Haemolymph is the analogue ofblood for most arthropods. An arthropod has anopen circulatory system, with a body cavity called ahaemocoel through which haemolymph circulates to the interiororgans. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. They have ladder-likenervous systems, with pairedventralnerve cords running through all segments and forming pairedganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and theirbrains are formed by fusion of the ganglia of these segments and encircle theesophagus. Therespiratory andexcretory systems of arthropods vary, depending as much on their environment as on thesubphylum to which they belong.
Arthropods use combinations ofcompound eyes andpigment-pitocelli for vision. In most species, the ocelli can only detect the direction from which light is coming, andthe compound eyes are the main source of information, but the main eyes ofspiders 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 assetae that project through their cuticles. Similarly, their reproduction and development are varied; allterrestrial species useinternal fertilization, but this is sometimes by indirect transfer of thesperm via an appendage or the ground, rather than by direct injection. Aquatic species use either internal orexternal fertilization. Almost all arthropods lay eggs, with many species giving birth to live young after the eggs have hatched inside the mother; but a few are genuinelyviviparous, such asaphids. Arthropod hatchlings vary from miniature adults to grubs andcaterpillars that lack jointed limbs and eventually undergo a totalmetamorphosis to produce the adult form. The level of maternal care for hatchlings varies from nonexistent to the prolonged care provided bysocial insects.
The evolutionary ancestry of arthropods dates back to theCambrian period. The group is generally regarded asmonophyletic, and many analyses support the placement of arthropods withcycloneuralians (or their constituent clades) in a superphylumEcdysozoa. Overall, however, thebasal 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 aspollinators of crops. Some species are known to spread severe disease to humans,livestock, andcrops.
Terrestrial arthropods are often called bugs.[Note 1] The term is also occasionally extended to colloquial names for freshwater or marinecrustaceans (e.g.,Balmain bug,Moreton Bay bug,mudbug) and used by physicians and bacteriologists for disease-causing germs (e.g.,superbugs),[8] but entomologists reserve this term for a narrow category of "true bugs", insects of the orderHemiptera.[8]
Arthropoda is the largest animalphylum, with the estimates of the number of arthropod species varying from 1,170,000 to 5~10 million and accounting for over 80 percent of all known living animal species.[14][15] One arthropodsub-group, theinsects, includes moredescribed species than any othertaxonomic class.[16] The total number of species remains difficult to determine. This is due to the census modeling assumptions projected onto other regions in order to scale up from counts at specific locations applied to the whole world. A study in 1992 estimated that there were 500,000 species of animals and plants inCosta Rica alone, of which 365,000 were arthropods.[16]
They are important members of marine, freshwater, land and airecosystems and one of only two major animal groups that have adapted to life in dry environments; the other isamniotes, whose living members are reptiles, birds and mammals.[17] Both the smallest and largest arthropods arecrustaceans. Thesmallest belong to the classTantulocarida, some of which are less than 100 micrometres (0.0039 in) long.[18] Thelargest are species in the classMalacostraca, with the legs of theJapanese spider crab potentially spanning up to 4 metres (13 ft)[19] and theAmerican lobster reaching weights over 20 kg (44 lbs).
Theembryos of all arthropods are segmented, built from a series of repeated modules. Thelast common ancestor of living arthropods probably consisted of a series of undifferentiated segments, each with a pair ofappendages that functioned as limbs. However, all known living and fossil arthropods have grouped segments intotagmata in which segments and their limbs are specialized in various ways.[17]
The three-part appearance of manyinsect bodies and the two-part appearance ofspiders is a result of this grouping.[21] There are no external signs of segmentation inmites.[17] Arthropods also have two body elements that are not part of this serially repeated pattern of segments, anocular somite at the front, where the mouth and eyes originated,[17][22] and atelson at the rear, behind theanus.
