Arthropod Explained

An arthropod (from Greek Greek, Ancient (to 1453);: ἄρθρον arthron, "joint" and Greek, Ancient (to 1453);: πούς pous, "foot" (gen. ποδός)) is an invertebrate animal having an exoskeleton (external skeleton), a segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,[1] which includes insects, arachnids, myriapods, and crustaceans. The term Arthropoda as originally proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora.Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Arthropods are bilaterally symmetrical and their body possesses an external skeleton. Some species have wings.

Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments. They have over a million described species, making up more than 80 percent of all described living animal species, some of which, unlike most other animals, are very successful in dry environments. Arthropods range in size from the microscopic crustacean Stygotantulus up to the Japanese spider crab.

Arthropods' primary internal cavity is a haemocoel, which accommodates their internal organs, and through which their haemolymph – analogue of blood – circulates; they have open circulatory systems. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and their brains are formed by fusion of the ganglia of these segments and encircle the esophagus. The respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong. Their vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many bristles known as setae that project through their cuticles. Arthropods' methods of reproduction and development are diverse; all terrestrial species use internal fertilization, but this is often by indirect transfer of the sperm via an appendage or the ground, rather than by direct injection.

The evolutionary ancestry of arthropods dates back to the Cambrian period. The group is generally regarded as monophyletic, and many analyses support the placement of arthropods with cycloneuralians (or their constituent clades) in a superphylum Ecdysozoa. Overall, however, the basal relationships of animals are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated. Aquatic species use either internal or external fertilization. Almost all arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and eventually undergo a total metamorphosis to produce the adult form. The level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by scorpions.

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.


The word arthropod comes from the Greek árthron, "joint", and pous (gen. podos (ποδός)), i.e. "foot" or "leg", which together mean "jointed leg".[2]


Arthropods are invertebrates with segmented bodies and jointed limbs. The exoskeleton or cuticles consists of chitin, a polymer of glucosamine. The cuticle of many crustaceans, beetle mites, 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.[3]


Estimates of the number of arthropod species vary between 1,170,000 and 5 to 10 million and account for over 80 per cent 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 ecosystems, and are one of only two major animal groups that have adapted to life in dry environments; the other is amniotes, whose living members are reptiles, birds and mammals.[4] One arthropod sub-group, insects, 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 70g. Some living crustaceans are much larger; for example, the legs of the Japanese spider crab may span up to 4m (13feet), with the heaviest of all living arthropods being the American lobster, topping out at over 20 kg (44 lbs).



The three-part appearance of many insect bodies and the two-part appearance of spiders is a result of this grouping;[5] in fact there are no external signs of segmentation in mites. Arthropods also have two body elements that are not part of this serially repeated pattern of segments, an acron at the front, ahead of the mouth, and a telson at the rear, behind the anus. The eyes are mounted on the acron.

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 appendages, with the upper branch acting as a gill while the lower branch was used for locomotion.[6] In some segments of all known arthropods the appendages have been modified, for example to form gills, mouth-parts, antennae for collecting information, or claws for grasping; arthropods are "like Swiss Army knives, 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.


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


See main article: Arthropod exoskeleton. Arthropod exoskeletons are made of cuticle, a non-cellular material secreted by the epidermis. Their cuticles vary in the details of their structure, but generally consist of three main layers: the epicuticle, a thin outer waxy coat that moisture-proofs the other layers and gives them some protection; the exocuticle, which consists of chitin and chemically hardened proteins; and the endocuticle, which consists of chitin and unhardened proteins. The exocuticle and endocuticle together are known as the procuticle. 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 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. 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 exoskeleton is cheaper to build than an all-organic one of comparable strength.


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


See main article: Ecdysis.

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.[8]



Internal organs


Respiration and circulation




Nervous system


Excretory system






Compound eyes consist of fifteen to several thousand independent ommatidia, columns that are usually hexagonal in cross section. Each ommatidium is an independent sensor, with its own light-sensitive cells and often with its own lens and cornea. Compound eyes have a wide field of view, and can detect fast movement and, in some cases, the 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 20cm (10inches) 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.




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 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. Aquatic arthropods may breed by external fertilization, as for example frogs do, or by internal fertilization, where the ova remain in the female's body and the sperm must somehow be inserted. All known terrestrial arthropods use internal fertilization. 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.[15]

Most arthropods lay eggs, but scorpions are 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 caterpillars, which do not have segmented limbs or hardened cuticles, and metamorphose into adult forms by entering an inactive phase in which the larval tissues are broken down and re-used to build the adult body. 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 larvae that have only three segments and pairs of appendages.


See also: Phylogeny of insects.

Last common ancestor

The last common ancestor of all arthropods is reconstructed as a modular organism with each module covered by its own sclerite (armor plate) and bearing a pair of biramous limbs. However, whether the ancestral limb was uniramous or biramous is far from a settled debate.This Ur-arthropod had a ventral mouth, pre-oral antennae and 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.[16]

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 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.[17] [18] A fossil of Marrella from the Burgess Shale has provided the earliest clear evidence of moulting.

The earliest fossil crustaceans date from about in the Cambrian, and fossil 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.

