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Temporal range: 435–0  Ma
Arachnida collage.pngAraneaeScorpionesPseudoscorpiones
Representatives of the 12 extant orders of arachnids
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Clade: Arachnomorpha
Subphylum: Chelicerata
Class: Arachnida
Lamarck, 1801

Arachnida ( /əˈræknɪdə/ ) is a class of joint-legged arthropods, in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegaroons. [1]


Adult arachnids have eight legs attached to the cephalothorax, although the frontmost pair of legs in some species has converted to a sensory function, while in other species, different appendages can grow large enough to take on the appearance of extra pairs of legs. The term is derived from the Greek word ἀράχνη (aráchnē, 'spider'), from the myth of the hubristic human weaver Arachne, who was turned into a spider. [2]

Almost all extant arachnids are terrestrial, living mainly on land. However, some inhabit freshwater environments and, with the exception of the pelagic zone, marine environments as well. They comprise over 110,000 named species, of which 51,000 are species of spiders. [3] [4]


Basic characteristics of arachnids include four pairs of legs (1) and a body divided into two tagmata: the cephalothorax (2) and the abdomen (3) Spider-characteristics.png
Basic characteristics of arachnids include four pairs of legs (1) and a body divided into two tagmata: the cephalothorax (2) and the abdomen (3)

Almost all adult arachnids have eight legs, unlike adult insects which all have six legs. However, arachnids also have two further pairs of appendages that have become adapted for feeding, defense, and sensory perception. The first pair, the chelicerae, serve in feeding and defense. The next pair of appendages, the pedipalps, have been adapted for feeding, locomotion, and/or reproductive functions. In scorpions, pseudoscorpions, and ricinuleids the pedipalps ends in a pair of pinchers, and in whip scorpions, Schizomida, Amblypygi, and most harvestmen, they are raptorial and used for prey capture. [5] In Solifugae, the palps are quite leg-like, so that these animals appear to have ten legs. The larvae of mites and Ricinulei have only six legs; a fourth pair usually appears when they moult into nymphs. However, mites are variable: as well as eight, there are adult mites with six or, like in Eriophyoidea, even four legs. [6] [7] And while the adult males in some members of Podapolipidae have six legs, the adult females have only a single pair. [8]

Arachnids are further distinguished from insects by the fact they do not have antennae or wings. Their body is organized into two tagmata, called the prosoma, or cephalothorax, and the opisthosoma, or abdomen. (However, there is currently neither fossil nor embryological evidence that arachnids ever had a separate thorax-like division, so the validity of the term cephalothorax, which means a fused cephalon, or head, and thorax, has been questioned. There are also arguments against use of 'abdomen', as the opisthosoma of many arachnids contains organs atypical of an abdomen, such as a heart and respiratory organs. [9] ) The prosoma, or cephalothorax, is usually covered by a single, unsegmented carapace. The abdomen is segmented in the more primitive forms, but varying degrees of fusion between the segments occur in many groups. It is typically divided into a preabdomen and postabdomen, although this is only clearly visible in scorpions, and in some orders, such as the Acari, the abdominal sections are completely fused. [10] A telson is present in scorpions, where it has been modified to a stinger, and into a flagellum in the Palpigradi, Schizomida (very short) and whip scorpions. [11] At the base of the flagellum in the two latter groups there are gland who produce acetic acid as a chemical defense. [12] Except for a pair of pectines in scorpions, [13] and the spinnerets in spiders, the abdomen has no appendages. [14]

Like all arthropods, arachnids have an exoskeleton, and they also have an internal structure of cartilage-like tissue, called the endosternite, to which certain muscle groups are attached. The endosternite is even calcified in some Opiliones. [15]


Most arachnids lack extensor muscles in the distal joints of their appendages. Spiders and whipscorpions extend their limbs hydraulically using the pressure of their hemolymph. [16] Solifuges and some harvestmen extend their knees by the use of highly elastic thickenings in the joint cuticle. [16] Scorpions, pseudoscorpions and some harvestmen have evolved muscles that extend two leg joints (the femur-patella and patella-tibia joints) at once. [17] [18] The equivalent joints of the pedipalps of scorpions though, are extended by elastic recoil. [19]

