Chelicerata split fromMandibulata by the mid-Cambrian, as evidenced bystem-group chelicerates likeHabeliida andMollisonia present by this time.[2] The surviving marine species include the four species ofxiphosurans (horseshoe crabs), and possibly the 1,300 species ofpycnogonids (sea spiders), if the latter are indeed chelicerates. On the other hand, there are over 77,000 well-identified species of air-breathing chelicerates, and there may be about 500,000 unidentified species.
Like allarthropods, chelicerates havesegmented bodies with jointed limbs, all covered in acuticle made ofchitin andproteins. The cheliceratebody plan consists of twotagmata, theprosoma and theopisthosoma – excepting themites, which have lost any visible division between these sections. Thechelicerae, which give the group its name, are the onlyappendages that appear before the mouth. In most sub-groups, they are modest pincers used to feed. However, spiders' chelicerae form fangs that most species use to injectvenom into prey. The group has theopen circulatory system typical of arthropods, in which a tube-like heart pumps blood through thehemocoel, which is the major body cavity. Marine chelicerates have gills, while the air-breathing forms generally have bothbook lungs andtracheae. In general, theganglia of living chelicerates'central nervous systems fuse into large masses in the cephalothorax, but there are wide variations and this fusion is very limited in theMesothelae, which are regarded as the oldest and most basal group of spiders. Most chelicerates rely on modifiedbristles for touch and for information about vibrations, air currents, and chemical changes in their environment. The most active hunting spiders also have very acute eyesight.
Chelicerates were originally predators, but the group has diversified to use all the major feeding strategies: predation,parasitism,herbivory,scavenging andeating decaying organic matter. Althoughharvestmen can digest solid food, the guts of most modern chelicerates are too narrow for this, and they generally liquidize their food by grinding it with their chelicerae andpedipalps and flooding it with digestiveenzymes. To conserve water, air-breathing chelicerates excrete waste as solids that are removed from their blood byMalpighian tubules, structures that alsoevolved independently ininsects.[3]
While the marine horseshoe crabs rely onexternal fertilization, air-breathing chelicerates use internal but usually indirect fertilization. Many species use elaboratecourtship rituals to attract mates. Most lay eggs that hatch as what look like miniature adults, but all scorpions and a few species of mites keep the eggs inside their bodies until the young emerge. In most chelicerate species the young have to fend for themselves, but in scorpions and some species of spider the females protect and feed their young.
Theevolutionary origins of chelicerates from the early arthropods have been debated for decades. Although there is considerable agreement about the relationships between most chelicerate sub-groups, the inclusion of the Pycnogonida in this taxon has been questioned, and the exact position of scorpions is still controversial, though they were long considered the most basal of the arachnids.[4]
Although the venom of a few spider and scorpion species can be very dangerous to humans, medical researchers are investigating the use of these venoms for the treatment of disorders ranging fromcancer toerectile dysfunction. The medical industry also uses the blood of horseshoe crabs as a test for the presence of contaminantbacteria. Mites can causeallergies in humans, transmit several diseases to humans and theirlivestock, and are serious agriculturalpests.
Formation of anterior segments across arthropod taxa based on previous hypothesis.[9] Note the antenna-bearing somite 1 was thought to be lost in Chelicerata.
Formation of anterior segments across arthropod taxa based on gene expression and neuroanatomical observations,[10][11] Note the chelicera(Ch) and chelifore(Chf) arose from somite 1 and thus correspond to the first antenna(An/An1) of other arthropods.
The Chelicerata arearthropods as they have:segmented bodies with jointed limbs, all covered in acuticle made ofchitin andproteins; heads that are composed of several segments that fuse during the development of theembryo; a much reducedcoelom; ahemocoel through which theblood circulates, driven by a tube-like heart.[9] Chelicerates' bodies consist of twotagmata, sets of segments that serve similar functions: the foremost one, called theprosoma orcephalothorax, and the rear tagma is called theopisthosoma orabdomen.[12] However, in theAcari (mites and ticks) there is no visible division between these sections.[13]
Theprosoma is formed in the embryo by fusion of the ocular somite (referred as "acron" in previous literatures), which carries the eyes andlabrum,[11] with six post-ocular segments (somite 1 to 6),[10] which all have paired appendages. It was previously thought that chelicerates had lost the antennae-bearing somite 1,[14] but later investigations reveal that it is retained and corresponds to a pair ofchelicerae or chelifores,[15] small appendages that often formpincers. Somite 2 has a pair ofpedipalps that in most sub-groups perform sensory functions, while the remaining fourcephalothorax segments (somite 4 to 6) have pairs of legs.[10] In basal forms the ocular somite has a pair ofcompound eyes on the sides and four pigment-cupocelli ("little eyes") in the middle.[12] The mouth is between somite 1 and 2 (chelicerae and pedipalps).
