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Chelicerata
Chelicerata
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Chelicerata
Temporal range: Middle Cambrian – Present, 508–0 Mya Possible Fortunian record
PycnogonidaXiphosuraEurypteridAraneaeScorpionAcari
Left to right, top to bottom: Ammothea hilgendorfi (Pycnogonida), Limulus polyphemus (Xiphosura), Eurypterus remipes (Eurypterida), Araneus diadematus (Araneae), Buthus occitanus (Scorpiones), Trombidium holosericeum (Acari)
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Arthropoda
Clade: Arachnomorpha
Subphylum: Chelicerata
Heymons, 1901
Groups
Synonyms
  • Cheliceriformes Schram and Hedgpeth, 1978

The subphylum Chelicerata (from Neo-Latin, from French chélicère, from Ancient Greek χηλή (khēlḗ) 'claw, chela' and κέρας (kéras) 'horn')[1] constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, and mites, among many others), as well as a number of extinct lineages, such as the eurypterids (sea scorpions) and chasmataspidids.

Chelicerata split from Mandibulata by the mid-Cambrian, as evidenced by stem-group chelicerates like Habeliida and Mollisonia present by this time.[2] The surviving marine species include the four species of xiphosurans (horseshoe crabs), and possibly the 1,300 species of pycnogonids (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 all arthropods, chelicerates have segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins. The chelicerate body plan consists of two tagmata, the prosoma and the opisthosoma – excepting the mites, which have lost any visible division between these sections. The chelicerae, which give the group its name, are the only appendages 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 inject venom into prey. The group has the open circulatory system typical of arthropods, in which a tube-like heart pumps blood through the hemocoel, which is the major body cavity. Marine chelicerates have gills, while the air-breathing forms generally have both book lungs and tracheae. In general, the ganglia 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 the Mesothelae, which are regarded as the oldest and most basal group of spiders. Most chelicerates rely on modified bristles 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 and eating decaying organic matter. Although harvestmen 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 and pedipalps and flooding it with digestive enzymes. To conserve water, air-breathing chelicerates excrete waste as solids that are removed from their blood by Malpighian tubules, structures that also evolved independently in insects.[3]

While the marine horseshoe crabs rely on external fertilization, air-breathing chelicerates use internal but usually indirect fertilization. Many species use elaborate courtship 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.

The evolutionary 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]

Venom has evolved three times in the chelicerates; spiders, scorpions and pseudoscorpions, or four times if the hematophagous secretions produced by ticks are included. In addition there have been unverified descriptions of venom glands in Solifugae.[5] Chemical defense has been found in whip scorpions, shorttailed whipscorpions, harvestmen, beetle mites and sea spiders.[6][7][8]

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 from cancer to erectile dysfunction. The medical industry also uses the blood of horseshoe crabs as a test for the presence of contaminant bacteria. Mites can cause allergies in humans, transmit several diseases to humans and their livestock, and are serious agricultural pests.

Description

[edit]
Four types of arthropods showing the acron and 9 head and/or body segments. Trilobites and chelicerates are shown with 7 head segments, and crustaceans and tracheates with 5 head segments. Of these, the first head segment of chelicerates and the second head segment of trachates is lost in development. All four start with an acron at the anterior end bearing compound eyes. All have nephridia on some or all head segments, some of which are lost in development in chelicerates. All—other than chelicerates—have antennae on the first head segment, and crustaceans also have antennae on the second head segment. Only chelicerans have chelicera, on the second head segment and first body segment, and pedipalps, on the third body segment. Crustaceans have mandibles on the third head segment and maxillae on each of the fourth and fifth head segments. Trilobites and chelicerates bear legs on all remaining head segments, but crustaceans and tracheates have legs on the anterior body segments.
A
L
L
L
L
L
L
x
C
P
L
L
L
L
Ci
A
A
Mnd
Mx
Mx
L
L
L
L
L
A
x
Mnd
Mx
Mx
L
L
L
L
    = acron
    = segments contributing to the head
    = body segments
x = lost during development
    = eyes
    = nephridia
O = nephridia lost during development
L = Leg
Mnd = Mandible
Mx = Maxilla
Four types of arthropods showing the acron and 9 head and/or body segments. Trilobites and chelicerates are shown with 7 head segments, and crustaceans and tracheates with 5 head segments. Of these, the first head segment of chelicerates and the second head segment of trachates is lost in development. All four start with an acron at the anterior end bearing compound eyes. All have nephridia on some or all head segments, some of which are lost in development in chelicerates. All—other than chelicerates—have antennae on the first head segment, and crustaceans also have antennae on the second head segment. Only chelicerans have chelicera, on the second head segment and first body segment, and pedipalps, on the third body segment. Crustaceans have mandibles on the third head segment and maxillae on each of the fourth and fifth head segments. Trilobites and chelicerates bear legs on all remaining head segments, but crustaceans and tracheates have legs on the anterior body segments.
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.

Segmentation and cuticle

[edit]

The Chelicerata are arthropods as they have: segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins; heads that are composed of several segments that fuse during the development of the embryo; a much reduced coelom; a hemocoel through which the blood circulates, driven by a tube-like heart.[9] Chelicerates' bodies consist of two tagmata, sets of segments that serve similar functions: the foremost one, called the prosoma or cephalothorax, and the rear tagma is called the opisthosoma or abdomen.[12] However, in the Acari (mites and ticks) there is no visible division between these sections.[13]

The prosoma is formed in the embryo by fusion of the ocular somite (referred as "acron" in previous literatures), which carries the eyes and labrum,[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 of chelicerae or chelifores,[15] small appendages that often form pincers. Somite 2 has a pair of pedipalps that in most sub-groups perform sensory functions, while the remaining four cephalothorax segments (somite 4 to 6) have pairs of legs.[10] In basal forms the ocular somite has a pair of compound eyes on the sides and four pigment-cup ocelli ("little eyes") in the middle.[12] The mouth is between somite 1 and 2 (chelicerae and pedipalps).

