. Chitin ( carbon, C8H13O5N)n ( ) is a long-chain polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical su ...

chitin
and chemically hardened
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s; and the endocuticle, which consists of chitin and unhardened proteins. The exocuticle and endocuticle together are known as the
procuticle The cuticle forms the major part of the integument In biology, integument is the natural covering of an organism or an organ, such as its skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, ...
. 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. The exoskeletons of most aquatic
crustacean Crustaceans (Crustacea ) form a large, diverse arthropod taxon which includes such animals as crabs, lobsters, crayfish, Caridea, shrimp, krill, Dendrobranchiata, prawns, woodlice, barnacles, copepods, amphipoda, amphipods and mantis shrimp. The ...
s are biomineralized with
calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together ...

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. Biomineralization generally affects the exocuticle and the outer part of the endocuticle. Two recent hypotheses about the evolution of biomineralization in arthropods and other groups of animals propose that it provides tougher defensive armor, and that it allows animals to grow larger and stronger by providing more rigid skeletons; and in either case a mineral-organic
composite Composite or compositing may refer to: Materials * Composite material, a material that is made from several different substances ** Metal matrix composite, composed of metal and other parts ** Cermet, a composite of ceramic and metallic materials * ...

composite
exoskeleton is cheaper to build than an all-organic one of comparable strength. The cuticle may 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 Feathers are epidermal growths that form a distinctive outer covering, or plumage Plumage ( "feather") is a layer of feather Feathers are epidermal growths that form a distinctive outer covering, or plumage, on dinosaurs, both avi ...

feather
-like setae to increase the surface area of swimming appendages and to
filter Filter, filtering or filters may refer to: Science and technology Device * Filter (chemistry), a device which separates solids from fluids (liquids or gases) by adding a medium through which only the fluid can pass ** Filter (aquarium), critical ...
food particles out of water; aquatic insects, which are air-breathers, use thick
felt Felt is a textile material that is produced by matting, condensing and pressing fibers together. Felt can be made of natural fibers such as wool Wool is the textile fiber obtained from sheep and other animals, including Cashmere wool, ca ...
-like coats of setae to trap air, extending the time they can spend under water; heavy, rigid setae serve as defensive spines. Although all arthropods use muscles attached to the inside of the exoskeleton to flex their limbs, some still use
hydraulic Hydraulics (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is a ...

hydraulic
pressure to extend them, a system inherited from their pre-arthropod ancestors; for example, all spiders extend their legs hydraulically and can generate pressures up to eight times their resting level.


Moulting

The exoskeleton cannot stretch and thus restricts growth. Arthropods therefore replace their exoskeletons by undergoing
ecdysis Ecdysis is the moulting In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (ofte ...
(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. Ruppert, Fox & Barnes (2004), pp. 523–524 The developmental stages between each moult (ecdysis) until sexual maturity is reached is called an
instar An instar (, from the Latin ''īnstar'', "form", "likeness") is a developmental stage of arthropods An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly ...
. 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
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
s that digests the endocuticle and thus detaches the old cuticle. This phase begins when the
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
has secreted a new
epicuticle The cuticle forms the major part of the integument In biology, integument is the natural covering of an organism or an organ, such as its skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, ...
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. 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.


Internal organs

Arthropod bodies are also segmented internally, and the nervous, muscular, circulatory, and excretory systems have repeated components. Arthropods come from a lineage of animals that have a
coelom The coelom (or celom) is the main body cavity A body cavity is any space or compartment, or potential space Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social ...
, 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 The circulatory system, also called the cardiovascular system or the vascular system, is an organ system that permits blood to circulate and transport nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. T ...
, a cavity that runs most of the length of the body and through which
blood Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers t ...

blood
flows. Ruppert, Fox & Barnes (2004), pp. 527–528


Respiration and circulation

Arthropods have open
circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A biological system is a complex network Network and networking may refer to: Arts, entertai ...
s, although most have a few short, open-ended
arteries An artery (plural arteries) () is a blood vessel that takes blood away from the heart to one or more parts of the body (tissues, lungs, brain etc.). Most arteries carry oxygenated blood; the two exceptions are the pulmonary arteries, pulmonary ...

