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Arachnida () is a
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently f ...
of joint-legged
invertebrate Invertebrates are animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular resp ...
animal Animals (also called Metazoa) are organisms that form the Animalia. With few exceptions, animals , , are , can , and grow from a hollow sphere of , the , during . Over 1.5 million animal have been —of which around 1 million are —b ...

animal
s (
arthropod An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart fr ...
s), in the
subphylum In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Convention (norm), convention in zoology that rules the formal scientific name, scientific naming of organisms treated as animals. It is als ...
Chelicerata The subphylum Chelicerata (from New Latin New Latin (also called Neo-Latin or modern Latin) is the revival of Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. ...
. Arachnida includes orders containing
spider Spiders ( Araneae) are air-breathing s that have eight legs, with fangs generally able to inject , and that extrude . They are the largest order of s and rank seventh in total species diversity among all of organisms.Sebastin, P.A. & Peter, ...

spider
s (the largest order),
scorpion Scorpions are predatory arachnids of the Order (biology), order Scorpiones. They have eight legs, and are easily recognized by a pair of Chela (organ), grasping pincers and a narrow, segmented tail, often carried in a characteristic forward curv ...

scorpion
s,
tick Ticks (suborder Ixodida) are parasitic s that are part of the superorder . Adult ticks are approximately 3 to 5 mm in length depending on age, sex, species, and "fullness". Ticks are external s, living by of mammals, birds, and sometim ...

tick
s,
mite Mites are small arachnid Arachnida () is a Class (biology), class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes orders containing spiders (the largest order), scorpions, ticks, mites, op ...
s,
harvestmen The Opiliones (formerly Phalangida) are an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, ...

harvestmen
, and
solifuges
solifuges
. In 2019, a
molecular phylogenetic Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominately in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...
study also placed
horseshoe crabs Horseshoe crabs are marine and brackish water arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Eua ...

horseshoe crabs
in Arachnida. Almost all adult arachnids have eight
legs A leg is a weight-bearing and animal locomotion, locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, ...
, although the front pair of legs in some species has converted to a sensory function, while in other species, different
appendage An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are co ...
s can grow large enough to take on the appearance of extra pairs of legs. The term is derived from the
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
word (''aráchnē''), from the myth of the hubristic human weaver
Arachne Arachne (; from , cognate with Latin ) is the protagonist of a tale in Greek mythology known primarily from the version told by the Roman poet Ovid (43 BCE–17 CE), which is the earliest extant source for the story. In Book Six of his ...

Arachne
, who was turned into a spider. Almost all
extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, s ...
arachnids are
terrestrial Terrestrial refers to things related to land Land is the solid surface of the Earth that is not permanently covered by water. The vast majority of human activity throughout history has occurred in land areas that support agriculture A ...
, living mainly on land. However, some inhabit freshwater environments and, with the exception of the
pelagic zone The pelagic zone consists of the water column A water column is a Concept, conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004) ...
, marine environments as well. They comprise over 100,000 named
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

species
.


Morphology

Almost all adult arachnids have eight legs, unlike adult
insect Insects (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Rom ...

insect
s 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
pedipalp Pedipalps (commonly shortened to palps or palpi) are the second pair of appendage An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism In biology, an organism (from Ancient Greek ...
s, have been adapted for feeding, locomotion, and/or
reproductive Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual orga ...

reproductive
functions. In
Solifugae Solifugae is an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, an ...

Solifugae
, the palps are quite leg-like, so that these animals appear to have ten legs. The
larva A larva (plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect developmental biology, development such as insects, amphibians, or cnidarians typically have a larval phase of the ...
e of mites and
Ricinulei Ricinulei is an order of arachnids. Like most arachnids, they are predatory, eating small arthropods. In older works they are sometimes referred to as Podogona. , 76 extant species of ricinuleids have been described worldwide, all in the single f ...

Ricinulei
have only six legs; a fourth pair usually appears when they
moult In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...
into
nymph A nymph ( grc, νύμφη, nýmphē, el, script=Latn, nímfi, label=; , ) in is a minor female . Different from es, nymphs are generally regarded as personifications of nature, are typically tied to a specific place or landform, and are usual ...
s. However, mites are variable: as well as eight, there are adult mites with six or even four legs. Arachnids are further distinguished from insects by the fact they do not have
antennae Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two coll ...
or
wings A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoil An airfoil (American English Am ...

wings
. Their body is organized into two tagmata, called the
prosoma The cephalothorax, also called prosoma in some groups, is a Tagma (biology), tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms ''prosoma'' and ''opisthosoma'' are equiva ...
, or
cephalothorax The cephalothorax, also called prosoma in some groups, is a Tagma (biology), tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms ''prosoma'' and ''opisthosoma'' are equiva ...
, and the
opisthosoma 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 (arachnids, horseshoe crabs and others). Although it is similar in most respects to a ...
, or
abdomen The abdomen (colloquially called the belly, tummy, midriff or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the Trunk (anatomy), ...