The opisthosoma consists of thirteen or fewer segments, may or may not end with a telson.[10] In some taxa such as scorpion and eurypterid the opisthosoma is divided into two groups, mesosoma and metasoma.[10] The abdominal appendages of modern chelicerates are missing or heavily modified[12] – for example in spiders the remaining appendages form spinnerets that extrude silk,[16] while those of horseshoe crabs (Xiphosura) form gills.[17][10]

Like all arthropods, chelicerates' bodies and appendages are covered with a tough cuticle made mainly of chitin and chemically hardened proteins. Since this cannot stretch, the animals must molt 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

[edit]

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, while pedipalps 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 fossil eurypterids and modern harvestmen form large claws that extended ahead of the body,[19] while scorpions' 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 as syringes to inject sperm into the females' reproductive openings when mating.[16]

    Nervous system
    Digestive & excretory
system
    Circulatory system
    Respiratory system
    Reproductive system
  1 Chelicera
  2 Venom gland
  3 Brain
  4 Pumping stomach
  5 Forward aorta branch
  6 Digestive cecum
  7 Heart
  8 Midgut
10 Stercoral pocket
11 Rear aorta
15 Ovary (female)
18 Legs
Spider's main organs[21]

Body cavities and circulatory systems

[edit]

As in all arthropods, the chelicerate body has a very small coelom restricted to small areas round the reproductive and excretory systems. The main body cavity is a hemocoel 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. Although arteries direct the blood to specific parts of the body, they have open ends rather than joining directly to veins, and chelicerates therefore have open circulatory systems as is typical for arthropods.[22]

Respiratory systems

[edit]

These depend on individual sub-groups' environments. Modern terrestrial chelicerates generally have both book lungs, which deliver oxygen and remove waste gases via the blood, and tracheae, which do the same without using the blood as a transport system.[23] The living horseshoe crabs are aquatic and have book gills that lie in a horizontal plane. For a long time it was assumed that the extinct eurypterids had gills, but the fossil evidence was ambiguous. However, a fossil of the 45 millimetres (1.8 in) long eurypterid Onychopterella, from the Late Ordovician 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]

Feeding and digestion

[edit]

The guts of most modern chelicerates are too narrow to take solid food.[23] All scorpions and almost all spiders are predators that "pre-process" food in preoral cavities formed by the chelicerae and the bases of the pedipalps.[16][20] However, one predominantly herbivore spider species is known,[25] and many supplement their diets with nectar and pollen.[26] Many of the Acari (ticks and mites) are blood-sucking parasites, but there are many predatory, herbivore and scavenger sub-groups. All the Acari have a retractable feeding assembly that consists of the chelicerae, pedipalps and parts of the exoskeleton, 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 toothed gnathobases that both grind the food and push it towards the mouth.[17] This is how the earliest arthropods are thought to have fed.[28]

Excretion

[edit]

Horseshoe crabs convert nitrogenous wastes to ammonia and dump it via their gills, and excrete other wastes as feces via the anus. They also have nephridia ("little kidneys"), which extract other wastes for excretion as urine.[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 and Malpighian tubules. The tubules filter wastes out of the blood and dump them into the hindgut as solids, a system that has evolved independently in insects and several groups of arachnids.[23]

Nervous system

[edit]
  Cephalothorax ganglia fused into brain Abdominal ganglia fused into brain
Horseshoe crabs All First two segments only
Scorpions All None
Mesothelae First two pairs only None
Other arachnids All All

Chelicerate nervous systems are based on the standard arthropod model of a pair of nerve cords, each with a ganglion per segment, and a brain 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 bears antennae 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 of horseshoe crabs include all the ganglia of the prosoma plus those of the first two opisthosomal segments, while the other opisthosomal segments retain separate pairs of ganglia.[17] In most living arachnids, except scorpions 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 the Mesothelae, 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]

Senses

[edit]

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 called setae, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of taste and smell, often by means of setae.[33]

Living chelicerates have both compound eyes (only in horseshoe crabs, as the compound eye in the other clades has been reduced to a cluster of no more than five pairs of ocelli), 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 be homologous 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 of dragonflies,[35] able to see in both colors and UV-light.[36]

Reproduction

[edit]
Female scorpion Vaejovis cashi carrying its young (white)

Horseshoe crabs use external fertilization; the sperm and ova meet outside the parents' bodies. Despite being aquatic, they spawn on land in the intertidal 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] Their trilobite-like larvae 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 they molt.[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 for Opiliones 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 his spermatophore (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 the pedipalps 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 some mites are viviparous, 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 therefore matrotrophic.[45]

Levels of parental care for the young range from zero to prolonged. Scorpions carry their young on their backs until the first molt, and in a few semi-social species the young remain with their mother.[46] Some spiders care for their young, for example a wolf 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 even regurgitate food.[47]

Evolutionary history

[edit]

Fossil record

[edit]

There are large gaps in the chelicerates' fossil record because, like all arthropods, their exoskeletons are organic and hence their fossils are rare except in a few lagerstätten where conditions were exceptionally suited to preserving fairly soft tissues. The Burgess Shale animals like Sidneyia from about 505 million years ago have been classified as chelicerates, the latter because its appendages resemble those of the Xiphosura (horseshoe crabs). However, cladistic analyses that consider wider ranges of characteristics place neither as chelicerates. There is debate about whether Fuxianhuia from earlier in the Cambrian period, about 525 million years ago, was a chelicerate. Another Cambrian fossil, Kodymirus, was originally classified as an aglaspid but may have been a eurypterid 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]

Reconstruction of Mollisonia plenovenatrix, the oldest known arthropod with confirmed chelicerae

Sanctacaris, member of the family Sanctacarididae from the Burgess Shale of Canada, represents the oldest occurrence of a confirmed chelicerate, Middle Cambrian in age.[49] Although its chelicerate nature has been doubted for its pattern of tagmosis (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]

Holotype of the xiphosuran Lunataspis aurora

The eurypterids have left few good fossils and one of the earliest confirmed eurypterid, Pentecopterus decorahensis, appears in the Middle Ordovician period 467.3 million years ago, making it the oldest eurypterid.[51] Until recently the earliest known xiphosuran fossil dated from the Late Llandovery stage of the Silurian 436 to 428 million years ago,[52] but in 2008 an older specimen described as Lunataspis aurora was reported from about 445 million years ago in the Late Ordovician.[53]

The oldest known arachnid is the trigonotarbid Palaeotarbus jerami, from about 420 million years ago in the Silurian period, and had a triangular cephalothorax and segmented abdomen, as well as eight legs and a pair of pedipalps.[54]