"Arachnida" from Ernst Haeckel's Kunstformen der Natur, 1904 Haeckel Arachnida.jpg
"Arachnida" from Ernst Haeckel's Kunstformen der Natur, 1904


There are characteristics that are particularly important for the terrestrial lifestyle of arachnids, such as internal respiratory surfaces in the form of tracheae, or modification of the book gill into a book lung, an internal series of vascular lamellae used for gas exchange with the air. [20] While the tracheae are often individual systems of tubes, similar to those in insects, ricinuleids, pseudoscorpions, and some spiders possess sieve tracheae, in which several tubes arise in a bundle from a small chamber connected to the spiracle. This type of tracheal system has almost certainly evolved from the book lungs, and indicates that the tracheae of arachnids are not homologous with those of insects. [21]

Further adaptations to terrestrial life are appendages modified for more efficient locomotion on land, internal fertilisation, special sensory organs, and water conservation enhanced by efficient excretory structures as well as a waxy layer covering the cuticle.

The excretory glands of arachnids include up to four pairs of coxal glands along the side of the prosoma, and one or two pairs of Malpighian tubules, emptying into the gut. Many arachnids have only one or the other type of excretory gland, although several do have both. The primary nitrogenous waste product in arachnids is guanine. [21]

Arachnid blood is variable in composition, depending on the mode of respiration. Arachnids with an efficient tracheal system do not need to transport oxygen in the blood, and may have a reduced circulatory system. In scorpions and some spiders, however, the blood contains haemocyanin, a copper-based pigment with a similar function to haemoglobin in vertebrates. The heart is located in the forward part of the abdomen, and may or may not be segmented. Some mites have no heart at all. [21]

Diet and digestive system

Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. But ticks, and many mites, are parasites, some of which are carriers of disease. The diet of mites also include tiny animals, fungi, plant juices and decomposing matter. [22] Almost as varied is the diet of harvestmen, where we will find predators, decomposers and omnivores feeding on decaying plant and animal matter, droppings, animals and mushrooms. [23] [24] [25] The harvestmen and some mites, such as the house dust mite, are also the only arachnids able to ingest solid food, which exposes them to internal parasites, [26] although it is not unusual for spiders to eat their own silk. And one species of spider is mostly herbivorous. [27] Scorpions, spiders and pseudoscorpions secrete venom from specialized glands to kill prey or defend themselves. [28] Their venom also contains pre-digestive enzymes that helps breaking down the prey. [29] [30] [31] The saliva of ticks contains anticoagulants and anticomplements, and several species produce a neurotoxin. [32] [33]

Arachnids produce digestive enzymes in their stomachs, and use their pedipalps and chelicerae to pour them over their dead prey. The digestive juices rapidly turn the prey into a broth of nutrients, which the arachnid sucks into a pre-buccal cavity located immediately in front of the mouth. Behind the mouth is a muscular, sclerotised pharynx, which acts as a pump, sucking the food through the mouth and on into the oesophagus and stomach. In some arachnids, the oesophagus also acts as an additional pump.

The stomach is tubular in shape, with multiple diverticula extending throughout the body. The stomach and its diverticula both produce digestive enzymes and absorb nutrients from the food. It extends through most of the body, and connects to a short sclerotised intestine and anus in the hind part of the abdomen. [21]


Arachnids have two kinds of eyes: the lateral and median ocelli. The lateral ocelli evolved from compound eyes and may have a tapetum, which enhances the ability to collect light. With the exception of scorpions, which can have up to five pairs of lateral ocelli, there are never more than three pairs present. The median ocelli develop from a transverse fold of the ectoderm. The ancestors of modern arachnids probably had both types, but modern ones often lack one type or the other. [26] The cornea of the eye also acts as a lens, and is continuous with the cuticle of the body. Beneath this is a transparent vitreous body, and then the retina and, if present, the tapetum. In most arachnids, the retina probably does not have enough light sensitive cells to allow the eyes to form a proper image. [21]

In addition to the eyes, almost all arachnids have two other types of sensory organs. The most important to most arachnids are the fine sensory hairs that cover the body and give the animal its sense of touch. These can be relatively simple, but many arachnids also possess more complex structures, called trichobothria.