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 Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino Jr., Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont Co ...
, or head, and
thorax The thorax or chest is a part of the anatomy of humans, mammals, other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main Tagma (biology), divisions ...

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.) 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 Acari (or Acarina ) are a taxon of arachnids that contains mites and ticks. The diversity of the Acari is extraordinary and their fossil record, fossil history goes back to at least the early Devonian period. Acarologists (people who study ...

Acari
, the abdominal sections are completely fused. A
telson Image:Penaeus diagram telson.png, upright=1.4, Diagram highlighting the telson of the prawn ''Litopenaeus setiferus'' The telson is the posterior-most division of the body of an arthropod. Depending on the definition, the telson is either consider ...
is present in scorpions, where it has been modified to a stinger, and in the Schizomida, and Palpigradi. Like all arthropods, arachnids have an
exoskeleton An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular ...

exoskeleton
, and they also have an internal structure of
cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic fibers (or yellow fibers) are an essential component of the extracellular matrix In biology Biology is the natural science that studies life and living org ...

cartilage
-like tissue, called the endosternite, to which certain muscle groups are attached. The endosternite is even calcified in some
Opiliones The Opiliones (formerly Phalangida) are an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, ...

Opiliones
.


Locomotion

Most arachnids lack
extensor Motion (physics), Motion, the process of movement, is described using specific anatomical terminology, anatomical terms. Motion includes movement of Organ (anatomy), organs, joints, limbs, and specific sections of the body. The terminology used d ...
muscles in the
distal Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. Terms used generally derive from Latin or Greek language, Greek roots and used to describe something in its standard anatomical position. This ...
joints of their appendages. Spiders and s extend their limbs hydraulically using the pressure of their
hemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...
. Solifuges and some
harvestmen The Opiliones (formerly Phalangida) are an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, ...

harvestmen
extend their knees by the use of highly elastic thickenings in the joint cuticle.
Scorpion Scorpions are predatory arachnids of the Order (biology), order Scorpiones. They have eight legs, and are easily recognized by a pair of Chela (organ), grasping pincers and a narrow, segmented tail, often carried in a characteristic forward curv ...

Scorpion
s,
pseudoscorpion A pseudoscorpion, also known as a false scorpion or book scorpion, are arachnid Arachnida () is a Class (biology), class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes orders containing spid ...

pseudoscorpion
s and some harvestmen have evolved muscles that extend two leg joints (the femur-patella and patella-tibia joints) at once. The equivalent joints of the pedipalps of scorpions though, are extended by elastic recoil.


Physiology

There are characteristics that are particularly important for the terrestrial lifestyle of arachnids, such as internal respiratory surfaces in the form of
trachea The trachea, also known as the windpipe, is a cartilaginous Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue, rubber-like padding that covers and protects the ends of long bone A bone is a Stiffness, rigid tissue ( ...
e, or modification of the
book gill 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 connect ...
into a
book lung A book lung is a type of respiration organ used for atmospheric gas exchange that is present in many arachnid Arachnida () is a class of joint-legged invertebrate Invertebrates are animals that neither possess nor develop a vertebral colum ...
, an internal series of
vascular The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an Biological system, organ system that permits blood to circulate and transport nutrien ...
lamellae used for
gas exchange Gas exchange is the physical process by which gases move passively by Diffusion#Diffusion vs. bulk flow, diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid ...

gas exchange
with the air. 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. Further adaptations to terrestrial life are
appendage An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are co ...
s 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 gland The coxal gland is a gland found in some arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthrop ...
s along the side of the prosoma, and one or two pairs of
Malpighian tubule The Malpighian tubule system is a type of excretory and osmoregulation, osmoregulatory system found in some insects, myriapods, arachnids and tardigrades. The system consists of branching tubules extending from the alimentary canal that absorbs ...
s, 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 Guanine () (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical meaning ...

guanine
. 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 Hemoglobin or haemoglobin (spelling differences) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein in the red blood cel ...

haemoglobin
in vertebrates. The
heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste ...

heart
is located in the forward part of the abdomen, and may or may not be segmented. Some mites have no heart at all.