Attercopus fimbriunguis, from 386 million years ago in the Devonian period, bears the earliest known silk-producing spigots, and was therefore hailed as a spider,[55] but it lacked spinnerets 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 the Cretaceous Period.[58] Several Carboniferous spiders were members of the Mesothelae, a basal group now represented only by the Liphistiidae,[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 Late Silurian Proscorpius has been classified as a scorpion, but differed significantly from modern scorpions: it appears wholly aquatic since it had gills rather than book lungs or tracheae; its mouth was completely under its head and almost between the first pair of legs, as in the extinct eurypterids and living horseshoe crabs.[60] Fossils of terrestrial scorpions with book lungs have been found in Early Devonian rocks from about 402 million years ago.[61] The oldest species of scorpion found as of 2021 is Dolichophonus loudonensis, which lived during the Silurian, in present-day Scotland.[62]

Relationships with other arthropods

[edit]

The "traditional" view of arthropod phylogeny shows chelicerates as less closely related to the other major living groups (crustaceans; hexapods, which includes insects; and myriapods, which includes centipedes and millipedes) than these other groups are to each other. Recent research since 2001, using both molecular phylogenetics (the application of cladistic analysis to biochemistry, especially to organisms' DNA and RNA) and detailed examination of how various extant arthropods' nervous systems develop in the embryos, 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 as trilobites results in a swing back to the "traditional" view, wherein trilobites are placed as the sister-group of the Tracheata (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]

Total group Arthropoda

"Gilled lobopodians" (Pambdelurion, Kerygmachela)

Opabinia

Radiodonta (e.g Anomalocaris)

Deuteropoda
Total group Chelicerata

Megacheira

Habeliida

Mollisoniida

Chelicerata crown group

"Great appendage bivalved forms" (Occacaris, Forfexicaris)

Isoxyida

Artiopoda (including Trilobita)

Mandibulata

Fuxianhuiida

Myriapoda (millipedes, centipedes, etc)

Hymenocarina

Pancrustacea (crustaceans, insects, etc)


Major sub-groups

[edit]
Chelicerata

Xiphosura (horseshoe crabs)

Arachnida

Scorpiones

Opiliones (harvestmen)

Pseudoscorpiones

Solifugae (sun spiders)

Palpigradi (microwhip scorpions)

Trigonotarbida†

Araneae (spiders)

Haptopoda†

Amblypygi (whip spiders)

Uropygi (whip scorpions)

Schizomida

Ricinulei (hooded tickspiders)

Anactinotrichida

Acariformes (mites)

Shultz (2007)'s evolutionary family tree of arachnids[65] – † marks extinct groups.

It is generally agreed that the Chelicerata contain the classes Arachnida (spiders, scorpions, mites, etc.), Xiphosura (horseshoe crabs) and Eurypterida (sea scorpions, extinct).[65] The extinct Chasmataspidida may be a sub-group within Eurypterida.[65][66] The Pycnogonida (sea spiders) were traditionally classified as chelicerates, but some features suggest they may be representatives of the earliest arthropods from which the well-known groups such as chelicerates evolved.[67]

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 of DNA 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 the cladogram on the right, but also noted that many uncertainties remain.[69] In recent analyses the clade Tetrapulmonata 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 Late Silurian Proscorpius have been classified by paleontologists as scorpions, but described as wholly aquatic as they had gills rather than book lungs or tracheae. Their mouths are also completely under their heads and almost between the first pair of legs, as in the extinct eurypterids and living horseshoe crabs.[60] This presents a difficult choice: classify Proscorpius 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 other arachnids,[24] so that either scorpions are not arachnids or "arachnids" are not monophyletic.[60] Cladistic analyses have recovered Proscorpius within the scorpions,[57] based on reinterpretation of the species' breathing apparatus.[75] This is reflected also in the reinterpretation of Palaeoscorpius 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 genera Eurypterus, Parastylonurus, Rhenopterus and Stoermeropterus) concluded that the Xiphosura, as presently understood, was paraphyletic (a group sharing a last 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 group Sclerophorata within the clade Dekatriata (composed of sclerophorates and chasmataspidids). 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 the Prosomapoda, a group including the Xiphosurida (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]

Arachnomorpha

Diversity

[edit]

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 living Pycnogonida (if they are chelicerates[67]) and Xiphosura are marine animals that breathe dissolved oxygen, 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 small invertebrates. However, many species feed as parasites, herbivores, scavengers and detritivores.[13][27][80]

Diversity of living chelicerates
Group Described species[80][83][84] Diet
Pycnogonida (sea-spiders) 500 Carnivorous[80]
Araneae (spiders) 50,300 Carnivorous;[80] 1 herbivore[25]
Acari (mites and ticks) 32,000 Carnivorous, parasitic, herbivore, detritivore[13][80]
Opiliones (harvestmen) 6,500 Carnivorous, herbivore, detritivore[27]
Pseudoscorpiones (false scorpions) 3,200 Carnivorous[85]
Scorpiones (scorpions) 1,400 Carnivorous[20]
Solifugae (sunspiders) 900 Carnivorous, omnivorous[86]
Schizomida (small whipscorpions) 180 Carnivorous[87]
Amblypygi (whipspiders) 100 Carnivorous[88]
Uropygi (whipscorpions) 90 Carnivorous[89]
Palpigradi (micro whipscorpions) 60  
Xiphosura (horseshoe crabs) 4 Carnivorous[80]
Ricinulei 60 Carnivorous[90]

Interaction with humans

[edit]
A microscopic mite Lorryia formosa.