Finally, slit sense organs are slit-like pits covered with a thin membrane. Inside the pit, a small hair touches the underside of the membrane, and detects its motion. Slit sense organs are believed to be involved in proprioception, and possibly also hearing. [21]


Courtship behavior of Thelyphonus sp. Vinegaroons in courtship (Thelyphonus sp) (8390306848).jpg
Courtship behavior of Thelyphonus sp.

Arachnids may have one or two gonads, which are located in the abdomen. The genital opening is usually located on the underside of the second abdominal segment. In most species, the male transfers sperm to the female in a package, or spermatophore. The males in harvestmen and some mites have a penis. [34] Complex courtship rituals have evolved in many arachnids to ensure the safe delivery of the sperm to the female. [21] Members of many orders exhibit sexual dimorphism. [35]

Arachnids usually lay yolky eggs, which hatch into immatures that resemble adults. Scorpions, however, are either ovoviviparous or viviparous, depending on species, and bear live young. Also some mites are ovoviviparous and viviparous, even if most lay eggs. [36] In most arachnids only the females provide parental care, with harvestmen being one of the few exceptions. [37] [38]

Taxonomy and evolution


The phylogenetic relationships among the main subdivisions of arthropods have been the subject of considerable research and dispute for many years. A consensus emerged from about 2010 onwards, based on both morphological and molecular evidence. Extant (living) arthropods are a monophyletic group and are divided into three main clades: chelicerates (including arachnids), pancrustaceans (the paraphyletic crustaceans plus insects and their allies), and myriapods (centipedes, millipedes and allies). [39] [40] [41] [42] [43] The three groups are related as shown in the cladogram below. [41] Including fossil taxa does not fundamentally alter this view, although it introduces some additional basal groups. [44]


Chelicerata (sea spiders, horseshoe crabs and arachnids) Nymphon signatum 194389384 (white background).jpg Limulus polyphemus (aquarium) (white background).jpg Aptostichus simus Monterey County.jpg


Myriapoda (centipedes, millipedes, and allies) Scolopendra japonica aozumukade Da Ban Fu Sheng Ju Shan Chan .jpg Andrognathus corticarius A.jpg

Pancrustacea (crustaceans and hexapods) Lobster png by absurdwordpreferred d2xqhvd.png Platycheirus angustatus (Syrphidae) - (male imago), Elst (Gld), the Netherlands - 2.jpg

The extant chelicerates comprise two marine groups: sea spiders and horseshoe crabs, and the terrestrial arachnids. These have been thought to be related as shown below. [40] [43] (Pycnogonida (sea spiders) may be excluded from the chelicerates, which are then identified as the group labelled "Euchelicerata". [45] ) A 2019 analysis nests Xiphosura deeply within Arachnida. [46]


Pycnogonida (sea spiders) Nymphon signatum 194389384 (white background).jpg


Xiphosura (horseshoe crabs) Limulus polyphemus (aquarium) (white background).jpg

Arachnida Aptostichus simus Monterey County.jpg

Discovering relationships within the arachnids has proven difficult as of March 2016, with successive studies producing different results. A study in 2014, based on the largest set of molecular data to date, concluded that there were systematic conflicts in the phylogenetic information, particularly affecting the orders Acariformes, Parasitiformes and Pseudoscorpiones, which have had much faster evolutionary rates. Analyses of the data using sets of genes with different evolutionary rates produced mutually incompatible phylogenetic trees. The authors favoured relationships shown by more slowly evolving genes, which demonstrated the monophyly of Chelicerata, Euchelicerata and Arachnida, as well as of some clades within the arachnids. The diagram below summarizes their conclusions, based largely on the 200 most slowly evolving genes; dashed lines represent uncertain placements. [43]