Arthropods provide the earliest identifiable fossils of land animals, from about in the Late Silurian,[19] and terrestrial tracks from about appear to have been made by arthropods. Arthropods were well pre-adapted to colonize land, because their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water. Around the same time the aquatic, scorpion-like eurypterids became the largest ever arthropods, some as long as 2.5m (08.2feet).

The oldest known 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 spinnerets means it was not one of the true spiders, 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 scorpions with gills appear in the Silurian and Devonian periods, and the earliest fossil of an air-breathing scorpion with book lungs dates from the Early Carboniferous period.

The oldest definitive insect fossil is the Devonian Rhyniognatha hirsti, dated at, but its mandibles are of a type found only in winged insects, which suggests that the earliest insects appeared in the Silurian period. The 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



Further analysis and discoveries in the 1990s reversed this view, and led to acceptance that arthropods are monophyletic, in other words they do share a common ancestor that was itself an arthropod.[20] For example, Graham Budd's analyses of Kerygmachela in 1993 and of Opabinia in 1996 convinced him that these animals were similar to onychophorans and to various Early Cambrian "lobopods", and he presented an "evolutionary family tree" that showed these as "aunts" and "cousins" of all arthropods. 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").


Relationships of Ecdysozoa to each other and to annelids, etc., including euthycarcinoids





See also: List of arthropod orders.

Arthropods belong to phylum Euarthropoda.[1] The phylum is sometimes called Arthropoda, but strictly this term denotes a (putative - see Tactopoda) clade that also encompasses the phylum Onychophora.

Euarthropoda is typically subdivided into five subphyla, of which one is extinct:

  2. Chelicerates include horseshoe crabs, spiders, mites, scorpions and related organisms. They are characterised by the presence of chelicerae, appendages just above / in front of the mouth. Chelicerae appear in scorpions and horseshoe crabs as tiny claws that they use in feeding, but those of spiders have developed as fangs that inject venom.
  3. Myriapods comprise millipedes, centipedes, 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.


The 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 paraphyletic, with Hexapoda having evolved from within it, so that Crustacea and Hexapoda form a clade, Pancrustacea. The position of Myriapoda, Chelicerata 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 trilobites 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.


Interaction with humans

See also: Entomophagy and Pollinator decline.

Crustaceans such as crabs, lobsters, crayfish, 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 Indians of southern Venezuela, after the highly irritant hairs – the spider's main defense system – are removed. Humans also 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 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.[23] 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 Mayans. While the region was under 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.


Diseases transmitted by insects! Disease !! Insect !! Cases per year !! Deaths per year
Anopheles mosquito 267 M 1 to 2 M
Aedes mosquito 4,432 1,177
Culex mosquito 250 M unknown

Although arthropods are the most numerous phylum on Earth, and thousands of arthropod species are venomous, they inflict relatively few serious bites and stings on humans. Far more serious are the effects on humans of diseases carried by blood-sucking insects. Other blood-sucking insects infect livestock with diseases that kill many animals and greatly reduce the usefulness of others. Ticks can cause tick paralysis and several parasite-borne diseases in humans. A few of the closely related mites also infest humans, causing intense itching, and others cause 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 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.

See also



External links

Notes and References

  1. Reference showing that Euarthropoda is a phylum: 10.1038/nature13576. 25132546 . 2014Natur.514..363S . 514 . 7522 . Hallucigenia's onychophoran-like claws and the case for Tactopoda . 2014 . Nature . 363–366 . Smith Martin R.
  2. Web site: Arthropoda. Online Etymology Dictionary.
  3. Kovoor. J.. Natural calcification of the prosomatic endosternite in the Phalangiidae (Arachnida:Opiliones).. Calcified Tissue Research. 1978. 26. 3. 267–9. 750069. 10.1007/BF02013269.
  4. [#refRuppert|Ruppert, Fox & Barnes (2004)]
  5. [#refGould|Gould (1990)]
  6. Web site: Giant sea creature hints at early arthropod evolution. 2015-03-11.
  7. Summarised in Gould (1990), pp. 107–121.
  8. [#refRuppert|Ruppert, Fox & Barnes (2004)]
  9. [#refRuppert|Ruppert, Fox & Barnes (2004)]
  10. [#refRuppert|Ruppert, Fox & Barnes (2004)]
  11. [#refRuppert|Ruppert, Fox & Barnes (2004)]
  12. [#refRuppert|Ruppert, Fox & Barnes (2004)]
  13. [#refRuppert|Ruppert, Fox & Barnes (2004)]
  14. [#refRuppert|Ruppert, Fox & Barnes (2004)]
  15. [#refRuppert|Ruppert, Fox & Barnes (2004)]
  16. Web site: Arthropod ancestor had the mouth of a penis worm - Natural History Museum.
  18. [#refGould|Gould (1990)]
  19. Russell J.. Garwood. Greg. Edgecombe. 2011 . Early Terrestrial Animals, Evolution, and Uncertainty . Evolution: Education and Outreach . 4 . 3 . 489–501 . 10.1007/s12052-011-0357-y .
  20. The book is
  21. Web site: Misunderstood worm-like fossil finds its place in the Tree of Life. 17 August 2014.
  22. Web site: Arthropoda . 2007-02-07.
  23. Free summary at