In the past, Native Americans ate the flesh of horseshoe 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 for livestock 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 dangerous bacteria, and to detect spinal meningitis and some cancers.[91]

Cooked tarantula spiders are considered a delicacy in Cambodia,[92] and by the Piaroa Indians of southern Venezuela.[93] Spider venoms may be a less polluting alternative to conventional pesticides as they are deadly to insects but the great majority are harmless to vertebrates.[94] Possible medical uses for spider venoms are being investigated, for the treatment of cardiac arrhythmia,[95] Alzheimer's disease,[96] strokes,[97] and erectile dysfunction.[98]

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 of genetic engineering.[99] Spider silk proteins have been successfully produced in transgenic goats' milk,[100] tobacco leaves,[101] silkworms,[102][103][104] and bacteria,[99][105][106] and recombinant 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 from jellyfish 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 in Mexico, 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 and parasites that can cause diseases in humans, while the saliva of a few species can directly cause tick 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 as rodents may infest humans if their normal hosts are eliminated.[113] Three species of mite are a threat to honey bees and one of these, Varroa destructor, has become the largest single problem faced by beekeepers worldwide.[114] Mites cause several forms of allergic diseases, including hay fever, asthma and eczema, and they aggravate atopic dermatitis.[115] Mites are also significant crop pests, although predatory mites may be useful in controlling some of these.[80][116]

References

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Bibliography

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Chelicerata

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Chelicerata is a of the phylum Arthropoda, comprising an ancient and diverse lineage of joint-legged distinguished by the absence of antennae and the presence of specialized fang-like mouthparts called . These arthropods exhibit a divided into two main tagmata: the prosoma and the opisthosoma, with the prosoma typically bearing six pairs of appendages including , pedipalps (or homologues), and walking legs in most groups, though pycnogonids show variations. All appendages are uniramous (unbranched), and chelicerates lack mandibles, relying instead on for feeding, which can function in piercing, grasping, or manipulating prey. Recent phylogenomic studies recognize Pycnogonida () as the sister group to all other chelicerates (Euchelicerata), within which (horseshoe crabs) is nested inside a paraphyletic Arachnida (arachnids, including spiders, scorpions, mites, ticks, and other orders). Arachnids represent the most species-rich group, with approximately 53,000 species of spiders (Araneae) and over 60,000 species of mites and ticks (Acari) as of 2025, contributing to a total diversity exceeding 120,000 described living species across the . Chelicerates inhabit a wide range of environments, from marine and freshwater habitats to terrestrial ecosystems, where they play key ecological roles as predators, parasites, and decomposers. Evolutionarily, Chelicerata originated during the period around 510 million years ago, likely branching from early stem groups such as the great appendage arthropods. Fossil records reveal a rich history, with ancient forms like eurypterids (sea scorpions) dominating seas, while modern diversity has shifted toward terrestrial arachnids. As one of the most biodiverse and ecologically significant groups—second only to —chelicerates have adapted through innovations in silk production, venom, and sensory structures, influencing ecosystems and human interactions via medically important species like venomous spiders and disease-vectoring ticks.

Anatomy and Physiology

Body Plan and Segmentation

Chelicerata is a of Arthropoda distinguished by the absence of antennae, the presence of as the first pair of appendages, and a body divided into two main tagmata: the anterior prosoma () and the posterior opisthosoma (abdomen).30672-9) This division reflects a fundamental tagmosis, where originally similar segments fuse and specialize for distinct functions, with the prosoma typically handling locomotion and feeding while the opisthosoma accommodates respiration, , and . The prosoma generally comprises six segments, often fused into a single sclerite, bearing the and five additional pairs of appendages. The opisthosoma consists of 12 to 13 segments, which may be reduced, fused, or visible as distinct somites depending on the , resulting in a total of 18 to 19 body segments across chelicerates. Tagmosis in chelicerates involves evolutionary modifications where prosomal segments integrate sensory and manipulative roles, and opisthosomal segments adapt for internal organ support, often leading to a compact or annulated appearance. The chelicerate exoskeleton is a chitin-based reinforced by sclerotization through protein cross-linking and sometimes calcium salts, providing structural support and protection while allowing flexibility at joints. Growth occurs via periodic molting, or , where the old is enzymatically softened and shed, regulated by hormones that trigger apolysis and new cuticle formation. Variations in body plan segmentation are evident across chelicerate clades; for instance, in horseshoe crabs (), the opisthosoma features pronounced, flap-like book gills derived from limb-bearing segments, contributing to a more segmented appearance.30672-9) In contrast, (Pycnogonida) exhibit an elongated prosoma with extended appendages and a highly reduced opisthosoma, often comprising only a few fused segments for gonadal and digestive functions. These adaptations highlight the plasticity of chelicerate tagmosis in response to diverse habitats.

Appendages and Mouthparts

Chelicerates are characterized by a distinctive set of appendages on the prosoma, totaling six pairs: a pair of , a pair of pedipalps, and four pairs of walking legs. These appendages are jointed structures covered by a chitinous , adapted primarily for feeding, locomotion, and sensory perception, with variations across the major clades such as Arachnida, , and Pycnogonida. The and pedipalps, in particular, represent key innovations that distinguish chelicerates from other arthropods, having evolved from ancestral walking limbs through modifications in developmental , such as the dachshund domain, which influences appendage segmentation and identity. The , the foremost pair of appendages, are paired, preoral structures typically modified into pincer- or fang-like forms specialized for feeding. In spiders (Araneae), they function as fangs that stab prey to inject and , while in scorpions (Scorpiones), they are robust pincers used to grasp and crush food items. These appendages are innervated by the tritocerebrum, the posterior division of the arthropod , reflecting their deutocerebral origin and homology to the second antennae of mandibulates. Evolutionarily, chelicerae derive from biramous walking limbs of early arthropods, with reductions in branching and elongation of the claw-like distal segment occurring in the chelicerate stem lineage, as evidenced by fossil great-appendage arthropods from the period. The pedipalps, the second pair of prosomal appendages, exhibit greater morphological diversity and functional versatility across chelicerates. In horseshoe crabs (), they serve primarily sensory roles, aiding in manipulation of food and environmental exploration with their leg-like structure. In spiders, pedipalps are often enlarged and chelate, functioning in prey capture and, in males, as modified intromittent organs during reproduction, showing pronounced . Scorpions feature enlarged, pincer-like pedipalps that conduct from associated glands, enhancing their predatory efficiency. This variability stems from evolutionary modifications in regulation, particularly the labial gene, which influences pedipalp identity and differentiation from walking legs. The four pairs of walking legs on the prosoma are adapted for locomotion, typically comprising a coxa, , , , , metatarsus, and tarsus, though segment counts vary slightly among groups. In arachnids, the tarsi often bear sensory setae for chemoreception and mechanoreception during navigation. (Pycnogonida) display elongated, paddle-like walking legs suited for marine perambulation over substrates, with some species exhibiting integration for feeding. These legs, like other prosomal appendages, attach to the segmented prosoma, which provides the underlying framework for their mobility. Opisthosomal appendages are generally reduced or internalized in most chelicerates, reflecting a trend toward abdominal simplification compared to the prosoma. In spiders, the posterior opisthosomal segments give rise to spinnerets, modified appendages that produce for web-building and other functions. Scorpions possess pectines on the second opisthosomal segment, comb-like structures with sensory capabilities. In contrast, xiphosurans like horseshoe crabs retain prominent, flap-like opisthosomal appendages structured as book gills, consisting of stacked lamellae derived from ancestral limb exopods. These structures highlight the evolutionary reduction of abdominal limbs in terrestrial lineages versus retention in aquatic forms.