Hubbardia pentapeltis (Schizomida) Hubbardia pentapeltis female.jpg
Hubbardia pentapeltis (Schizomida)

Tetrapulmonata, here consisting of Araneae, Amblypygi and Uropygi (Thelyphonida s.s.) (Schizomida was not included in the study), received strong support. Somewhat unexpectedly, there was support for a clade comprising Opiliones, Ricinulei and Solifugae, a combination not found in most other studies. [43] In early 2019, a molecular phylogenetic analysis placed the horseshoe crabs, Xiphosura, as the sister group to Ricinulei. It also grouped pseudoscorpions with mites and ticks, which the authors considered may be due to long branch attraction. [46] The addition of Scorpiones to produce a clade called Arachnopulmonata was also well supported. Pseudoscorpiones may also belong here, as all six orders share the same ancient whole genome duplication, [47] and analyses support pseudoscorpions as the sister group of scorpions. [48] Genetic analysis has not yet been done for Ricinulei, Palpigradi, or Solifugae, but horseshoe crabs have gone through two whole genome duplications, which gives them five Hox clusters with 34 Hox genes, the highest number found in any invertebrate, yet it is not clear if the oldest genome duplication is related to the one in Arachnopulmonata. [49] [50]

More recent phylogenomic analyses that have densely sampled both genomic datasets and morphology have supported horseshoe crabs as nested inside Arachnida, suggesting a complex history of terrestrialization. [51] [52] Morphological analyses including fossils tend to recover the Tetrapulmonata, including the extinct group the Haptopoda, [53] [54] [55] [56] [57] but recover other ordinal relationships with low support.

Fossil history

Fossil Goniotarbus angulatus (Phalangiotarbida) Goniotarbus angulatus holotype fossil dorsal ventral.jpg
Fossil Goniotarbus angulatus (Phalangiotarbida)
Fossil of Kreischeria (Trigonotarbida) Kreischeria Vienna.jpg
Fossil of Kreischeria (Trigonotarbida)

The Uraraneida are an extinct order of spider-like arachnids from the Devonian and Permian. [58]

A fossil arachnid in 100 million year old (mya) amber from Myanmar, Chimerarachne yingi , has spinnerets (to produce silk); it also has a tail, like the Palaeozoic Uraraneida, some 200 million years after other known fossils with tails. The fossil resembles the most primitive living spiders, the mesotheles. [59] [53]


Eukoenenia spelaea (Palpigradi) Live Eukoenenia spelaea in its cave habitat.png
Eukoenenia spelaea (Palpigradi)

The subdivisions of the arachnids are usually treated as orders. Historically, mites and ticks were treated as a single order, Acari. However, molecular phylogenetic studies suggest that the two groups do not form a single clade, with morphological similarities being due to convergence. They are now usually treated as two separate taxa – Acariformes, mites, and Parasitiformes, ticks – which may be ranked as orders or superorders. The arachnid subdivisions are listed below alphabetically; numbers of species are approximate.

Extant forms
Extinct forms

It is estimated that 110,000 arachnid species have been described, and that there may be over a million in total. [4]

See also

Related Research Articles

<span class="mw-page-title-main">Merostomata</span> Class of arthropods

Merostomata is a class of chelicerate arthropods that contains the extinct Eurypterida and the extant Xiphosura. The term was originally used by James Dwight Dana to refer to Xiphosura only, but was emended by Henry Woodward to cover both groups.

<span class="mw-page-title-main">Chelicerata</span> Subphylum of arthropods

The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids, as well as a number of extinct lineages, such as the eurypterids and chasmataspidids.

<span class="mw-page-title-main">Pseudoscorpion</span> Order of arachnids

Pseudoscorpions, also known as false scorpions or book scorpions, are small, scorpion-like arachnids belonging to the order Pseudoscorpiones, also known as Pseudoscorpionida or Chelonethida.