Originally, it seems that each appendage-bearing segment had two separate pairs of appendages: an upper, unsegmentedexite and a lower, segmented endopod. These would later fuse into a single pair ofbiramous appendages united by a basal segment (protopod or basipod), with the upper branch acting as agill while the lower branch was used for locomotion.[23][24][20] The appendages of mostcrustaceans and some extinct taxa such astrilobites have another segmented branch known asexopods, but whether these structures have a single origin remain controversial.[25][26][20] In some segments of all known arthropods, the appendages have been modified, for example to form gills, mouth-parts,antennae for collecting information,[21] or claws for grasping;[27] arthropods are "likeSwiss Army knives, each equipped with a unique set of specialized tools."[17] 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.[17]
0:● Ocular somite
1-2...: ∎ Somites of head tagma (head / cephalon / prosoma)
Alignment of anterior body segments and appendages across various arthropod taxa, based on the observations until the mid 2010s. Head regions in black.[22][28]
The most conspicuous specialization of segments is in the head. The four major groups of arthropods –Chelicerata (sea spiders,horseshoe crabs andarachnids),Myriapoda (symphylans,pauropods,millipedes andcentipedes),Pancrustacea (oligostracans,copepods,malacostracans,branchiopods,hexapods, etc.), and the extinctTrilobita – have heads formed of various combinations of segments, with appendages that are missing or specialized in different ways.[17][28] Despite myriapods and hexapods both having similar head combinations, hexapods are deeply nested within crustacea while myriapods are not, so these traits are believed to have evolved separately. In addition, some extinct arthropods, such asMarrella, belong to none of these groups, as their heads are formed by their own particular combinations of segments and specialized appendages.[29]
Working out the evolutionary stages by which all these different combinations could have appeared is so difficult that it has long been known as "Thearthropod head problem".[30] In 1960, R. E. Snodgrass even hoped it would not be solved, as he found trying to work out solutions to be fun.[Note 2]
Illustration of an idealized arthropod exoskeleton.
Arthropod exoskeletons are made ofcuticle, a non-cellular material secreted by theepidermis.[17] Their cuticles vary in the details of their structure, but generally consist of three main layers: theepicuticle, a thin outerwaxy coat that moisture-proofs the other layers and gives them some protection; theexocuticle, which consists ofchitin and chemically hardenedproteins; and theendocuticle, which consists of chitin and unhardened proteins. The exocuticle and endocuticle together are known as theprocuticle.[32] 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.[17]
The exoskeletons of most aquaticcrustaceans arebiomineralized withcalcium 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.[33] Biomineralization generally affects the exocuticle and the outer part of the endocuticle.[32] Two recent hypotheses about the evolution of biomineralization in arthropods and other groups of animals propose that it provides tougher defensive armor,[34] and that it allows animals to grow larger and stronger by providing more rigid skeletons;[35] and in either case a mineral-organiccomposite exoskeleton is cheaper to build than an all-organic one of comparable strength.[35][36]
The cuticle may havesetae (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 usefeather-like setae to increase the surface area of swimming appendages and tofilter food particles out of water; aquatic insects, which are air-breathers, use thickfelt-like coats of setae to trap air, extending the time they can spend under water; heavy, rigid setae serve as defensive spines.[17]
Although all arthropods use muscles attached to the inside of the exoskeleton to flex their limbs, some still usehydraulic pressure to extend them, a system inherited from their pre-arthropod ancestors;[37] for example, all spiders extend their legs hydraulically and can generate pressures up to eight times their resting level.[38]
Acicada climbing out of itsexuviae while attached to tree
The exoskeleton cannot stretch and thus restricts growth. Arthropods, therefore, replace their exoskeletons by undergoingecdysis (moulting), or shedding the old exoskeleton, theexuviae, after growing a new one that is not yet hardened. Moulting cycles run nearly continuously until an arthropod reaches full size. The developmental stages between each moult (ecdysis) until sexual maturity is reached is called aninstar. 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.[39]
In the initial phase of moulting, the animal stops feeding and its epidermis releases moulting fluid, a mixture ofenzymes that digests theendocuticle and thus detaches the old cuticle. This phase begins when theepidermis has secreted a newepicuticle 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.[39]
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 bypredators. Moulting may be responsible for 80 to 90% of all arthropod deaths.[39]
Arthropod bodies are also segmented internally, and the nervous, muscular, circulatory, and excretory systems have repeated components.[17] Arthropods come from a lineage of animals that have acoelom, 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 ahydrostatic 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 ahemocoel, a cavity that runs most of the length of the body and through whichblood flows.[40]
Respiration and circulation in amyodocopidostracod. Simplified transverse section through anterior body and carapace, showing gaseous diffusion through the inner lamella of the carapace (yellow arrows)
Arthropods have opencirculatory systems. Most have a few short, open-endedarteries. In chelicerates and crustaceans, the blood carriesoxygen to the tissues, whilehexapods use a separate system oftracheae. Many crustaceans and a few chelicerates andtracheates userespiratory pigments to assist oxygen transport. The most common respiratory pigment in arthropods iscopper-basedhemocyanin; this is used by many crustaceans and a fewcentipedes. A few crustaceans and insects use iron-basedhemoglobin, the respiratory pigment used byvertebrates. As with other invertebrates, the respiratory pigments of those arthropods that have them are generally dissolved in the blood and rarely enclosed incorpuscles as they are in vertebrates.[40]
The heart is a muscular tube that runs just under the back and for most of the length of the hemocoel. It contracts in ripples that run from rear to front, pushing blood forwards. Sections not being squeezed by the heart muscle are expanded either by elasticligaments or by smallmuscles, 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.[40]
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 havebook lungs.[41] Tracheae, systems of branching tunnels that run from the openings in the body walls, deliver oxygen directly to individual cells in many insects, myriapods andarachnids.[42]
Central nervous system of a nectiopodremipede, showing the presence of both deutocerebrum (dc) and ventral nerve cord (vnc) organized by segmented ganglia.