Theopisthosoma consists of thirteen or fewer segments, may or may not end with atelson.[10] In some taxa such asscorpion andeurypterid the opisthosoma is divided into two groups,mesosoma andmetasoma.[10] The abdominal appendages of modern chelicerates are missing or heavily modified[12] – for example inspiders the remaining appendages formspinnerets that extrudesilk,[16] while those ofhorseshoe crabs (Xiphosura) formgills.[17][10]
Like all arthropods, chelicerates' bodies and appendages are covered with a toughcuticle made mainly of chitin and chemically hardened proteins. Since this cannot stretch, the animals mustmolt to grow. In other words, they grow new but still soft cuticles, then cast off the old one and wait for the new one to harden. Until the new cuticle hardens the animals are defenseless and almost immobilized.[18]
Chelicerae and pedipalps are the two pairs of appendages closest to the mouth; they vary widely in form and function and the consistent difference between them is their position in the embryo and corresponding neurons:chelicerae are deutocerebral and arise from somite 1, ahead of the mouth, whilepedipalps are tritocerebral and arise from somite 2, behind the mouth.[12][10][11]
The chelicerae ("claw horns") that give the sub-phylum its name normally consist of three sections, and the claw is formed by the third section and a rigid extension of the second.[12][19] However, spiders' have only two sections, and the second forms a fang that folds away behind the first when not in use.[16] The relative sizes of chelicerae vary widely: those of some fossileurypterids and modernharvestmen form large claws that extended ahead of the body,[19] whilescorpions' are tiny pincers that are used in feeding and project only slightly in front of the head.[20]
In basal chelicerates, the pedipalps are unspecialized and subequal to the posterior pairs of walking legs.[10] However, in sea spider and arachnids, the pedipalps are more or less specialized for sensory[12] or prey-catching function[10] – for example scorpions have pincers[20] and male spiders have bulbous tips that act assyringes to injectsperm into the females' reproductive openings when mating.[16]
As in all arthropods, the chelicerate body has a very smallcoelom restricted to small areas round the reproductive and excretory systems. The main body cavity is ahemocoel that runs most of the length of the body and through which blood flows, driven by a tubular heart that collects blood from the rear and pumps it forward. Althougharteries direct the blood to specific parts of the body, they have open ends rather than joining directly toveins, and chelicerates therefore have opencirculatory systems as is typical for arthropods.[22]
These depend on individual sub-groups' environments. Modern terrestrial chelicerates generally have bothbook lungs, which deliver oxygen and remove waste gases via the blood, andtracheae, which do the same without using the blood as a transport system.[23] The livinghorseshoe crabs are aquatic and havebook gills that lie in a horizontal plane. For a long time it was assumed that the extincteurypterids had gills, but the fossil evidence was ambiguous. However, a fossil of the 45 millimetres (1.8 in) long eurypteridOnychopterella, from the LateOrdovician period, has what appear to be four pairs of vertically oriented book gills whose internal structure is very similar to that of scorpions' book lungs.[24]
The guts of most modern chelicerates are too narrow to take solid food.[23] Allscorpions and almost allspiders arepredators that "pre-process" food in preoral cavities formed by thechelicerae and the bases of thepedipalps.[16][20] However, one predominantlyherbivore spider species is known,[25] and many supplement their diets withnectar andpollen.[26] Many of theAcari (ticks and mites) are blood-suckingparasites, but there are many predatory, herbivore andscavenger sub-groups. All the Acari have a retractable feeding assembly that consists of the chelicerae, pedipalps and parts of theexoskeleton, and which forms a preoral cavity for pre-processing food.[13]
Harvestmen are among the minority of living chelicerates that can take solid food, and the group includes predators, herbivores and scavengers.[27]Horseshoe crabs are also capable of processing solid food, and use a distinctive feeding system. Claws at the tips of their legs grab small invertebrates and pass them to a food groove that runs from between the rearmost legs to the mouth, which is on the underside of the head and faces slightly backwards. The bases of the legs form toothedgnathobases that both grind the food and push it towards the mouth.[17] This is how the earliestarthropods are thought to have fed.[28]
Horseshoe crabs convertnitrogenous wastes toammonia and dump it via their gills, and excrete other wastes asfeces via theanus. They also havenephridia ("little kidneys"), which extract other wastes for excretion asurine.[17] Ammonia is so toxic that it must be diluted rapidly with large quantities of water.[29] Most terrestrial chelicerates cannot afford to use so much water and therefore convert nitrogenous wastes to other chemicals, which they excrete as dry matter. Extraction is by various combinations of nephridia andMalpighian tubules. The tubules filter wastes out of the blood and dump them into the hindgut as solids, a system that has evolvedindependently ininsects and several groups ofarachnids.[23]