<span class="mw-page-title-main">Opiliones</span> Order of arachnids (harvestmen/daddy longlegs)

The Opiliones are an order of arachnids colloquially known as harvestmen, harvesters, harvest spiders, or daddy longlegs. As of April 2017, over 6,650 species of harvestmen have been discovered worldwide, although the total number of extant species may exceed 10,000. The order Opiliones includes five suborders: Cyphophthalmi, Eupnoi, Dyspnoi, Laniatores, and Tetrophthalmi, which were named in 2014.

<span class="mw-page-title-main">Ricinulei</span> Order of spider-like animals

Ricinulei is a small order of arachnids. Like most arachnids, they are predatory, eating small arthropods. They occur today in west-central Africa (Ricinoides) and the Americas as far north as Texas. As of 2021, 91 extant species of ricinuleids have been described worldwide, all in the single family Ricinoididae. In older works they are sometimes referred to as Podogona. Due to their obscurity they do not have a proper common name, though in academic literature they are occasionally referred to as hooded tickspiders.

<span class="mw-page-title-main">Book lung</span> Type of lung commonly found in arachnids

A book lung is a type of respiration organ used for atmospheric gas exchange that is present in many arachnids, such as scorpions and spiders. Each of these organs is located inside an open ventral abdominal, air-filled cavity (atrium) and connects with its surroundings through a small opening for the purpose of respiration.

<span class="mw-page-title-main">Sea spider</span> Order of marine arthropods

Sea spiders are marine arthropods of the order Pantopoda, belonging to the class Pycnogonida, hence they are also called pycnogonids. They are cosmopolitan, found in oceans around the world. The over 1,300 known species have leg spans ranging from 1 mm (0.04 in) to over 70 cm (2.3 ft). Most are toward the smaller end of this range in relatively shallow depths; however, they can grow to be quite large in Antarctic and deep waters.

<span class="mw-page-title-main">Solifugae</span> Order of arachnids

Solifugae is an order of animals in the class Arachnida known variously as camel spiders, wind scorpions, sun spiders, or solifuges. The order includes more than 1,000 described species in about 147 genera. Despite the common names, they are neither true scorpions nor true spiders. Most species of Solifugae live in dry climates and feed opportunistically on ground-dwelling arthropods and other small animals. The largest species grow to a length of 12–15 cm (5–6 in), including legs. A number of urban legends exaggerate the size and speed of the Solifugae, and their potential danger to humans, which is negligible.

<i>Plesiosiro</i> Extinct genus of arachnids

Plesiosiro is an extinct arachnid genus known exclusively from nine specimens from the Upper Carboniferous of Coseley, Staffordshire, United Kingdom. The genus is monotypic, represented only by the species Plesiosiro madeleyi described by Reginald Innes Pocock in his important 1911 monograph on British Carboniferous arachnids. It is the only known member of the order Haptopoda.

<span class="mw-page-title-main">Trigonotarbida</span> Extinct order of arachnids

The order Trigonotarbida is a group of extinct arachnids whose fossil record extends from the late Silurian to the early Permian. These animals are known from several localities in Europe and North America, as well as a single record from Argentina. Trigonotarbids can be envisaged as spider-like arachnids, but without silk-producing spinnerets. They ranged in size from a few millimetres to a few centimetres in body length and had segmented abdomens (opisthosoma), with the dorsal exoskeleton (tergites) across the backs of the animals' abdomens, which were characteristically divided into three or five separate plates. Probably living as predators on other arthropods, some later trigonotarbid species were quite heavily armoured and protected themselves with spines and tubercles. About seventy species are currently known, with most fossils originating from the Carboniferous coal measures.