arteries
. In chelicerates and crustaceans, the blood carries
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
to the tissues, while hexapods use a separate system of tracheae. Many crustaceans, but few chelicerates and tracheates, use
respiratory pigmentA respiratory pigment is a metalloprotein that serves a variety of important functions, its main being O2 transport. Other functions performed include O2 storage, CO2 transport, and transportation of substances other than respiratory gases. There ar ...
s to assist oxygen transport. The most common respiratory pigment in arthropods is
copper Copper is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

copper
-based
hemocyanin Hemocyanins (also spelled haemocyanins and abbreviated Hc) are proteins that transport oxygen throughout the bodies of some invertebrate animals. These metalloproteins contain two copper atoms that reversibly bind a single oxygen molecule (O2). ...
; this is used by many crustaceans and a few
centipede Centipedes (from the New Latin New Latin (also called Neo-Latin or modern Latin) is the revival of Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally s ...

centipede
s. A few crustaceans and insects use iron-based
hemoglobin Hemoglobin or haemoglobin (spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from , , "through" and , , "I speak") is used in two distinct wa ...

hemoglobin
, the respiratory pigment used by
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...
s. As with other invertebrates, the respiratory pigments of those arthropods that have them are generally dissolved in the blood and rarely enclosed in as they are in vertebrates. 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
ligament A ligament is the Connective tissue#Types, fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body incl ...

ligament
s or by small
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

muscle
s, 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. 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 lung A book lung is a type of respiration organ used for atmospheric gas exchange that is present in many arachnid Arachnida () is a class of joint-legged invertebrate Invertebrates are animals that neither possess nor develop a vertebral colum ...
s. 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
arachnid Arachnida () is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytica ...

arachnid
s.


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
ganglia A ganglion is a group of neuron cell bodies in the peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In Biology, biology, the nervous system is a Complex system, high ...

ganglia
from which sensory and
motor An engine or motor is a machine A machine is a man-made device that uses power to apply forces and control movement to perform an action. Machines can be driven by animals and people A people is a plurality of person A person ( ...
nerves run to other parts of the segment. Although the pairs of ganglia in each segment often appear physically fused, they are connected by
commissure A commissure () is the location at which two objects abut or are joined. The term is used especially in the fields of anatomy and biology. * The most common usage of the term refers to the brain's commissures, of which there are five. Such a commi ...
s (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 The segmental ganglia (singular: s. ganglion) are ganglia of the annelid and arthropod central nervous system that lie in the segmented ventral nerve cord. The ventral nerve cord itself is a chain of (Metamerism (biology), metameric) segmental gangl ...
are incorporated into the subesophageal ganglia, which occupy most of the space in the cephalothorax (front "super-segment"). Ruppert, Fox & Barnes (2004), pp. 531–532


Excretory system

There are two different types of arthropod excretory systems. In aquatic arthropods, the end-product of biochemical reactions that
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

nitrogen
is
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

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. Ruppert, Fox & Barnes (2004), pp. 529–530 Various groups of terrestrial arthropods have independently developed a different system: the end-product of nitrogen metabolism is
uric acid Uric acid is a heterocyclic compound A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different chemical element, elements as members of its ring(s). Heterocyclic chemistry is the branch of orga ...

uric acid
, which can be excreted as dry material; the Malpighian tubule system filters the uric acid and other nitrogenous waste out of the blood in the hemocoel, and dumps these materials into the hindgut, from which they are expelled as feces. Most aquatic arthropods and some terrestrial ones also have organs called nephridia ("little kidneys"), which extract other wastes for excretion as urine.


Senses

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
seta In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...

seta
e, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of taste and Olfaction, 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 Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two coll ...
of most hexapods include sensor packages that monitor humidity, moisture and temperature. Ruppert, Fox & Barnes (2004), pp. 532–537


Optical

Most arthropods have sophisticated visual systems that include one or more usually both of
compound eye as imaged by an electron microscope A compound eye is a Eye, visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidium, ommatidia, which are tiny independent photoreception units that consist of ...

compound eye
s and pigment-cup
ocelli A simple eye (sometimes called a pigment pit) refers to a form of eye Eyes are organs of the visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photorecept ...
("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
spider Spiders (order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, and the ...

spider
s are pigment-cup ocelli that are capable of forming images, and those of jumping spiders can rotate to track prey. Compound eyes consist of fifteen to several thousand independent ommatidia, columns that are usually hexagonal in Cross section (geometry), cross section. Each ommatidium is an independent sensor, with its own light-sensitive cells and often with its own lens (anatomy), lens and cornea. Compound eyes have a wide field of view, and can detect fast movement and, in some cases, the light polarization, polarization of light. 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 are most important to most arthropods. 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. 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. 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.