Living arthropods have paired main nerve cords running along their bodies below the gut, and in each segment the cords form a pair ofganglia from whichsensory andmotor nerves run to other parts of the segment. Although the pairs of ganglia in each segment often appear physically fused, they are connected bycommissures (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 ofsubesophageal ganglia, under and behind the esophagus. Spiders take this process a step further, as all thesegmental ganglia are incorporated into the subesophageal ganglia, which occupy most of the space in the cephalothorax (front "super-segment").[43]
There are two different types of arthropod excretory systems. In aquatic arthropods, the end-product of biochemical reactions thatmetabolisenitrogen isammonia, 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.[41] 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.[44] Various groups of terrestrial arthropods have independently developed a different system: the end-product of nitrogen metabolism isuric acid, which can be excreted as dry material; theMalpighian 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 asfeces.[44] Most aquatic arthropods and some terrestrial ones also have organs callednephridia ("littlekidneys"), which extract other wastes for excretion asurine.[44]
The stiffcuticles 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, mostlysetae, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents oftaste andsmell, often by means of setae. Pressure sensors often take the form of membranes that function aseardrums, but are connected directly to nerves rather than toauditory ossicles. Theantennae of most hexapods include sensor packages that monitorhumidity, moisture and temperature.[45]
Most arthropods lack balance andacceleration sensors, and rely on their eyes to tell them which way is up. The self-righting behavior ofcockroaches is triggered when pressure sensors on the underside of the feet report no pressure. However, manymalacostracan crustaceans havestatocysts, which provide the same sort of information as the balance and motion sensors of the vertebrateinner ear.[45]
Theproprioceptors 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.[45]
Arthropod eyesHead of awasp with three ocelli (center), and compound eyes at the left and right
Most arthropods have sophisticated visual systems that include one or more usually both ofcompound eyes and pigment-cupocelli ("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 ofspiders are pigment-cup ocelli that are capable of forming images,[45] and those ofjumping spiders can rotate to track prey.[46]
Compound eyes consist of fifteen to several thousand independentommatidia, columns that are usuallyhexagonal incross section. Each ommatidium is an independent sensor, with its own light-sensitive cells and often with its ownlens andcornea.[45] Compound eyes have a wide field of view, and can detect fast movement and, in some cases, thepolarization of light.[47] On the other hand, the relatively large size of ommatidia makes the images rather coarse, and compound eyes are shorter-sighted than those of birds and mammals – although this is not a severe disadvantage, as objects and events within 20 cm (8 in) are most important to most arthropods.[45] 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 andultra-violet.[45]
A few arthropods, such asbarnacles, arehermaphroditic, that is, each can have the organs of bothsexes. However, individuals of most species remain of one sex their entire lives.[48] A few species ofinsects and crustaceans can reproduce byparthenogenesis, especially if conditions favor a "population explosion". However, most arthropods rely onsexual reproduction, and parthenogenetic species often revert to sexual reproduction when conditions become less favorable.[49] The ability to undergomeiosis is widespread among arthropods including both those that reproduce sexually and those that reproduceparthenogenetically.[50] Although meiosis is a major characteristic of arthropods, understanding of its fundamental adaptive benefit has long been regarded as an unresolved problem,[51] that appears to have remained unsettled.
