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
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Clade: Arachnomorpha
Subphylum: Chelicerata
Heymons, 1901
Groups
Synonyms
  • Cheliceriformes Schram and Hedgpeth, 1978

The subphylum Chelicerata (from

chasmataspidids
.

Chelicerata split from

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

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,

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 recently been questioned (see below), 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 undocumented descriptions of venom glands in Solifugae.

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

allergies in humans, transmit several diseases to humans and their livestock, and are serious agricultural pests
.

Description

Trilobitomorpha
A
L
L
L
L
L
L
x
C
P
L
L
L
L
Ci
Crustacea
A
A
Mnd
Mx
Mx
L
L
L
L
L
Tracheata
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
C =
Chelicera
Ci =
Chilarium
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

The Chelicerata are

Acari (mites and ticks) there is no visible division between these sections.[13]

The

ocelli ("little eyes") in the middle.[12]
The mouth is between somite 1 and 2 (chelicerae and pedipalps).

The

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

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]

The large chelicerae of the pterygotid eurypterid Acutiramus cummingsi, with pedipalps undiferentiated from the walking legs
The whip spider Phrynichus phipsoni, with short chelicerae and large specialized pedipalps

Chelicerae and pedipalps

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.

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
  9
Malphigian tubules
10 Stercoral pocket
11 Rear aorta
12
Spinneret
14
Trachea
15 Ovary (female)
18 Legs
Spider's main organs[21]

Body cavities and circulatory systems

As in all arthropods, the chelicerate body has a very small

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

These depend on individual sub-groups' environments. Modern terrestrial chelicerates generally have both

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

The guts of most modern chelicerates are too narrow to take solid food.

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

Horseshoe crabs convert

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

  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

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

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

As with other arthropods, chelicerates'

smell, often by means of setae.[33]

Living chelicerates have both

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

Female scorpion Vaejovis cashi carrying its young (white)

molt.[17]

Also the sea spiders have external fertilization. The male and female 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]

Being air-breathing animals, although many mites have become secondary aquatic,[40] the arachnids use internal fertilization. Except for opiliones and some mites, where the male have a penis used for direct fertilization,[41] 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 store his sperm in appendages modified into sperm transfer organs, such as the pedipalps in male spiders, which is inserted into the female genital openings during copulation.[16] Courtship rituals are common, especially in species where the male risk being eaten before mating. 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).[42][43][44][45] 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.[46]

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.[47] 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.[48]

Evolutionary history

Fossil record

There are large gaps in the chelicerates'

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.[49]

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

book gills.[51]

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.[52] Until recently the earliest known xiphosuran fossil dated from the Late Llandovery stage of the Silurian 436 to 428 million years ago,[53] but in 2008 an older specimen described as Lunataspis aurora was reported from about 445 million years ago in the Late Ordovician.[54]

The oldest known

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.[55]

spinnerets and hence was not a true spider.[57] Rather, it was likely sister group to the spiders, a clade which has been named Serikodiastida.[58] Close relatives of the group survived through to the Cretaceous Period.[59] Several Carboniferous spiders were members of the Mesothelae, a basal group now represented only by the Liphistiidae,[56] and fossils suggest taxa closely related to the spiders, but which were not true members of the group were also present during this Period.[60]

The Late

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.[61] Fossils of terrestrial scorpions with book lungs have been found in Early Devonian rocks from about 402 million years ago.[62] The oldest species of scorpion found as of 2021 is Dolichophonus loudonensis, which lived during the Silurian, in present-day Scotland.[63]

Relationships with other arthropods

Arthropoda
Pancrustacea
Paradoxopoda

Myriapoda

Chelicerata

 A recent view of chelicerate
phylogeny[64]

The "traditional" view of the arthropod "family tree" shows chelicerates as less closely related to the other major living groups (

Tracheata (hexapods plus myriapods) and chelicerates as least closely related to the other groups.[67]

Major sub-groups

It is generally agreed that the Chelicerata contain 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.[70]

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).[71] An analysis in 2007 using only anatomical features produced the cladogram on the right, but also noted that many uncertainties remain.[72] In recent analyses the clade Tetrapulmonata is reliably recovered, but other ordinal relationships remain in flux.[59][73][60][74][75][76][77]

The position of scorpions is particularly controversial. Some early fossils such as the Late

Cladistic analyses have recovered Proscorpius within the scorpions,[58] based on reinterpretation of the species' breathing apparatus.[78] This is reflected also in the reinterpretation of Palaeoscorpius as a terrestrial animal.[79]

A 2013 phylogenetic analysis

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.,[77] but others still retrieve a monophyletic arachnida.[81]

Arachnomorpha

†Fuxianhuia

†Antennulata

†Emeraldella

†

Trilobitomorpha

†Sidneyia

†Megacheira
Chelicerata
Pycnogonida
Euchelicerata

Diversity

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.

parasites, herbivores, scavengers and detritivores.[13][27][82]

Diversity of living chelicerates
Group Described species[82][85][86] Diet
Pycnogonida
(sea-spiders) 		500 Carnivorous[82]
Araneae
(spiders) 		50,300 Carnivorous;[82] 1 herbivore[25]
Acari
(mites and ticks) 		32,000 Carnivorous, parasitic, herbivore, detritivore[13][82]
Opiliones (harvestmen) 6,500 Carnivorous, herbivore, detritivore[27]
Pseudoscorpiones
(false scorpions) 		3,200 Carnivorous[87]
Scorpiones
(scorpions) 		1,400 Carnivorous[20]
Solifugae (sunspiders) 900 Carnivorous,
omnivorous[88]
Schizomida (small whipscorpions) 180 Carnivorous[89]
Amblypygi (whipspiders) 100 Carnivorous[90]
Uropygi (whipscorpions) 90 Carnivorous[91]
Palpigradi (micro whipscorpions) 60  
Xiphosura (horseshoe crabs) 4 Carnivorous[82]
Ricinulei 60 Carnivorous[92]

Interaction with humans

A microscopic mite Lorryia formosa.

In the past,

spinal meningitis and some cancers.[93]

Cooked

cardiac arrhythmia,[97] Alzheimer's disease,[98] strokes,[99] and erectile dysfunction.[100]

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

transgenic goats' milk,[102]
tobacco leaves,[103]
silkworms,[104][105][106]
and bacteria,[101][107][108] and recombinant spider silk is now available as a commercial product from some biotechnology companies.[106]

In the 20th century, there were about 100 reliably reported deaths from spider bites,[109] compared with 1,500 from jellyfish stings.[110] 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.[111] On the other hand, medical uses of scorpion venom are being investigated for treatment of brain cancers and bone diseases.[112][113]

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.[114]

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

predatory mites may be useful in controlling some of these.[82][118]

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