Chelicerate nervous systems are based on the standard arthropod model of a pair ofnerve cords, each with aganglion per segment, and abrain formed by fusion of the ganglia just behind the mouth with those ahead of it.[30] If one assume that chelicerates lose the first segment, which bearsantennae in other arthropods, chelicerate brains include only one pair of pre-oral ganglia instead of two.[12] However, there is evidence that the first segment is indeed available and bears the cheliceres.[31][15]
There is a notable but variable trend towards fusion of other ganglia into the brain. The brains ofhorseshoe crabs include all the ganglia of theprosoma plus those of the first two opisthosomal segments, while the other opisthosomal segments retain separate pairs of ganglia.[17] In most livingarachnids, exceptscorpions if they are true arachnids,all the ganglia, including those that would normally be in the opisthosoma, are fused into a single mass in the prosoma and there are no ganglia in the opisthosoma.[23] However, in theMesothelae, which are regarded as the most basal living spiders, the ganglia of the opisthosoma and the rear part of the prosoma remain unfused,[32] and in scorpions the ganglia of the cephalothorax are fused but the abdomen retains separate pairs of ganglia.[23]
As with other arthropods, chelicerates'cuticles 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, spiders and other arthropods have modified their cuticles into elaborate arrays of sensors. Various touch and vibration sensors, mostly bristles calledsetae, 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.[33]
Living chelicerates have bothcompound eyes (only inhorseshoe crabs, as the compound eye in the other clades has been reduced to a cluster of no more than five pairs ofocelli), mounted on the sides of the head, plus pigment-cup ocelli ("little eyes"), mounted in the middle. These median ocelli-type eyes in chelicerates are assumed to behomologous with the crustacean nauplius eyes and the insect ocelli.[34] The eyes of horseshoe crabs can detect movement but not form images.[17] At the other extreme,jumping spiders have a very wide field of vision,[16] and their main eyes are ten times as acute as those ofdragonflies,[35] able to see in both colors and UV-light.[36]
Horseshoe crabs useexternal fertilization; thesperm andova meet outside the parents' bodies. Despite being aquatic, they spawn on land in theintertidal zone on the beach.[37] The female digs a depression in the wet sand, where she will release her eggs. The male, usually more than one, then releases his sperm onto them.[38] Theirtrilobite-likelarvae have full sets of appendages and eyes. Initially the horseshoe crab larvae begin with two pairs of book-gills, later gaining three more pairs of book-gills as theymolt.[17]
Sea spiders also reproduce via external fertilization. The male and female sea spiders release their sperm and eggs into the water where fertilization occurs. The male then collects the eggs and carries them around under his body.[39]
Except forOpiliones and some mites, where the male has a penis used for direct fertilization,[40] fertilization in arachnids is indirect. Indirect fertilization happens in two ways: the male deposit hisspermatophore (package of sperm) on the ground, which is then picked up by the female, or the male stores his sperm in appendages modified into sperm transfer organs, such as thepedipalps in male spiders, which are inserted into the female genital openings during copulation.[16]Courtship rituals are common, especially in species where the male risks being eaten before mating.[citation needed] Most arachnids lay eggs, but all scorpions and somemites areviviparous, giving birth to live young (even more mites are ovoviviparous, but most are oviparous).[41][42][43][44] Female pseudoscorpions carry their eggs in a brood pouch on the belly, where the growing embryos feeds on a nutritive fluid provided by the mother during development, and are thereforematrotrophic.[45]
Levels of parental care for the young range from zero to prolonged. Scorpions carry their young on their backs until the firstmolt, and in a few semi-social species the young remain with their mother.[46] Some spiders care for their young, for example awolf spider's brood cling to rough bristles on the mother's back,[16] and females of some species respond to the "begging" behavior of their young by giving them their prey, provided it is no longer struggling, or evenregurgitate food.[47]