Aquatic arthropods may breed by external fertilization, as for examplehorseshoe crabs do,[52] or byinternal fertilization, where theova remain in the female's body and thesperm must somehow be inserted. All known terrestrial arthropods use internal fertilization.Opiliones (harvestmen),millipedes, and some crustaceans use modified appendages such asgonopods orpenises to transfer the sperm directly to the female. However, most maleterrestrial arthropods producespermatophores, waterproof packets ofsperm, 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 complexcourtship rituals look likely to be successful.[48]
Most arthropods lay eggs,[48] but scorpions areovoviviparous: they produce live young after the eggs have hatched inside the mother, and are noted for prolonged maternal care.[53] 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 assilverfish, the hatchlings do not feed and may be helpless until after their first moult. Many insects hatch as grubs orcaterpillars, which do not have segmented limbs or hardened cuticles, andmetamorphose into adult forms by entering an inactive phase in which the larval tissues are broken down and re-used to build the adult body.[54]Dragonfly larvae have the typical cuticles and jointed limbs of arthropods but are flightless water-breathers with extendable jaws.[55] Crustaceans commonly hatch as tinynauplius larvae that have only three segments and pairs of appendages.[48]
Based on the distribution of sharedplesiomorphic features in extant and fossil taxa, thelast common ancestor of all arthropods is inferred to have been as a modular organism with each module covered by its ownsclerite (armor plate) and bearing a pair of biramouslimbs.[56] However, whether the ancestral limb wasuniramous or biramous is far from a settled debate.This Ur-arthropod had aventral mouth, pre-oral antennae anddorsal eyes at the front of the body. It was assumed to have been a non-discriminatorysediment feeder, processing whatever sediment came its way for food,[56] but fossil findings hint that the last common ancestor of both arthropods andPriapulida shared the same specialized mouth apparatus: a circular mouth with rings of teeth used for capturing animal prey.[57]
It has been proposed that theEdiacaran animalsParvancorina andSpriggina, from around555 million years ago, were arthropods,[58][59][60] but later study shows that their affinities of being origin of arthropods are not reliable.[61] Small arthropods with bivalve-like shells have been found in Early Cambrian fossil beds dating541 to 539 million years ago in China and Australia.[62][63][64][65] The earliest Cambriantrilobite fossils are about 520 million years old, but the class was already quite diverse and worldwide, suggesting that they had been around for quite some time.[66] In theMaotianshan shales, which date back to 518 million years ago, arthropods such asKylinxia andErratus have been found that seem to representtransitional fossils between stem (e.g.Radiodonta such asAnomalocaris) and true arthropods.[67][68][24] Re-examination in the 1970s of theBurgess Shale fossils from about505 million years ago identified many arthropods, some of which could not be assigned to any of the well-known groups, and thus intensified the debate about theCambrian explosion.[69][70][71] A fossil ofMarrella from the Burgess Shale has provided the earliest clear evidence ofmoulting.[72]
Kylinxia may be a key transitional fossil between stem-arthropods and true arthropods.[67]Yicaris is one of the earliest crustaceans that have been discovered.
Arthropods provide the earliest identifiable fossils of land animals, from about419 million years ago in the LateSilurian,[41] and terrestrial tracks from about450 million years ago appear to have been made by arthropods.[78] Arthropods possessed attributes that were easycoopted for life on land; their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water.[79] Around the same time the aquatic, scorpion-likeeurypterids became the largest ever arthropods, some as long as 2.5 m (8 ft 2 in).[80]