Diet and digestive system

Arachnids are mostly
carnivorous A carnivore , meaning "meat Meat is animal flesh that is eaten as food. Humans have hunted and killed animals for meat since prehistoric times. The advent of civilization allowed the domestication of animals such as chickens, sheep, rabbi ...
, feeding on the pre-digested bodies of insects and other small animals. Only in the
harvestmen The Opiliones (formerly Phalangida) are an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, ...

harvestmen
and among
mites Mites are small arachnid Arachnida () is a Class (biology), class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes orders containing spiders (the largest order), scorpions, ticks, mites, opi ...

mites
, such as the
house dust mite House dust mites (HDM, or simply dust mites) are mites found in association with dust in dwellings. They are known for causing an dust mite allergy, allergy. The main species are: * ''Dermatophagoides farinae'' (American house dust mite) * ''D ...

house dust mite
, is there ingestion of solid food particles, and thus exposure to internal parasites, although it is not unusual for spiders to eat their own silk. Several groups secrete
venom Venom is a type of poison In , poisons are that can cause , injury or harm to , , , and usually by chemical reactions or other on the scales, when an organism is exposed to a sufficient quantity. In a metaphorical broader use of term it ...

venom
from specialized
gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek participle , "setting in motion") is any member of a class of signaling molecules in multicellular organisms, ...

gland
s to kill prey or enemies. Several mites and ticks are
parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is adaptation (biology), adapted structurally to this w ...
s, some of which are carriers of
disease A disease is a particular abnormal condition that negatively affects the structure A structure is an arrangement and organization of interrelated elements in a material object or system A system is a group of Interaction, interacting ...
. Arachnids produce digestive juices 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 The pharynx (plural: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea – the tubes going down to the stomach and the lungs. It is found in vertebrates and invertebrates, thou ...

pharynx
, which acts as a pump, sucking the food through the mouth and on into the
oesophagus The esophagus (American English) or oesophagus (British English; both ), informally known as the food pipe or gullet, is an Organ (anatomy), organ in vertebrates through which food passes, aided by Peristalsis, peristaltic contractions, from t ...

oesophagus
and
stomach The stomach is a muscular, in the of humans and many other animals, including several s. The stomach has a dilated structure and functions as a vital organ. In the digestive system the stomach is involved in the second phase of digestion, ...

stomach
. In some arachnids, the oesophagus also acts as an additional pump. The stomach is tubular in shape, with multiple
diverticula In medicine Medicine is the Art (skill), art, science, and Praxis (process) , practice of caring for a patient and managing the diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment or Palliative care , palliation of their ...
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 The gastrointestinal tract, (GI tract, GIT, digestive tract, digestion tract, alimentary canal) is the tract from the mouth to the anus which includes all the organs of the digestive system The human digestive system consists of the human ...

intestine
and
anus The anus (from Latin ''wikt:en:anus#Latin, anus'' meaning "ring", "circle") is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that ...

anus
in the hind part of the abdomen.


Senses

Arachnids have two kinds of eyes: the lateral and median
ocelli A simple eye (sometimes called a pigment pit) refers to a form of eye Eyes are organs of the visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photorecept ...
. The lateral ocelli evolved from
compound eye as imaged by an electron microscope A compound eye is a Eye, visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidium, ommatidia, which are tiny independent photoreception units that consist of ...

compound eye
s 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 ectoderm is one of the three primary germ layer A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrate Vertebrates () comprise all species of animal Animals (also ...

ectoderm
. The ancestors of modern arachnids probably had both types, but modern ones often lack one type or the other. The
cornea The cornea is the transparent Transparency, transparence or transparent most often refer to transparency and translucency, the physical property of allowing the transmission of light through a material. They may also refer to: Literal uses * ...

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 The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a Focus (optics), focused two-dimensional image of the visual world o ...

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. 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 Trichobothria (singular trichobothrium) are elongate setae ("hairs") present in Arachnida, arachnids, various orders of insects, and myriapods that function in the detection of airborne vibrations and currents, and electrical charge . In 1883, F ...
. 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 Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense A sense is a biological system A biological system is a complex biological network, network which connects several biologically relevant entities. Biological ...
, and possibly also hearing.