There are large gaps in the chelicerates'fossil record because, like allarthropods, theirexoskeletons areorganic and hence their fossils are rare except in a fewlagerstätten where conditions were exceptionally suited to preserving fairly soft tissues. TheBurgess Shale animals likeSidneyia from about505 million years ago have been classified as chelicerates, the latter because its appendages resemble those of theXiphosura (horseshoe crabs). However,cladistic analyses that consider wider ranges of characteristics place neither as chelicerates. There is debate about whetherFuxianhuia from earlier in theCambrian period, about525 million years ago, was a chelicerate. Another Cambrian fossil,Kodymirus, was originally classified as anaglaspid but may have been aeurypterid and therefore a chelicerate. If any of these was closely related to chelicerates, there is a gap of at least 43 million years in the record between true chelicerates and their nearest not-quite chelicerate relatives.[48]
Sanctacaris, member of the familySanctacarididae from the Burgess Shale ofCanada, represents the oldest occurrence of a confirmed chelicerate, Middle Cambrian in age.[49] Although its chelicerate nature has been doubted for its pattern oftagmosis (how the segments are grouped, especially in the head),[48] a restudy in 2014 confirmed its phylogenetic position as the oldest chelicerate.[49] Another fossil of the site,Mollisonia, is considered a basal chelicerate and it has the oldest known chelicerae and proto-book gills.[50]
The eurypterids have left few good fossils and one of the earliest confirmed eurypterid,Pentecopterus decorahensis, appears in the MiddleOrdovician period467.3 million years ago, making it the oldest eurypterid.[51]Until recently the earliest knownxiphosuran fossil dated from the Late Llandovery stage of theSilurian436 to 428 million years ago,[52] but in 2008 an older specimen described asLunataspis aurora was reported from about445 million years ago in the LateOrdovician.[53]
Attercopus fimbriunguis, from386 million years ago in theDevonian period, bears the earliest known silk-producing spigots, and was therefore hailed as a spider,[55] but it lackedspinnerets and hence was not a true spider.[56] Rather, it was likely sister group to the spiders, a clade which has been named Serikodiastida.[57] Close relatives of the group survived through to theCretaceous Period.[58] SeveralCarboniferous spiders were members of theMesothelae, a basal group now represented only by theLiphistiidae,[55] and fossils suggest taxa closely related to the spiders, but which were not true members of the group were also present during this Period.[59]
The LateSilurianProscorpius has been classified as a scorpion, but differed significantly from modern scorpions: it appears wholly aquatic since it hadgills rather thanbook lungs ortracheae; its mouth was completely under its head and almost between the first pair of legs, as in the extincteurypterids and livinghorseshoe crabs.[60] Fossils of terrestrial scorpions withbook lungs have been found in EarlyDevonian rocks from about402 million years ago.[61] The oldest species of scorpion found as of 2021 isDolichophonus loudonensis, which lived during the Silurian, in present-day Scotland.[62]
The "traditional" view of arthropod phylogeny shows chelicerates as less closely related to the other major living groups (crustaceans;hexapods, which includesinsects; andmyriapods, which includescentipedes andmillipedes) than these other groups are to each other. Recent research since 2001, using bothmolecular phylogenetics (the application of cladistic analysis tobiochemistry, especially to organisms'DNA andRNA) and detailed examination of how various extant arthropods'nervous systems develop in theembryos, suggests that chelicerates are most closely related to myriapods, while hexapods and crustaceans are each other's closest relatives. However, analysis including extinct arthropods such astrilobites results in a swing back to the "traditional" view, wherein trilobites are placed as the sister-group of theTracheata (hexapods plus myriapods) and chelicerates as least closely related to the other groups.[63]
Cladogram after O'Flynn et al, 2023, showing possible relationships of Chelicerata to living and extinct arthropod groups:[64]
However, the structure of "family tree" relationships within the Chelicerata has been controversial ever since the late 19th century. An attempt in 2002 to combine analysis ofDNA features of modern chelicerates and anatomical features of modern and fossil ones produced credible results for many lower-level groups, but its results for the high-level relationships between major sub-groups of chelicerates were unstable, in other words minor changes in the inputs caused significant changes in the outputs of the computer program used (POY).[68] An analysis in 2007 using only anatomical features produced thecladogram on the right, but also noted that many uncertainties remain.[69] In recent analyses the cladeTetrapulmonata is reliably recovered, but other ordinal relationships remain in flux.[58][70][59][71][72][73][74]