The oldest knownarachnid is thetrigonotarbidPalaeotarbus jerami, from about420 million years ago in the Silurian period.[81][Note 3]Attercopus fimbriunguis, from386 million years ago in theDevonian period, bears the earliest known silk-producing spigots, but its lack ofspinnerets means it was not one of the truespiders,[83] which first appear in the LateCarboniferous over299 million years ago.[84] TheJurassic andCretaceous periods provide a large number of fossil spiders, including representatives of many modern families.[85] The oldest knownscorpion isDolichophonus, dated back to436 million years ago.[86] Lots of Silurian and Devonian scorpions were previously thought to begill-breathing, hence the idea that scorpions were primitively aquatic and evolved air-breathingbook lungs later on.[87] However subsequent studies reveal most of them lacking reliable evidence for an aquatic lifestyle,[88] while exceptional aquatic taxa (e.g.Waeringoscorpio) most likely derived from terrestrial scorpion ancestors.[89]
The oldest fossil record ofhexapod is obscure, as most of the candidates are poorly preserved and their hexapod affinities had been disputed. An iconic example is the DevonianRhyniognatha hirsti, dated at396 to 407 million years ago, itsmandibles are thought to be a type found only inwinged insects, which suggests that the earliest insects appeared in the Silurian period.[90] However later study shows thatRhyniognatha most likely represent a myriapod, not even a hexapod.[91] The unequivocal oldest known hexapod is thespringtailRhyniella, from about410 million years ago in the Devonian period, and thepalaeodictyopteranDelitzschala bitterfeldensis, from about325 million years ago in the Carboniferous period, respectively.[91] TheMazon Creek lagerstätten from the Late Carboniferous, about300 million years ago, include about 200 species, some gigantic by modern standards, and indicate that insects had occupied their main modernecological niches asherbivores,detritivores andinsectivores. Socialtermites andants first appear in the EarlyCretaceous, and advanced social bees have been found in Late Cretaceous rocks but did not become abundant until the MiddleCenozoic.[92]
From 1952 to 1977, zoologistSidnie Manton and others argued that arthropods arepolyphyletic, 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: thechelicerates, includingspiders andscorpions; the crustaceans; and theuniramia, consisting ofonychophorans,myriapods andhexapods. These arguments usually bypassedtrilobites, as the evolutionary relationships of this class were unclear. Proponents of polyphyly argued the following: that the similarities between these groups are the results ofconvergent evolution, as natural consequences of having rigid, segmentedexoskeletons; 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 havebiramous limbs with separate gill and leg branches, while the other two groups haveuniramous limbs in which the single branch serves as a leg.[94]
includes living groups and extinct forms such astrilobites
Simplified summary of Budd's (1996) "broad-scale" cladogram[93]
Further analysis and discoveries in the 1990s reversed this view, and led to acceptance that arthropods aremonophyletic, in other words they are inferred to share a common ancestor that was itself an arthropod.[95][96] For example,Graham Budd's analyses ofKerygmachela in 1993 and ofOpabinia 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.[93][97] 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").[98]
A contrary view was presented in 2003, when Jan Bergström andHou Xian-guang 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. In 2006, they suggested that arthropods were more closely related tolobopods andtardigrades than to anomalocarids.[99] In 2014, it was found that tardigrades were more closely related to arthropods than velvet worms.[100]
Relationships of Ecdysozoa to each other and to annelids,etc.,[101][failed verification] including euthycarcinoids[102]
Higher up the "family tree", theAnnelida have traditionally been considered the closest relatives of the Panarthropoda, since both groups have segmented bodies, and the combination of these groups was labelledArticulata. There had been competing proposals that arthropods were closely related to other groups such asnematodes,priapulids andtardigrades, but these remained minority views because it was difficult to specify in detail the relationships between these groups.
In the 1990s,molecular phylogenetic analyses ofDNA sequences produced a coherent scheme showing arthropods as members of asuperphylum 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 andbrachiopods in another superphylum,Lophotrochozoa.
If the Ecdysozoa hypothesis is correct, then segmentation of arthropods and annelids either has evolvedconvergently 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.[103][101]
Summarizedcladogram of the relationships between extinct arthropod groups. For more, seeDeuteropoda.