Circulatory, Respiratory, and Excretory Systems

Chelicerates possess an open circulatory system in which is pumped by a dorsal ostiate heart into the hemocoel, a spacious that bathes the organs directly. This heart, located in the midline of the opisthosoma, receives through ostia and distributes it via anterior and posterior arteries, allowing nutrient and waste exchange without a closed vascular network. composition includes , a copper-containing protein that binds oxygen for transport, distinguishing chelicerates from other arthropods that may use . In some arachnids, accessory pulsatile organs at the bases of legs and pedipalps enhance local circulation, preventing stasis in appendages during activity. Respiratory adaptations in Chelicerata reflect transitions from aquatic to terrestrial habitats, with gas exchange occurring across specialized surfaces interfaced with the hemolymph. Aquatic chelicerates, such as horseshoe crabs (Xiphosura), employ book gills—stacked, flap-like lamellae in the opisthosoma that facilitate oxygen diffusion in water, supported by hemocyanin-mediated transport. Terrestrial arachnids primarily use book lungs, invaginated, air-filled sacs with thin lamellae that evolved from ancestral book gills, enabling efficient aerial respiration while minimizing water loss; these structures open via atrial slits on the ventral opisthosoma. Some arachnids, including spiders and solifuges, supplement book lungs with tracheae—branched, air-conducting tubes that penetrate tissues for direct oxygen delivery, an adaptation derived from gill-like precursors in the chelicerate lineage. Excretory processes in chelicerates manage nitrogenous waste and osmoregulation through segmentally arranged glands integrated with the hemolymph. In terrestrial arachnids, Malpighian tubules extend from the hindgut into the hemocoel, where they filter hemolymph to produce guanine as the primary nitrogenous waste—a sparingly soluble purine that conserves water by forming crystals rather than urea or ammonia. Aquatic chelicerates utilize coxal glands, paired structures at the leg bases in the prosoma, for waste elimination and ion regulation; these glands are homologous to the antennal (green) glands of crustaceans, reflecting shared arthropod ancestry. Coxal glands actively transport ions and water, aiding osmoregulation in variable salinities, while hemolymph circulates wastes from metabolic sites to these excretory organs and respiratory surfaces for coordinated homeostasis.

Nervous System and Sensory Organs

The nervous system of chelicerates features a centralized brain divided into three main regions: the protocerebrum, which processes visual input; the deutocerebrum, associated with chemosensory and mechanosensory functions; and the tritocerebrum, which innervates the chelicerae and is often enlarged to coordinate their manipulative actions. This tripartite structure connects posteriorly to a ventral nerve cord that runs through the body, with segmental ganglia fused in the prosoma to form a compact central mass, while the opisthosoma contains more dispersed nerve clusters for basic motor control. In arachnids, such as spiders and scorpions, this organization supports rapid sensory-motor integration, with the tritocerebrum particularly prominent for precise cheliceral movements during feeding. Sensory organs in chelicerates are adapted for diverse environmental cues, Most chelicerates, particularly arachnids and sea spiders (Pycnogonida), lack compound eyes and instead feature simple ocelli—typically 2 to 8 in number—located on the prosoma for basic light detection and orientation. In contrast, horseshoe crabs (Xiphosura) possess both compound eyes and simple ocelli. Chemoreceptors distributed on pedipalps and walking legs detect chemical signals from prey or mates, while arachnids possess specialized trichobothria, hair-like vibration sensors that enable detection of air currents and prey movements at a distance. Scorpions uniquely bear pectines, comb-like structures on the ventral prosoma that function as chemotactile organs, sweeping the substrate to sense pheromones and substrate vibrations during navigation and hunting. These sensory adaptations trace back to aquatic ancestors, where early visual systems in forms like eurypterids evolved from compound structures toward the simplified ocelli seen in modern terrestrial chelicerates. Brain size and neural complexity vary across chelicerates, with web-building spiders exhibiting relatively larger brains compared to cursorial hunters, correlating with the cognitive demands of precise silk manipulation and spatial planning in web construction. Neuromodulators such as octopamine play key roles in modulating neural activity, influencing arousal, motor patterns, and sensory processing in arachnids by acting on both central and peripheral neurons. This neural architecture underpins behavioral integration, including the sensory-motor circuits for hunting—where trichobothria and chemoreceptors guide predatory strikes—and mating signals, such as vibration-based courtship in scorpions detected via pectines.

Digestive System and Feeding Mechanisms

The digestive tract of chelicerates is a tubular alimentary canal divided into three main regions: the , , and , adapted for efficient processing of diverse sources such as prey tissues, fluids, and particulate matter. The , located primarily in the prosoma, begins with the opening into a preoral chamber, followed by a muscular that functions as a sucking to draw in . This is connected to a narrow and often a thin-walled sucking stomach or , which stores ingested material under pressure generated by prosoma-based mouthparts. These structures enable rapid of liquefied or suspended foods, distinguishing chelicerate from that of mandibulate arthropods. The , extending into the opisthosoma, is the primary site of enzymatic and nutrient absorption, featuring extensive diverticula or ceca that branch throughout the body to maximize surface area. These diverticula are lined with a specialized for secretion of and uptake of nutrients via and transport proteins; predominates, with enzymes breaking down proteins, , and carbohydrates externally before absorption. glands, analogous to a in crustaceans, produce these enzymes and also handle initial breakdown in some . In certain chelicerates, such as wood-feeding mites, symbiotic microbes in the assist in cellulose degradation, enabling utilization of material. The , comprising the intestine and , primarily reabsorbs water and ions from indigestible residues, forming compact before expulsion through the . Feeding mechanisms vary widely across chelicerates to match ecological niches. Liquid feeders, including most spiders and some scorpions, inject proteolytic and lipolytic enzymes via into prey, liquefying soft tissues for suction into the ; this extra-oral minimizes solid ingestion and optimizes nutrient extraction. In contrast, crushers like scorpions and horseshoe crabs use robust, toothed to masticate small prey or , allowing particulate ingestion that is then ground further in a gizzard-like structure before processing. , such as certain parasitic or microbivorous mites, employ chelate to strain fluids or fine particles from substrates like blood or fungal hyphae, often incorporating a pharyngeal for directed flow. Adaptations include post-feeding of mouthparts in some mites to deter predators after engorgement.