The position of scorpions is particularly controversial. Some early fossils such as the LateSilurianProscorpius have been classified by paleontologists as scorpions, but described as wholly aquatic as they hadgills rather thanbook lungs ortracheae. Their mouths are also completely under their heads and almost between the first pair of legs, as in the extincteurypterids and livinghorseshoe crabs.[60] This presents a difficult choice: classifyProscorpius and other aquatic fossils as something other than scorpions, despite the similarities; accept that "scorpions" are not monophyletic but consist of separate aquatic and terrestrial groups;[60] or treat scorpions as more closely related to eurypterids and possibly horseshoe crabs than to spiders and otherarachnids,[24] so that either scorpions are not arachnids or "arachnids" are not monophyletic.[60]Cladistic analyses have recoveredProscorpius within the scorpions,[57] based on reinterpretation of the species' breathing apparatus.[75] This is reflected also in the reinterpretation ofPalaeoscorpius as a terrestrial animal.[76]
A 2013 phylogenetic analysis[77] (the results presented in a cladogram below) on the relationships within the Xiphosura and the relations to other closely related groups (including the eurypterids, which were represented in the analysis by generaEurypterus,Parastylonurus,Rhenopterus andStoermeropterus) concluded that the Xiphosura, as presently understood, wasparaphyletic (a group sharing alast common ancestor but not including all descendants of this ancestor) and thus not a valid phylogenetic group. Eurypterids were recovered as closely related to arachnids instead of xiphosurans, forming the groupSclerophorata within the cladeDekatriata (composed of sclerophorates andchasmataspidids). This work suggested it is possible that Dekatriata is synonymous with Sclerophorata as the reproductive system, the primary defining feature of sclerophorates, has not been thoroughly studied in chasmataspidids. Dekatriata is in turn part of theProsomapoda, a group including theXiphosurida (the only monophyletic xiphosuran group) and other stem-genera. A recent phylogenetic analysis of the chelicerates places the Xiphosura within the Arachnida as the sister group of Ricinulei,[74][78] but others still retrieve a monophyletic Arachnida.[79]
Although well behind the insects, chelicerates are one of the most diverse groups of animals, with over 77,000 living species that have been described in scientific publications.[80] Some estimates suggest that there may be 130,000 undescribed species of spider and nearly 500,000 undescribed species of mites and ticks.[81] While the earliest chelicerates and the livingPycnogonida (if they are chelicerates[67]) andXiphosura are marine animals that breathe dissolvedoxygen, the vast majority of living species are air-breathers,[80] although a few spider species build "diving bell" webs that enable them to live under water.[82] Like their ancestors, most living chelicerates are carnivores, mainly on smallinvertebrates. However, many species feed asparasites,herbivores,scavengers anddetritivores.[13][27][80]
In the past,Native Americans ate the flesh ofhorseshoe crabs, and used the tail spines as spear tips and the shells to bail water out of their canoes. More recent attempts to use horseshoe crabs as food forlivestock were abandoned when it was found that this gave the meat a bad taste. Horseshoe crab blood contains a clotting agent,limulus amebocyte lysate, which is used to test antibiotics and kidney machines to ensure that they are free of dangerousbacteria, and to detectspinal meningitis and somecancers.[91]
Because spider silk is both light and very strong, but large-scale harvesting from spiders is impractical, work is being done to produce it in other organisms by means ofgenetic engineering.[99] Spider silk proteins have been successfully produced intransgenic goats' milk,[100]tobacco leaves,[101]silkworms,[102][103][104]and bacteria,[99][105][106] andrecombinant spider silk is now available as a commercial product from some biotechnology companies.[104]
In the 20th century, there were about 100 reliably reported deaths from spider bites,[107] compared with 1,500 fromjellyfish stings.[108] Scorpion stings are thought to be a significant danger in less-developed countries; for example, they cause about 1,000 deaths per year inMexico, but only one every few years in the USA. Most of these incidents are caused by accidental human "invasions" of scorpions' nests.[109] On the other hand, medical uses of scorpion venom are being investigated for treatment of brain cancers and bone diseases.[110][111]
Ticks are parasitic, and some transmit micro-organisms andparasites that can cause diseases in humans, while the saliva of a few species can directly causetick paralysis if they are not removed within a day or two.[112]
A few of the closely related mites also infest humans, some causing intense itching by their bites, and others by burrowing into the skin. Species that normally infest other animals such asrodents may infest humans if their normal hosts are eliminated.[113] Three species of mite are a threat tohoney bees and one of these,Varroa destructor, has become the largest single problem faced bybeekeepers worldwide.[114] Mites cause several forms of allergic diseases, includinghay fever,asthma andeczema, and they aggravateatopic dermatitis.[115] Mites are also significant crop pests, althoughpredatory mites may be useful in controlling some of these.[80][116]
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