Aside from the four major living groups (crustaceans,chelicerates,myriapods andhexapods), a number of fossil forms, mostly from the early Cambrian period, are difficult to place taxonomically, either from lack of obvious affinity to any of the main groups or from clear affinity to several of them.Marrella was the first one to be recognized as significantly different from the well-known groups.[29]
Modern interpretations of the basal, extinctstem-group of Arthropoda recognised the following groups, from most basal to most crownward:[105][104]
TheDeuteropoda is a recently established clade uniting the crown-group (living) arthropods with these possible "upper stem-group" fossils taxa.[105] The clade is defined by important changes to the structure of the head region such as the appearance of a differentiateddeutocerebral appendage pair, which excludes more basal taxa like radiodonts and "gilled lobopodians".[105]
Controversies remain about the positions of various extinct arthropod groups. Some studies recover Megacheira as closely related to chelicerates, while others recover them as outside the group containing Chelicerate and Mandibulata as stem-group euarthropods.[106] The placement of theArtiopoda (which contains the extinct trilobites and similar forms) is also a frequent subject of dispute.[107] The main hypotheses position them in the cladeArachnomorpha with the Chelicerates. However, one of the newer hypotheses is that the chelicerae have originated from the same pair of appendages that evolved into antennae in the ancestors ofMandibulata, which would place trilobites, which had antennae, closer to Mandibulata than Chelicerata, in the cladeAntennulata.[106][108] Thefuxianhuiids, usually suggested to be stem-group arthropods, have been suggested to be Mandibulates in some recent studies.[106] TheHymenocarina, a group of bivalved arthropods, previously thought to have been stem-group members of the group, have been demonstrated to be mandibulates based on the presence of mandibles.[104]
Radiodonts, Opabiniids, Gilled Lobopodians and the more traditional Lobopodians are all examples of basal stem-group arthropod lineages from the Cambrian
Thephylogeny of the major extant arthropod groups has been an area of considerable interest and dispute.[147] Recent studies strongly suggest that Crustacea, as traditionally defined, isparaphyletic, with Hexapoda having evolved from within it,[148][149] so that Crustacea and Hexapoda form a clade,Pancrustacea. The position ofMyriapoda,Chelicerata and Pancrustacea remains unclear as of April 2012[update]. In some studies, Myriapoda is grouped with Chelicerata (formingMyriochelata);[150][151] in other studies, Myriapoda is grouped with Pancrustacea (formingMandibulata),[148] or Myriapoda may be sister to Chelicerata plus Pancrustacea.[149]
The following cladogram shows the internal relationships between all the livingclasses of arthropods as of the late 2010s,[152][153][154] as well as the estimated timing for some of the clades:[155]
Insects and scorpions on sale in a food stall inBangkok, Thailand
Crustaceans such ascrabs,lobsters,crayfish,shrimp, andprawns have long been part of human cuisine, and are now raised commercially.[156] 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.[157][158] Cookedtarantulas are considered a delicacy inCambodia,[159][160][161] and by thePiaroa Indians of southernVenezuela, after the highly irritant hairs – the spider's main defense system – are removed.[162] Humans alsounintentionally eat arthropods in other foods,[163] and food safety regulations lay down acceptable contamination levels for different kinds of food material.[Note 4][Note 5] The intentional cultivation of arthropods and other small animals for human food, referred to asminilivestock, is now emerging inanimal husbandry as an ecologically sound concept.[167]Commercial butterfly breeding provides Lepidoptera stock tobutterfly conservatories, educational exhibits, schools, research facilities, and cultural events.
However, the greatest contribution of arthropods to human food supply is bypollination: 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.[168] Besides pollinating,bees producehoney, which is the basis of a rapidly growing industry and international trade.[169]
The red dyecochineal, produced from a Central American species of insect, was economically important to theAztecs andMayans.[170] While the region was underSpanish control, it becameMexico's second most-lucrative export,[171] and is now regaining some of the ground it lost to synthetic competitors.[172]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 inwoodworking and as afood additive. The blood of horseshoe crabs contains a clotting agent,Limulus Amebocyte Lysate, which is now used to test thatantibiotics and kidney machines are free of dangerousbacteria, and to detectspinal meningitis.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.[173] Recently insects have also gained attention as potential sources of drugs and other medicinal substances.[174]
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 forrobotics. The redundancy provided by segments allows arthropods andbiomimetic robots to move normally even with damaged or lost appendages.[175][176]
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 likemalaria carried byblood-sucking insects. Other blood-sucking insects infect livestock with diseases that kill many animals and greatly reduce the usefulness of others.[177]Ticks can causetick paralysis and severalparasite-borne diseases in humans.[178] A few of the closely relatedmites also infest humans, causing intense itching,[179] and others causeallergic diseases, includinghay fever,asthma, andeczema.[180]
^TheMuseum of New Zealand notes that "in everyday conversation",bug "refers to land arthropods with at least six legs, such as insects, spiders, and centipedes".[6] In a chapter on "Bugs That Are Not Insects", entomologist Gilbert Walbauer specifies centipedes, millipedes, arachnids (spiders,daddy longlegs, scorpions,mites,chiggers and ticks) as well as the few terrestrial crustaceans (sowbugs andpillbugs),[7] but argues that "including legless creatures such as worms, slugs, and snails among the bugs stretches the word too much".[8]
^"It would be too bad if the question of head segmentation ever should be finally settled; it has been for so long such fertile ground for theorizing that arthropodists would miss it as a field for mental exercise."[31]
^The fossil was originally namedEotarbus but was renamed when it was realized that aCarboniferous arachnid had already been namedEotarbus.[82]
^For a mention of insect contamination in an international food quality standard, see sections 3.1.2 and 3.1.3 of Codex 152 of 1985 of theCodex Alimentarius.[164]
^For examples of quantified acceptable insect contamination levels in food see the last entry (on "Wheat Flour") and the definition of "Extraneous material" inCodex Alimentarius,[165] and the standards published by the FDA.[166]
^Gravenhorst, J. L. C. (1843).Vergleichende Zoologie [Comparative Zoology] (in German). Breslau, (Prussia): Graß, Barth & Comp. p. foldout."Mit gegliederten Bewegungsorganen" (with articulated movement organs)
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^Mohrbeck, Inga; Martínez Arbizu, Pedro; Glatzel, Thomas (October 2010). "Tantulocarida (Crustacea) of the Southern Ocean deep sea, and the description of three new species ofTantulacus(Huys, Andersen & Kristensen, 1992)".Systematic Parasitology.77 (2):131–151.doi:10.1007/s11230-010-9260-0.PMID20852984.S2CID7325858.