Reproduction and Development

Chelicerates exhibit predominantly , with most species being dioecious, featuring distinct male and female individuals. is the norm across the , achieved through diverse mechanisms that prevent in terrestrial lineages. In arachnids such as spiders and scorpions, males typically transfer using modified pedipalps, which function as intromittent organs to deposit spermatophores or directly into the female's opisthosomal gonopore. In other groups like whip spiders () and sunspiders (), indirect transfer via stalked s deposited on the substrate is common, with females actively retrieving them. via occurs in select lineages, notably certain mites (Acari) like and some spiders such as Triaeris stenaspis, allowing unfertilized eggs to develop into females. Genital structures are located in the opisthosoma, with gonopores serving as the site for reception and egg extrusion. Male palpal bulbs in araneid spiders store and deliver , often requiring precise to avoid female aggression. In , produced by cheliceral glands facilitates cocoon formation around eggs or spermatophores during . Oviposition varies: most lay eggs externally, sometimes encased in or attached to substrates, while scorpions are ovoviviparous, retaining embryos internally until live birth. Brooding behaviors enhance offspring survival; female scorpions carry scorpling offspring on their backs for weeks post-birth, and male (Pycnogonida) use ovigers to brood eggs until hatching. Developmental patterns differ among chelicerate clades, reflecting adaptations to aquatic or terrestrial environments. Arachnids generally undergo direct development, hatching as miniature adults or juveniles without a free-living larval stage; spiderlings emerge with all eight legs and undergo iterative molts to reach maturity. In contrast, xiphosurans like horseshoe crabs exhibit indirect development, with embryos hatching as trilobite-like larvae that resemble ancient arthropods and undergo through multiple instars to the adult form. display a protonymphon larval stage, which is planktotrophic in some species before attaching and molting into juveniles. Extraembryonic tissues, such as serosa windows or sacs, support nutrient provision during embryogenesis in many chelicerates, aiding in the formation of segmented embryos. Life cycles involve sequential molting through instars, regulated by , which coordinate growth, maturation, and reproductive readiness. is typically achieved after several molts, with 5–10 instars in and up to 18 in scorpions. Sex determination is chromosomal, often involving multiple X chromosomes in (e.g., X₁X₂O system in males) or ZW heterogamety in acariform mites. Recent studies highlight ecdysteroids' role in arachnid reproduction; in the pseudoannulata, the receptor (EcR) and ultraspiracle (USP-1) mediate signaling for ovarian development and egg-laying. In spider mites, the spook influences , impacting reproductive output.

Evolutionary History

Fossil Record and Origins

The origins of Chelicerata trace back to the early period, with stem-group representatives emerging around 520 million years ago. Deposits from the Chengjiang biota in preserve great-appendage arthropods such as illecebrosa and Haikoucaris that exhibit morphological features, including raptorial frontal appendages homologous to , suggesting close affinity to the chelicerate lineage. The oldest unambiguous chelicerate fossil is Sanctacaris uncata from the mid-Cambrian of , dating to approximately 508 million years ago, which displays a divided with a prosoma bearing chelate appendages and an opisthosoma. Stem-chelicerates like Chasmataspis laurencii from the Middle of further illustrate early diversification, with biramous appendages and a xiphosuran-like tagmosis indicating transitional forms between marine basal groups and later clades. The and periods represent a peak in chelicerate diversity, particularly among eurypterids (sea scorpions), which dominated marine environments and attained giant sizes, with species like Jaekelopterus rhenaniae reaching up to 2.5 meters in length. A notable 2025 discovery is the first Silurian xiphosuran, Ciurcalimulus discobolus, from the Waldron Shale in , USA, dating to about 424 million years ago, which bridges a key gap in evolution. Arachnids emerged in the around 410–380 million years ago, with early terrestrial forms such as trigonotarbids and proto-spiders preserved in sites like the , including the oldest known impressions from approximately 410 million years ago. The and Permian saw further diversification, especially among arachnids and xiphosurans, amid expanding terrestrial habitats, though the end-Permian mass extinction around 252 million years ago eliminated many marine lineages, including the last eurypterids. Key fossil deposits highlight chelicerate evolution across periods, with Early Triassic xiphosurans like Vaderlimulus tricki from the Thaynes Group in Idaho, USA, providing insights into post-extinction recovery of horseshoe crab relatives. Amber inclusions preserve detailed arachnid anatomy, exemplified by a 140-million-year-old scorpion discovered in Jordanian amber in 2025, representing one of the earliest Cretaceous records. Exceptional preservation occurs in Lagerstätten such as the Carboniferous Mazon Creek biota in Illinois, where siderite concretions encase over 300 animal species, including spiny arachnids and eurypterid fragments, due to rapid burial in anoxic deltaic settings that favored the taphonomy of chitinous exoskeletons. The chelicerate fossil record encompasses over 2,000 described species, underscoring their evolutionary persistence from Cambrian seas to modern terrestrial ecosystems.

