Arachnids are a class (
Arachnida) of
joint-leggedAn arthropod is an invertebrate that has an exoskeleton , a segmented body, and jointed attachments called appendages. Arthropods are animals belonging to the Phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
invertebrateAn invertebrate is an animal without a vertebral column. The group includes 95% of all animal species — all animals except those in the Chordate subphylum Vertebrata ....
animalAnimals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously...
s in the subphylum
ChelicerataThe subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites. They originated as marine animals, possibly in the Cambrian period, but the first confirmed chelicerate fossils, eurypterids, date from in the...
. All arachnids have eight legs, although in some species the front pair may convert to a sensory function. The term is derived from the
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
word (
aráchnē), meaning "spider".
Most arachnids are terrestrial. However, some inhabit freshwater environments and, with the exception of the
pelagic zoneAny water in the sea that is not close to the bottom is in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means open sea....
, marine environments as well. They comprise over 100,000 named
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
, including
spiderSpiders are air-breathing chelicerate arthropods that have eight legs, and chelicerae modified into fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
s,
scorpionScorpions are predatory arthropod animals of the order Scorpiones within the class Arachnida. There are about 2,000 species of scorpions, found widely distributed south of about 49° N, except New Zealand and Antarctica...
s,
harvestmenHarvestmen are eight-legged invertebrate animals belonging to the order Opiliones in the class Arachnida, in the subphylum Chelicerata of the phylum Arthropoda. , over 6,400 species of harvestmen have been discovered worldwide, although the real number of extant species may exceed 10,000...
,
tickTick is the common name for the small arachnids in superfamily Ixodoidea that, along with other mites, constitute the Acarina. Ticks are ectoparasites , living by hematophagy on the blood of mammals, birds, and occasionally reptiles and amphibians...
s, and
miteMites, along with ticks, belong to the subclass Acarina and the class Arachnida. Mites are among the most diverse and successful of all the invertebrate groups. They have exploited an incredible array of habitats, and because of their small size most go totally unnoticed...
s.
Anatomy
Almost all adult arachnids have four pairs of legs, and arachnids may be easily distinguished from
insectInsects are arthropods, having a hard exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae. They are the most diverse group of animals on the planet and include more than a million species that are already described. Insects represent more than half of all...
s by this fact, since insects 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
pedipalpPedipalps , are the second pair of appendages of the prosoma in the subphylum Chelicerata. They are traditionally thought to be homologous with mandibles in Crustacea and insects, although more recent studies Pedipalps (commonly shortened to palps or palpi), are the second pair of appendages of the...
s have been adapted for feeding, locomotion, and/or reproductive functions. In
SolifugaeSolifugae is an order of Arachnida, containing more than 1,000 described species in about 140 genera. The name derives from Latin, and means those that flee from the sun. The order is also known by the names Solpugida, Solpugides, Solpugae, Galeodea and Mycetophorae...
, the palps are quite leg-like, so that these animals appear to have ten legs. The
larvaA larva is a young form of animal with indirect development, going through or undergoing metamorphosis ....
e of mites and
RicinuleiThe Order Ricinulei is a group of arachnids known as hooded tickspiders. In older works they are sometimes referred to as Podogona.As of 2007, approximately 60 species of ricinuleids have been described worldwide, all in the single family Ricinoididae They occur today in west-central Africa and...
have only six legs; the fourth pair appears when they
moultEcdysis is the moulting of the cuticula in arthropods and related groups . Since the cuticula of these animals is also the skeletal support of the body and is inelastic, it is shed during growth and a new, larger covering is formed...
into
nymphIn biology, a nymph is the immature form of some invertebrates, particularly insects, which undergoes in gradual metamorphosis before reaching its adult stage. Unlike a typical larva, a nymph's overall form already resembles that of the adult. In addition, while a nymph moults it never enters a...
s. However, there are also adult mites with six, or even four legs.
Arachnids are further distinguished from insects by the fact they have no
antennaeAntennae in biology have historically been paired appendages used for sensing in arthropods and crustaceans. More recently, the term has also been applied to cilium structures present in many cell types of eukaryotes....
or
wingsInsect wings are outgrowths of the insect exoskeleton that enable insects to fly. They are found on the second and third thoracic segments , and the two pairs are often referred to as the forewings and hindwings, respectively, though a few insects lack hindwings, even rudiments...
. Their body is organized into two
tagmaA tagma may refer to:*a tagma , a subdivision of the Byzantine army*a tagma , a grouping of segments...
called the prosoma, or
cephalothoraxThe cephalothorax is an anatomical term used in arachnids and malacostracan crustaceans for the first major body section. The remainder of the body is the abdomen , which may also bear lateral appendages as well as the tail, if present...
, and the
opisthosomaThe opisthosoma is the posterior portion of the arachnids body behind the prosoma . The number of segments and appendages on the opisthosoma vary. Scorpions have 13, but the first is only seen during its embryological development. Other arachnids have twelve or less...