Olfaction


Reproduction and development

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 their entire lives. A few species of
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
s 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. Aquatic animal, Aquatic arthropods may breed by external fertilization, as for example frogs do, or by
internal fertilization Internal fertilization is the union of an egg and sperm cell during sexual reproduction Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organism ...
, where the ovum, 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 Opiliones penis, penises to transfer the sperm directly to the female. However, most male terrestrial animal, 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. Ruppert, Fox & Barnes (2004), pp. 537–539 Most arthropods lay eggs, but scorpions are Ovoviparity, ovoviparous: they produce live young after the eggs have hatched inside the mother, and are noted for prolonged maternal care. 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 moult. Many insects hatch as grubs or
caterpillar Caterpillars ( ) are the larva, larval stage of members of the order Lepidoptera (the insect order comprising butterfly, butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawfly ...

caterpillar
s, which do not have segmented limbs or hardened cuticles, and metamorphosis, 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. Dragonfly larvae have the typical cuticles and jointed limbs of arthropods but are flightless water-breathers with extendable jaws. Crustaceans commonly hatch as tiny Nauplius (larva), nauplius larvae that have only three segments and pairs of appendages.


Evolutionary history


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 Arthropod leg, limbs. However, whether the ancestral limb was Arthropod leg#Biramous and uniramous, uniramous or biramous is far from a settled debate. This Ur-arthropod had a Ventral#Dorsal and ventral, ventral mouth, pre-oral antennae and Ventral#Dorsal and ventral, dorsal eyes at the front of the body. It was assumed it was a non-discriminatory sediment feeder, processing whatever sediment came its way for food, but fossil findings hints that the last common ancestor of both arthropods and priapulida shared the same specialized mouth apparatus; a circular mouth with rings of teeth used for capturing prey and was therefore carnivorous.


Fossil record

It has been proposed that the Ediacaran animals ''Parvancorina'' and ''Spriggina'', from around , were arthropods. Small arthropods with bivalve-like shells have been found in Early Cambrian fossil beds dating in China and Australia. The earliest Cambrian
trilobite Trilobites (; meaning "three lobes") are a group of marine s that form the Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the of the ( ...

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. Re-examination in the 1970s of the Burgess Shale fossils from about 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.Whittington, H. B. (1979). Early arthropods, their appendages and relationships. In M. R. House (Ed.), The origin of major invertebrate groups (pp. 253–268). The Systematics Association Special Volume, 12. London: Academic Press. Gould (1990) A fossil of ''
Marrella ''Marrella'' is an extinct genus of arthropod known from the middle Cambrian Burgess Shale of British Columbia. It is the most common animal represented in the Burgess Shale. History ''Marrella'' was the first fossil collected by Charles Doolit ...

Marrella
'' from the Burgess Shale has provided the earliest clear evidence of moulting. The earliest fossil crustaceans date from about in the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These peri ...
, and fossil
shrimp Shrimp are Decapoda, decapod crustaceans with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata. More narrow definitions may be restricted to Caridea, to smaller species of either group ...

shrimp
from about apparently formed a tight-knit procession across the seabed. Crustacean fossils are common from the Ordovician period onwards. They have remained almost entirely aquatic, possibly because they never developed excretory systems that conserve water. In 2020 scientists announced the discovery of ''Kylinxia'', a five-eyed ~5 cm long shrimp-like animal living Cambrian, 518 Mya that – with multiple distinctive features – appears to be a key ‘transitional fossil, missing link’ of the evolution from ''Anomalocaris'' to true arthropods and could be at the evolutionary root of true arthropods. Arthropods provide the earliest identifiable fossils of land animals, from about in the Late Silurian, and terrestrial tracks from about appear to have been made by arthropods. Arthropods were well preadaptation, 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. Around the same time the aquatic, scorpion-like eurypterids became the largest ever arthropods, some as long as . The oldest known
arachnid Arachnida () is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytica ...

arachnid
is the trigonotarbid ''Palaeotarbus jerami'', from about in the Silurian period. ''Attercopus fimbriunguis'', from in the Devonian period, bears the earliest known silk-producing spigots, but its lack of Spinneret (spider), spinnerets means it was not one of the true
spider Spiders (order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, and the ...

spider
s, which first appear in the Late Carboniferous over . The Jurassic and Cretaceous periods provide a large number of fossil spiders, including representatives of many modern families. Fossils of aquatic
scorpion Scorpions are predatory Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including hum ...

scorpion
s with
gill A gill () is a respiratory organ that many aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent ...
s appear in the Silurian and Devonian periods, and the earliest fossil of an air-breathing scorpion with
book lung A book lung is a type of respiration organ used for atmospheric gas exchange that is present in many arachnid Arachnida () is a class of joint-legged invertebrate Invertebrates are animals that neither possess nor develop a vertebral colum ...
s dates from the Early Carboniferous period. The oldest definitive insect fossil is the Devonian ''Rhyniognatha hirsti'', dated at , but its Mandible (insect mouthpart), mandibles are of a type found only in winged insects, which suggests that the earliest insects appeared in the Silurian period. The Mazon Creek fossils, Mazon Creek lagerstätten from the Late Carboniferous, about , 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 Middle Cenozoic.