Phylogenetic Relationships

Chelicerata represents one of the four principal monophyletic clades within Euarthropoda, positioned as the to , which encompasses and the clade (comprising and Crustacea). This arrangement, known as the Pancrustacea hypothesis, has been robustly supported by phylogenomic analyses of transcriptomic and genomic data across arthropods, contrasting with earlier morphological proposals that allied Chelicerata more closely with . The divergence between Chelicerata and is estimated to have occurred around 550 million years ago, during the late to early transition, marking a foundational split in arthropod evolution driven by ecological expansions in marine environments. Internally, Chelicerata's phylogeny remains contentious, particularly regarding the placement of Pycnogonida () and the composition of Euchelicerata. Traditional views positioned Pycnogonida as the basalmost chelicerate lineage, with Euchelicerata uniting (horseshoe crabs) and Arachnida as sister groups; however, phylogenomic studies from the , incorporating thousands of loci from transcriptomes and genomes, increasingly support Pycnogonida as a stem-group or outgroup to a more inclusive Euchelicerata, while nesting deeply within Arachnida as a derived . Key debates center on the relationships of extinct groups, such as Eurypterida (sea scorpions), which molecular and morphological evidence links as close relatives to Arachnida within the broader Arachnomorpha , and the rejection of monophyletic Merostomata (encompassing and Eurypterida). analyses calibrated with fossil constraints further indicate a radiation for major chelicerate lineages, with rapid diversification around 520–500 million years ago coinciding with the arthropod "explosion." Phylogenetic inference for Chelicerata integrates morphological traits, such as the homology of as a defining apomorphy, with molecular datasets emphasizing clusters that pattern body segmentation and appendage diversity. Recent transcriptomic studies from 2023–2025 have refined internal Arachnida relationships, confirming the of subclades like (uniting spiders, whip scorpions, and vinegaroons) through analyses of and synteny, while resolving long-standing polytomies with high statistical support. These approaches underscore the power of integrated phylogenomics to clarify chelicerate evolution, though ongoing debates highlight the need for expanded taxon sampling from underrepresented lineages.

Classification and Major Clades

Chelicerata is classified as a within the Arthropoda, encompassing several major classes that reflect its diverse evolutionary history. The extant representatives are primarily grouped into three classes: Arachnida, , and Pycnogonida, although the precise number of classes remains debated due to ongoing phylogenetic uncertainties regarding the placement of Pycnogonida as either a distinct class or more closely allied with other chelicerates. Arachnida dominates in with approximately 110,000 described species, while includes only 4 extant species, and Pycnogonida comprises around 1,300 species. Extinct classes, such as Eurypterida and , further illustrate the subphylum's ancient aquatic origins. The class Arachnida represents the largest and most morphologically diverse clade within Chelicerata, encompassing over 20 orders that are unified by key traits including a body divided into prosoma and opisthosoma, four pairs of walking legs, and the absence of antennae. Prominent orders include Araneae (spiders, characterized by silk-producing spinnerets and venomous fangs), Scorpiones (, distinguished by a metasoma ending in a ), and Acari (mites and ticks, notable for their highly modified body plan and parasitic lifestyles in many species). Recent phylogenetic revisions since 2020 have refined arachnid relationships, elevating certain groups to ordinal status and supporting clades like (including spiders and whip scorpions) and Arachnopulmonata (encompassing scorpions, , and lung-breathing arachnids), based on molecular and morphological data. These updates highlight the dynamic nature of arachnid , with Acari sometimes treated as a single order but increasingly recognized as comprising two major lineages ( and ) that may warrant separate ordinal rank. Xiphosura, commonly known as horseshoe crabs, is a small but phylogenetically significant class limited to four living in the family Limulidae, all confined to shallow marine environments. These organisms exhibit a distinctive limulid , featuring a horseshoe-shaped prosoma, a broad opisthosoma fused into a , book gills for respiration, and a long for locomotion and stability. As the sole surviving members of the broader Merostomata, xiphosurans provide critical insights into chelicerate , serving as a bridge between extinct aquatic forms and terrestrial arachnids. Pycnogonida, or , form a morphologically unique class adapted exclusively to marine habitats, with species characterized by a slender body, elongated for external feeding on prey or hosts, and highly reduced opisthosoma often incorporated into the prosoma. Their inclusion in Chelicerata is supported by shared and other traits, though some analyses question their exact position, placing them as the to all other chelicerates. Among extinct clades, Eurypterida, known as sea scorpions, were large aquatic predators reaching up to 2.5 meters in length, with a body plan featuring robust appendages, paddle-like swimming structures, and compound eyes adapted for underwater vision. This class, spanning the Ordovician to Permian periods, exemplifies early chelicerate diversification in marine ecosystems. Chasmataspidida, a transitional group from the Cambrian to Devonian, displayed intermediate features between xiphosurans and arachnids, such as a flattened body and short appendages, suggesting a pivotal role in the shift to terrestrial habitats.

Diversity and Ecology

Species Richness and Distribution

Chelicerata encompasses over 120,000 described as of 2025, representing one of the most biodiverse clades within Arthropoda. This richness is overwhelmingly dominated by the subclass Arachnida, which accounts for the vast majority, while the other major extant lineages exhibit far lower diversity. Within Arachnida, the order Araneae (spiders) includes approximately 52,000 , and Acari (mites and ticks) comprises over 54,000 , split between more than 42,000 in and over 12,400 in . In contrast, (horseshoe crabs) has only 4 extant , and Pycnogonida () around 1,400. Chelicerates exhibit a , inhabiting marine, freshwater, and terrestrial environments worldwide, though patterns vary markedly by lineage. Arachnids are predominantly terrestrial, with spiders and scorpions thriving across continents from deserts to forests, while some s occupy freshwater habitats such as streams and ponds. Xiphosurans and pycnogonids, conversely, are almost exclusively marine, with the former restricted to coastal shelf waters and the latter ranging from shallow intertidal zones to abyssal depths. Freshwater chelicerates remain rare overall, limited primarily to certain mite taxa. Biogeographically, chelicerate diversity peaks in tropical regions, where high is evident among spiders and scorpions, many confined to specific or locales in , , and . Pycnogonids show notable presence in polar waters, including species exhibiting polar . Invasive spread has also occurred, particularly with species like the Asian longhorned (Haemaphysalis longicornis), which has established populations in and , posing risks to and . Estimates suggest that undescribed chelicerate species could nearly double the current tally, with up to 500,000 additional mites and over 50,000 spiders potentially awaiting description, driven by cryptic diversity in understudied habitats. Recent surveys in the 2020s have accelerated discoveries, adding thousands of arachnid species—such as over 1,000 new spiders in alone—often through intensive fieldwork in biodiversity hotspots. These advances are bolstered by techniques, which have revealed hidden diversity and facilitated rapid identification in complex assemblages. Earlier counts, like the approximately 77,000 arachnids cited in pre-2020 sources, are now outdated due to this surge in taxonomic activity.