Reproduction

Arachnids may have one or two
gonad A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes (sex cells) and sex hormones of an organism. In the female of the species the reproductive cells are the egg cells, and in the male the reproductive cells are t ...
s, 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 A spermatophore or sperm ampulla is a capsule or mass containing spermatozoa A spermatozoon (pronounced , alternate spelling spermatozoön; plural spermatozoa; from grc, σπέρμα ("seed") and grc, ζῷον ("living being")) is a motile spe ...
. Complex courtship rituals have evolved in many arachnids to ensure the safe delivery of the sperm to the female. Members of many orders exhibit sexual dimorphism. Arachnids usually lay yolky
eggs Eggs are laid by female animals of many different species, including bird egg, birds, reptiles, amphibians, a few monotreme, mammals, and fish, and many of these have been eaten by humans for thousands of years. Bird and reptile eggs consist o ...
, which hatch into immatures that resemble adults. Scorpions, however, are either
ovoviviparous Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is an outmoded term used as a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animals have the embryos develop ...
or
viviparous Among animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aero ...
, depending on species, and bear live young. In most arachnids only the females provide parental care, with harvestmen being one of the few exceptions.


Taxonomy and evolution


Phylogeny

The
phylogenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...

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 300px, A cladogram of the primates, showing a ''monophyletic'' taxon: ''the simians'' (in yellow); a ''paraphyletic'' taxon: ''the prosimians'' (in cyan, including the red patch); and a ''polyphyletic'' group: ''the night-active primates, i.e., ...
group and are divided into three main clades: chelicerates (including arachnids), pancrustaceans (the
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a few—typically only one or two—Monophyly, monophyletic subgroups. The group is said to be paraphyleti ...
crustaceans plus insects and their allies), and myriapods (centipedes, millipedes and allies). The three groups are related as shown in the
cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics Cladistics (, from Greek language, Greek , ''kládos'', "branch") is an approach to Taxonomy (biology), biological classification in whi ...

cladogram
below. Including fossil taxa does not fundamentally alter this view, although it introduces some additional basal groups. 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. (Pycnogonida (sea spiders) may be excluded from the chelicerates, which are then identified as the group labelled "Euchelicerata".) A 2019 analysis nests Xiphosura deeply within Arachnida. Discovering relationships within the arachnids has proven difficult , 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 The Acariformes, also known as the Actinotrichida, are the most diverse of the two superorders of mite Mites are small arachnids (eight-legged arthropods). Mites are not a defined taxon, but the name is used for members of several groups of ...
,
Parasitiformes Parasitiformes is an order of Acari (treated as a suborder and superorder in outdated classifications). An alternative name is Anactinotrichida. Parasitiformes is one of two groups (orders) in Acari, the other being Acariformes (Actinotrichida).E ...
and , which have had much faster evolutionary rates. Analyses of the data using sets of genes with different evolutionary rates produced mutually incompatible
phylogenetic tree , based on completely sequenced genomes. A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram A diagram is a symbolic representati ...

phylogenetic tree
s. 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.
Tetrapulmonata Tetrapulmonata is a non-ranked supra-ordinal clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common a ...
, here consisting of
Araneae Spiders ( order Araneae) are air-breathing arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euar ...

Araneae
,
Amblypygi Amblypygi is an ancient order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or was ...

Amblypygi
and
Thelyphonida Thelyphonida is an arachnid Arachnida () is a Class (biology), class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes orders containing spiders (the largest order), scorpions, ticks, mites, o ...

Thelyphonida
(
Schizomida Schizomida (common name shorttailed whipscorpion) is an Order (biology), order of arachnids, generally less than in length. The order is not yet widely studied. As of 2005, more than 230 species of schizomids have been described worldwide, most ...
was not included in the study), received strong support. The addition of Scorpiones to produce a clade called Arachnopulmonata was also well supported. Pseudoscorpiones may also belong here, possibly as the sister of Scorpiones. Somewhat unexpectedly, there was support for a clade comprising
Opiliones The Opiliones (formerly Phalangida) are an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, ...

Opiliones
,
Ricinulei Ricinulei is an order of arachnids. Like most arachnids, they are predatory, eating small arthropods. In older works they are sometimes referred to as Podogona. , 76 extant species of ricinuleids have been described worldwide, all in the single f ...

Ricinulei
and
Solifugae Solifugae is an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, an ...

Solifugae
, a combination not found in most other studies. In early 2019, a molecular phylogenetic analysis placed the horseshoe crabs,
Xiphosura Xiphosura () is an order of arthropods related to arachnids. They are sometimes called horseshoe crabs (a name applied more specifically to the only extant family, Horseshoe crab, Limulidae). They first appeared in the Hirnantian (Late Ordovician) ...