<span class="mw-page-title-main">Tetrapulmonata</span> Clade of arachnids

Tetrapulmonata is a non-ranked supra-ordinal clade of arachnids. It is composed of the extant orders Uropygi, Schizomida, Amblypygi and Araneae (spiders). It is the only supra-ordinal group of arachnids that is strongly supported in molecular phylogenetic studies. Two extinct orders are also placed in this clade, Haptopoda and Uraraneida. In 2016, a newly described fossil arachnid, Idmonarachne, was also included in the Tetrapulmonata; as of March 2016 it has not been assigned to an order.

<span class="mw-page-title-main">Opisthosoma</span> Posterior body part of some arthropods

The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma (cephalothorax). It is a distinctive feature of the subphylum Chelicerata. Although it is similar in most respects to an abdomen, the opisthosoma is differentiated by its inclusion of the respiratory organs and the heart.

<span class="mw-page-title-main">Evolution of spiders</span> Origin from a chelicerate ancestor and diversification of spiders through geologic time

The evolution of spiders has been ongoing for at least 380 million years. The group's origins lie within an arachnid sub-group defined by the presence of book lungs ; the arachnids as a whole evolved from aquatic chelicerate ancestors. More than 45,000 extant species have been described, organised taxonomically in 3,958 genera and 114 families. There may be more than 120,000 species. Fossil diversity rates make up a larger proportion than extant diversity would suggest with 1,593 arachnid species described out of 1,952 recognized chelicerates. Both extant and fossil species are described annually by researchers in the field. Major developments in spider evolution include the development of spinnerets and silk secretion.

<span class="mw-page-title-main">Cyphophthalmi</span> Suborder of harvestmen/daddy longlegs

Cyphophthalmi is a suborder of harvestmen, colloquially known as mite harvestmen. Cyphophthalmi comprises 36 genera, and more than two hundred described species. The six families are currently grouped into three infraorders: the Boreophthalmi, Scopulophthalmi, and Sternophthalmi.

<span class="mw-page-title-main">Dromopoda</span> Subclass of arachnids

Dromopoda is a proposed subclass of the arachnids, including the Opiliones (harvestmen), Scorpions, Pseudoscorpions and Solifugae. The latter three are sometimes grouped as Novogenuata. Combined morphological and molecular analyses have shown Dromopoda to be monophyletic. However, a strictly molecular analysis did not support the monophyly of Dromopoda.

<i>Phalangium opilio</i> Species of harvestman/daddy longlegs

Phalangium opilio is a species of harvestman belonging to the family Phalangiidae.

<span class="mw-page-title-main">Opiliones anatomy</span>

Opiliones are an order of arachnids and share many common characteristics with other arachnids. However, several differences separate harvestmen from other arachnid orders such as spiders. The bodies of opiliones are divided into two tagmata : the abdomen (opisthosoma) and the cephalothorax (prosoma). Unlike spiders, the juncture between the abdomen and cephalothorax is often poorly defined. Harvestmen have chelicerae, pedipalps and four pairs of legs. Harvestmen were traditionally thought to have two eyes, except in the case of eyeless species. Developmental genetic work has shown that living species retain up to six eyes, including one pair of rudimentary median eyes and one pair of rudimentary lateral eyes.

<span class="mw-page-title-main">Harvestman phylogeny</span> Order of arachnids

Harvestmen (Opiliones) are an order of arachnids often confused with spiders, though the two orders are not closely related. Research on harvestman phylogeny is in a state of flux. While some families are clearly monophyletic, that is share a common ancestor, others are not, and the relationships between families are often not well understood.

<span class="mw-page-title-main">Tetrophthalmi</span> Suborder of harvestmen/daddy longlegs

Tetrophthalmi is an extinct suborder of Opiliones that had both median and lateral eyes. First described in 2014, it is known from two extinct species. Phylogenetic analysis suggests that this eye arrangement is the ancestral condition for harvestmen, placing Tetrophthalmi and Cyphophthalmi in a basal position within Opiliones.

Prashant P. Sharma is an Indian-American invertebrate biologist and a professor of Integrative Biology at the University of Wisconsin-Madison.


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