Habitats and Ecological Roles

Chelicerates occupy a wide array of habitats, reflecting their evolutionary diversification across marine, terrestrial, and parasitic environments. Arachnids, the dominant terrestrial group, thrive in , forests, and urban green spaces, where they exploit diverse microhabitats such as litter and mossy substrates. Marine chelicerates, including horseshoe crabs and (pycnogonids), are primarily benthic, inhabiting intertidal zones, estuaries, and deep-sea floors. Parasitic forms like ticks are obligate ectoparasites of vertebrates, questing in grassy or wooded areas to attach to hosts such as mammals and birds. In ecosystems, chelicerates fulfill critical roles as predators, decomposers, and intermediaries in food webs. Spiders and scorpions act as top invertebrate predators, regulating populations through predation and contributing to in natural and agricultural settings. Mites and other microarthropods serve as decomposers in soil, accelerating breakdown and nutrient cycling, particularly in litter where they enhance rates. Ticks, while parasitic, influence host dynamics by transmitting pathogens, indirectly shaping wildlife populations and disease . Chelicerates also engage in symbiotic interactions and serve as bioindicators. Certain mites form symbiotic associations with , aiding in dispersal and exchange within communities. Horseshoe crabs function as in estuarine ecosystems, providing essential for migratory birds and , and their populations indicate overall marine health due to sensitivity to habitat degradation. In polar regions, occupy prominent niches as predators on small , contributing to benthic community structure. As prey, chelicerates support higher trophic levels, serving as food for birds, , and amphibians across habitats. Some chelicerates exhibit specialized adaptations to extreme environments. scorpions conserve through low cuticular permeability and behavioral burrowing, minimizing respiratory and evaporative losses in arid conditions. Deep-sea pycnogonids adapt to low-oxygen depths by absorbing gases directly through their , with some lacking eyes to reduce energy demands in perpetual darkness. Invasive chelicerates, such as the ( hasselti), disrupt local food webs in non-native regions like by outcompeting indigenous predators.

Interactions with Humans

Economic and Medical Importance

Chelicerates, particularly horseshoe crabs, play a vital role in the through the extraction of (LAL) from their blood, which is used in endotoxin detection tests to ensure the sterility of injectable drugs and medical devices. This LAL testing market was valued at approximately $600 million in 2023. Prior to the 2020s, an average of about 500,000 horseshoe crabs were harvested each year for bleeding, with the process enabling critical that prevents bacterial contamination and has saved countless lives by averting in patients receiving pharmaceuticals. Spider silks, prized for their exceptional strength and biocompatibility, hold significant promise as biomaterials in medical applications such as , wound dressings, and systems. Efforts to commercialize spider silk production have included sericulture-like farming attempts using genetically modified silkworms, with companies achieving costs as low as $300 per kilogram through bioengineered methods. Venoms from scorpions and spiders are being researched for their pharmacological potential, including development of non-opioid painkillers; for instance, peptides from tarantula venom, such as ProTx-II, target sodium channels to provide relief for chronic pain conditions without addiction risks. Scorpion venoms also show anticancer properties, with molecules like BamazScplp1 from Amazonian scorpions demonstrating efficacy against breast cancer cells in 2025 studies, and chlorotoxin enabling targeted brain tumor imaging and therapy. Tick saliva contains anticoagulant proteins, such as Salp14, which inhibit blood clotting and complement pathways, offering leads for novel antithrombotic drugs in cardiovascular treatments. In agriculture, predatory mites like Phytoseiulus persimilis are deployed for biological control of pest mites, reducing crop losses and reliance, thereby enhancing economic in vegetable and production. Scorpions have cultural significance in , particularly in Chinese practices where the dried body of Buthus martensii (Quan Xie) is used to alleviate pain, convulsions, and rheumatic conditions.

Conservation and Threats

Chelicerates face multiple anthropogenic threats that contribute to declines across various taxa. Habitat loss due to and coastal development particularly affects arachnids, fragmenting their natural environments and reducing available refugia for like spiders and scorpions. Overharvesting poses a severe risk to xiphosurans, with horseshoe crabs exploited for bait, biomedical testing, and , leading to sharp drops in regions like the . Climate change exacerbates vulnerabilities for polar pycnogonids, such as , whose relies on cold, oxygen-rich waters that are warming and acidifying, potentially disrupting their physiological adaptations. Pesticides further threaten acarine mites, harming predatory essential for in agroecosystems through direct toxicity and sublethal effects on and . Conservation statuses highlight the urgency for chelicerates, with the classifying many assessed species as threatened, reflecting widespread endangerment from degradation and exploitation. The tri-spine (Tachypleus tridentatus) is listed as Endangered due to and breeding ground loss across its range. Scorpions benefit from some protected areas, such as national parks in Mexico's hotspots where is monitored, though many key populations remain unprotected. Recent 2025 analyses of mid-Cretaceous have revealed extinct chelicerate diversity, including a reinterpreted subfamily of scorpions, underscoring historical richness and the need to prevent further losses in modern lineages. Protective efforts include programs for xiphosurans, such as those by the Wetlands Institute and Maritime Aquarium, which release juveniles to bolster wild populations and support . farming for scorpions, practiced in regions like and , reduces pressure on wild stocks by producing medical-grade extracts from captive individuals, aiding conservation of overexploited species. Monitoring invasive ticks, such as the Asian longhorned tick (Haemaphysalis longicornis), involves passive surveillance networks by agencies like the USDA and state extensions, tracking distributions to mitigate disease spread and ecological impacts. Biomedical alternatives, like recombinant Factor C (rFC), are reducing horseshoe crab harvests by up to 90% in endotoxin testing, as endorsed by the U.S. Pharmacopeia in 2025, easing biomedical demands while preserving populations. Biodiversity hotspots, such as those harboring Australian trapdoor spiders (Idiopidae), emphasize targeted protections, with low-intensity fire management in south-western aiding survival of these long-lived endemics. The outlook for chelicerate conservation reveals significant gaps, particularly for mites due to their micro-scale habitats and understudied diversity, complicating broad-scale protections amid ongoing .

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