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 In phylogenetics In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiolo ...
. Morphological analyses including fossils tend to recover the Tetrapulmonata, including the extinct group the Haptopoda, but recover other ordinal relationships with low support.


Fossil history

The Uraraneida are an extinct order of spider-like arachnids from the Devonian and Permian. 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 Mesothelae, mesotheles.


Taxonomy

The subdivisions of the arachnids are usually treated as Order (biology), orders. Historically,
mite Mites are small arachnid Arachnida () is a Class (biology), class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes orders containing spiders (the largest order), scorpions, ticks, mites, op ...
s and
tick Ticks (suborder Ixodida) are parasitic s that are part of the superorder . Adult ticks are approximately 3 to 5 mm in length depending on age, sex, species, and "fullness". Ticks are external s, living by of mammals, birds, and sometim ...

tick
s 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. *
Acariformes The Acariformes, also known as the Actinotrichida, are the most diverse of the two superorders of mite Mites are small arachnids (eight-legged arthropods). Mites are not a defined taxon, but the name is used for members of several groups of ...
– mites (32,000 species) *
Amblypygi Amblypygi is an ancient order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or was ...

Amblypygi
– "blunt rump" tail-less whip scorpions with front legs modified into whip-like sensory structures as long as 25 cm or more (153 species) *
Araneae Spiders ( order Araneae) are air-breathing arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euar ...

Araneae
– spiders (40,000 species) *† Haptopoda – extinct arachnids apparently part of the
Tetrapulmonata Tetrapulmonata is a non-ranked supra-ordinal clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common a ...
, the group including spiders and whip scorpions (1 species) * Opilioacariformes – harvestman-like mites (10 genera) *
Opiliones The Opiliones (formerly Phalangida) are an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, ...

Opiliones
– phalangids, harvestmen or daddy-long-legs (6,300 species) * Palpigradi – microwhip scorpions (80 species) *
Parasitiformes Parasitiformes is an order of Acari (treated as a suborder and superorder in outdated classifications). An alternative name is Anactinotrichida. Parasitiformes is one of two groups (orders) in Acari, the other being Acariformes (Actinotrichida).E ...
– ticks (12,000 species) *†Phalangiotarbi – extinct arachnids of uncertain affinity (30 species) * Pseudoscorpionida – pseudoscorpions (3,000 species) *
Ricinulei Ricinulei is an order of arachnids. Like most arachnids, they are predatory, eating small arthropods. In older works they are sometimes referred to as Podogona. , 76 extant species of ricinuleids have been described worldwide, all in the single f ...

Ricinulei
– ricinuleids, hooded tickspiders (60 species) *
Schizomida Schizomida (common name shorttailed whipscorpion) is an Order (biology), order of arachnids, generally less than in length. The order is not yet widely studied. As of 2005, more than 230 species of schizomids have been described worldwide, most ...
– "split middle" whip scorpions with divided exoskeletons (220 species) *
Scorpion Scorpions are predatory arachnids of the Order (biology), order Scorpiones. They have eight legs, and are easily recognized by a pair of Chela (organ), grasping pincers and a narrow, segmented tail, often carried in a characteristic forward curv ...

Scorpion
es – scorpions (2,000 species) *
Solifugae Solifugae is an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, an ...

Solifugae
– solpugids, windscorpions, sun spiders or camel spiders (900 species) *
Thelyphonida Thelyphonida is an arachnid Arachnida () is a Class (biology), class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes orders containing spiders (the largest order), scorpions, ticks, mites, o ...

Thelyphonida
(also called Uropygi) – whip scorpions or vinegaroons, forelegs modified into sensory appendages and a long tail on abdomen tip (100 species) *†Trigonotarbida – extinct (late Silurian Cisuralian, Early Permian) *†Uraraneida – extinct spider-like arachnids, but with a "tail" and no Spinneret (spider), spinnerets (2 species) *
Xiphosura Xiphosura () is an order of arthropods related to arachnids. They are sometimes called horseshoe crabs (a name applied more specifically to the only extant family, Horseshoe crab, Limulidae). They first appeared in the Hirnantian (Late Ordovician) ...

Xiphosura
– horseshoe crabs (4 living species) It is estimated that 98,000 arachnid species have been described, and that there may be up to 600,000 in total.


See also

*Arachnophobia *Endangered spiders *Glossary of spider terms *List of extinct arachnids


References


External links


Arachnid
Natural History Museum, London {{Authority control Arachnids, Llandovery first appearances Extant Silurian first appearances