^Hejnol, Andreas; Scholtz, Gerhard (1 October 2004). "Clonal analysis of Distal-less and engrailed expression patterns during early morphogenesis of uniramous and biramous crustacean limbs".Development Genes and Evolution.214 (10):473–485.doi:10.1007/s00427-004-0424-2.ISSN1432-041X.PMID15300435.S2CID22426697.
^abWhittington, H. B. (1971), "Redescription ofMarrella splendens (Trilobitoidea) from the Burgess Shale, Middle Cambrian, British Columbia",Geological Survey of Canada Bulletin,209:1–24 Summarised inGould (1990), pp. 107–121.
^Schurko, A. M.; Mazur, D. J.; Logsdon, J. M. (February 2010). "Inventory and phylogenomic distribution of meiotic genes in Nasonia vitripennis and among diverse arthropods".Insect Molecular Biology.19 (Suppl 1):165–180.doi:10.1111/j.1365-2583.2009.00948.x.PMID20167026.S2CID11617147.
^abBergström, Jan; Hou, Xian-Guang (2005), "Early Palaeozoic non-lamellipedian arthropods", in Stefan Koenemann; Ronald A. Jenner (eds.),Crustacea and Arthropod Relationships, Crustacean Issues, vol. 16, Boca Raton:Taylor & Francis, pp. 73–93,doi:10.1201/9781420037548.ch4 (inactive 11 November 2024),ISBN978-0-8493-3498-6{{citation}}: CS1 maint: DOI inactive as of November 2024 (link)
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^Poschmann, Markus; Dunlop, Jason A.; Kamenz, Carsten; Scholtz, Gerhard (December 2008). "The Lower Devonian scorpionWaeringoscorpio and the respiratory nature of its filamentous structures, with the description of a new species from the Westerwald area, Germany".Paläontologische Zeitschrift.82 (4):418–436.Bibcode:2008PalZ...82..418P.doi:10.1007/BF03184431.ISSN0031-0220.
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^Izquierdo-López, Alejandro; Caron, Jean-Bernard (August 2024). "The Cambrian Odaraia alata and the colonization of nektonic suspension-feeding niches by early mandibulates".Proceedings of the Royal Society B: Biological Sciences.291 (2027).doi:10.1098/rspb.2024.0622.ISSN1471-2954.PMC 11463219.PMID39043240.
^Pulsipher, M. A.; Anderson, E. P.; Wright, L. S.; Kluessendorf, J.; Mikulic, D. G.; Schiffbauer, J. D. (2022). "Description ofAcheronauta gen. nov., a possible mandibulate from the Silurian Waukesha Lagerstätte, Wisconsin, USA".Journal of Systematic Palaeontology.20 (1). 2109216.Bibcode:2022JSPal..20....1P.doi:10.1080/14772019.2022.2109216.S2CID252839113.
^Wilson, Heather M.; Almond, John E. (February 2001). "New Euthycarcinoids and an Enigmatic Arthropod from the British Coal Measures".Palaeontology.44 (1):143–156.Bibcode:2001Palgy..44..143W.doi:10.1111/1475-4983.00174.
^Fayers, S. R.; Trewin, N. H.; Morrissey, L. (May 2010). "A large arthropod from the Lower Old Red Sandstone (Early Devonian) of Tredomen Quarry, south Wales: ARTHROPOD FROM THE LOWER ORS".Palaeontology.53 (3):627–643.doi:10.1111/j.1475-4983.2010.00951.x.
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