Evolutionary family tree

From 1952 to 1977, zoologist Sidnie Manton and others argued that arthropods are Polyphyly, polyphyletic, in other words, that 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
spider Spiders (order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, and the ...

spider
s and
scorpion Scorpions are predatory Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including hum ...

scorpion
s; the crustaceans; and the uniramia, consisting of onychophorans,
myriapod Myriapoda (Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycen ...
s and hexapods. These arguments usually bypassed
trilobite Trilobites (; meaning "three lobes") are a group of marine s that form the Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the of the ( ...

trilobite
s, 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
exoskeleton An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular ...

exoskeleton
s; 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 The arthropod leg is a form of jointed of s, usually used for . Many of the terms used for arthropod leg segments (called podomeres) are of origin, and may be confused with terms for bones: ''coxa'' (meaning , plural ''coxae''), ''trochanter'', '' ...
limbs with separate gill and leg branches, while the other two groups have uniramous limbs in which the single branch serves as a leg.
Simplified summary of Budd's "broad-scale" cladogram (1996)
Further analysis and discoveries in the 1990s reversed this view, and led to acceptance that arthropods are
monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...

monophyletic
, in other words they do share a common ancestor that was itself an arthropod. 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 The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These peri ...
"lobopods", and he presented an "evolutionary family tree" that showed these as "aunts" and "cousins" of all arthropods. 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"). 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''. In 2006, they suggested that arthropods were more closely related to lobopods and tardigrades than to anomalocarids. In 2014, research indicated that tardigrades were more closely related to arthropods than velvet worms.
Relationships of Ecdysozoa to each other and to annelids, etc., including euthycarcinoids
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 Hypothesis, 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, molecular phylogenetic analyses of DNA sequences produced a coherent scheme showing arthropods as members of a superphylum labelled Ecdysozoa ("animals that moult"), 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 at 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 Convergent evolution, 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.


Classification

Arthropods belong to phylum Euarthropoda. The phylum is sometimes called Arthropoda, but strictly this term denotes a (putative - see Tactopoda) clade that also encompasses the phylum
Onychophora Onychophora (from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following period ...

Onychophora
. Euarthropoda is typically scientific classification, subdivided into five subphylum, subphyla, of which one is extinct: # 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. # Chelicerata, Chelicerates include horseshoe crabs,
spider Spiders (order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, and the ...

spider
s,
mite Mites are small arachnid Arachnida () is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Cl ...
s,
scorpion Scorpions are predatory Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including hum ...

scorpion
s and related organisms. They are characterised by the presence of chelicerae, appendages just above / in front of the mouth. 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. # Myriapoda, Myriapods comprise millipedes,
centipede Centipedes (from the New Latin New Latin (also called Neo-Latin or modern Latin) is the revival of Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally s ...

centipede
s, and their relatives and have many body segments, each segment bearing one or two pairs of legs (or in a few cases being legless). They are sometimes grouped with the hexapods. # Crustaceans are primarily aquatic (a notable exception being woodlouse, woodlice) and are characterised by having Arthropod leg#Biramous and uniramous, biramous appendages. They include
lobster Lobsters are a family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and of society ...

lobster
s,
crab Crabs are decapod crustacean Crustaceans (Crustacea ) form a large, diverse arthropod taxon which includes such animals as crabs, lobsters, crayfish, Caridea, shrimp, krill, Dendrobranchiata, prawns, woodlice, barnacles, copepods, amphipoda ...

crab
s, barnacles, crayfish,
shrimp Shrimp are Decapoda, decapod crustaceans with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata. More narrow definitions may be restricted to Caridea, to smaller species of either group ...

shrimp
and many others. # Hexapoda, Hexapods comprise
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
s 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. Aside from these major groups, there are also a number of fossil forms, mostly from the Early
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These peri ...
, 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 ''Marrella'' is an extinct genus of arthropod known from the middle Cambrian Burgess Shale of British Columbia. It is the most common animal represented in the Burgess Shale. History ''Marrella'' was the first fossil collected by Charles Doolit ...

Marrella
'' was the first one to be recognized as significantly different from the well-known groups. The Phylogenetics, phylogeny of the major extant arthropod groups has been an area of considerable interest and dispute. Recent studies strongly suggest that Crustacea, as traditionally defined, is paraphyly, paraphyletic, with Hexapoda having evolved from within it, so that Crustacea and Hexapoda form a clade, Pancrustacea. The position of Myriapoda,
Chelicerata The subphylum Chelicerata (from New Latin New Latin (also called Neo-Latin or modern Latin) is the revival of Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. ...
and Pancrustacea remains unclear . In some studies, Myriapoda is grouped with Chelicerata (forming Myriochelata); in other studies, Myriapoda is grouped with Pancrustacea (forming Mandibulata), or Myriapoda may be sister to Chelicerata plus Pancrustacea.
Phylogenetic relationships of the major extant arthropod groups according to Regier et al. (2010); traditional subphyla in bold
The placement of the extinct
trilobite Trilobites (; meaning "three lobes") are a group of marine s that form the Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the of the ( ...

trilobite
s is also a frequent subject of dispute. 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. 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.


Interaction with humans

Crustaceans such as
crab Crabs are decapod crustacean Crustaceans (Crustacea ) form a large, diverse arthropod taxon which includes such animals as crabs, lobsters, crayfish, Caridea, shrimp, krill, Dendrobranchiata, prawns, woodlice, barnacles, copepods, amphipoda ...

crab
s,
lobster Lobsters are a family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and of society ...

lobster
s, crayfish,
shrimp Shrimp are Decapoda, decapod crustaceans with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata. More narrow definitions may be restricted to Caridea, to smaller species of either group ...

shrimp
, and prawns have long been part of human cuisine, and are now raised commercially. Insects and their grubs are at least as nutritious as meat, and are eaten both raw and cooked in many cultures, though not most European, Hindu, and Islamic cultures. Cooked tarantulas are considered a delicacy in Cambodia, and by the Piaroa , Piaroa Indians of southern Venezuela, after the highly irritant hairs – the spider's main defense system – are removed. Humans also Entomophagy#Unintentional ingestion, unintentionally eat arthropods in other foods, and food safety regulations lay down acceptable contamination levels for different kinds of food material. The intentional cultivation of arthropods and other small animals for human food, referred to as minilivestock, is now emerging in animal husbandry as an ecologically sound concept. Commercial butterfly breeding provides Lepidoptera stock to Butterfly house (conservatory), 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. Besides pollinating, bees produce honey, which is the basis of a rapidly growing industry and international trade. The red dye cochineal, produced from a Central American species of insect, was economically important to the Aztecs and Maya civilization, Mayans. While the region was under Spain, Spanish control, it became Mexico's second most-lucrative export, and is now regaining some of the ground it lost to synthetic competitors. Shellac, a resin secreted by a species of insect native to southern Asia, was historically used in great quantities for many applications in which it has mostly been replaced by synthetic resins, but it is still used in woodworking and as a food additive. 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. 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. Recently insects have also gained attention as potential sources of drugs and other medicinal substances. 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 biomimesis, biomimetic robots to move normally even with damaged or lost appendages. 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 malaria carried by blood-sucking insects. Other blood-sucking insects infect livestock with diseases that kill many animals and greatly reduce the usefulness of others. Ticks can cause tick paralysis and several parasite-borne diseases in humans. A few of the closely related
mite Mites are small arachnid Arachnida () is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Cl ...
s also infest humans, causing intense itching, and others cause allergy, allergic diseases, including hay fever, asthma, and eczema. Many species of arthropods, principally insects but also mites, are agricultural and forest pests. The mite ''Varroa destructor'' has become the largest single problem faced by beekeepers worldwide. Efforts to control arthropod pests by large-scale use of pesticides have caused long-term effects on human health and on biodiversity. Increasing arthropod Pesticide resistance, resistance to pesticides has led to the development of integrated pest management using a wide range of measures including biological control. Predatory mites may be useful in controlling some mite pests.


As predators

Even amongst arthropods usually thought of as obligate predators, flower, floral food sources (nectar and to a lesser degree pollen) are often useful adjunct sources. It was noticed in one study that adult ''Adalia bipunctata'' (predator and common biological control, biocontrol of ''Mediterranean flour moth, Ephestia kuehniella'') could survive on flowers but never completed the biological life cycle, life cycle, so a meta-analysis was done to find such an overall trend in previously published data, if it existed. In some cases floral resources are outright necessary. Overall, floral resources (and an imitation, i.e. sugar water) increase longevity and fecundity, meaning even predatory population numbers can depend on non-prey food abundance. Thus biocontrol success may surprisingly depend on nearby flowers.


See also

*Dorsal lobe *Invertebrate paleontology


Notes


References


Bibliography

* *


External links

*
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 {{good article Arthropods, Extant Cambrian first appearances