, or
abdomenIn vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
. The cephalothorax is derived from the fusion of the
cephalonCephalon, Inc. is a U.S. biopharmaceutical company co-founded in 1987 by Dr. Frank Baldino, Jr., a pharmacologist and former scientist with the DuPont Company, who continues to serve as its chairman and chief executive officer...
(head) and the
thoraxThe thorax is a division of an animal's body that lies between the head and the abdomen.In mammals, the thorax is the region of the body formed by the sternum, the thoracic vertebrae and the ribs. It extends from the neck to the diaphragm, and does not include the upper limbs. The heart and the...
, and 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.
Like all arthropods, arachnids have an
exoskeletonAn exoskeleton is an external skeleton that supports and protects an animal's body, in contrast to the internal endoskeleton of, for example, a human. Some animals, such as the tortoise, have both an endoskeleton and an exoskeleton...
, and they also have an internal structure of
cartilageCartilage is a stiff yet flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
-like tissue called the endosternite, to which certain muscle groups are attached. The endosternite is even calcified in some
OpilionesHarvestmen are eight-legged invertebrate animals belonging to the order Opiliones in the class Arachnida, in the subphylum Chelicerata of the phylum Arthropoda. , over 6,400 species of harvestmen have been discovered worldwide, although the real number of extant species may exceed 10,000...
.
Physiology
There are some important modifications that are particularly important for the terrestrial lifestyle of an arachnid, such as internal respiratory surfaces in the form of
tracheaTerrestrial arthropods have evolved an open respiratory system composed of spiracles, tracheae, and tracheoles to transport metabolic gasses to and from tissue. Some terrestrial woodlice have evolved pseudotrachea, a system which is also called corpus alatum, and is made up of air tubes that...
e, or modification of the book gill into a
book lungA book lung is a type of respiration organ used for atmospheric gas exchange and is found in arachnids, such as scorpions and spiders. Each of these organs is found inside a ventral abdominal cavity and connects with the surroundings through a small opening. Book lungs are not related to the lungs...
, an internal series of
vascularIn zoology, "vascular" means "related to blood vessels", which are part of the Circulatory system. An organ or tissue that is vascularized is heavily endowed with blood vessels and thus richly supplied with blood....
lamellaLamella is a term for a platelike structure, appearing in multiples, that occurs in various situations, such as biology or materials sciences. It implies a thin layer , the same derivation as for `laminate'...
e used for
gas exchangeGas exchange or respiration takes place at a respiratory surface—a boundary between the external environment and the interior of the body. For unicellular organisms the respiratory surface is governed by Fick's law, which determines that respiratory surfaces must have:*a large surface area*a thin...
with the air. While the tracheae are often individual systems of tubes, similar to those in insects, ricnuleids, pseudoscorpions, and some spiders possess sieve tracheae, in which several tubes arise in a bundle from a small chamber connected to the
spiracleSpiracles are small openings on the surface of some animals that usually lead to respiratory systems.In elasmobranchs , a spiracle is found behind each eye, and is often used to pump water through the gills while the animal is at rest .-Spiracles in insects:Insects and some more advanced spiders...
. 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
appendageAn appendage in the broadest sense is an additional or subsidiary part existing on, or added to, something which can generally still function if the appendage has never existed or is later provided or grown, or will still perform a primary function if the appendage is removed.- Biological context...
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 glandThe coxal gland is a gland in some arthropods for collecting and excreting urine. The excretion can take place at the base of the legs or, in some higher crustaceans, of the antennae or maxillae .The green gland is divided into three parts namely the end sac, the...
s 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
guanineGuanine is one of the five main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, thymine, and uracil. In DNA, guanine is paired with cytosine. With the formula C
5H
5N
5O, guanine is a derivative of purine, consisting of a fused...
.
The blood of arachnids 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
heartThe heart is a muscular organ found in all vertebrates that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
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
carnivorousA carnivore , meaning 'meat eater' , is an animal that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of vertebrate and/or invertebrate animal tissue, whether through predation or scavenging...
, feeding on the pre-digested bodies of insects and other small animals. Only in the harvestmen and among
mitesMITES, or Minority Introduction to Engineering and Science, is a six-week summer program for rising high school seniors held at the Massachusetts Institute of Technology. Its purpose is to expose students from minority, or otherwise disadvantaged backgrounds, to the fields of science and engineering...
, such as the
house dust miteThe house dust mite , is a cosmopolitan guest in human habitation. Dust mites feed on organic detritus such as flakes of shed human skin and flourish in the stable environment of dwellings. House dust mites are a common cause of asthma and allergic symptoms worldwide...
, 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
venomVenom is any of a variety of toxins used by certain types of animals. Generally, venom is injected by such means as a bite or a sting.-The distinction between venom and poison:...
from specialized
glandA gland is an organ in an animal's body that synthesizes a substance for release such as hormones or breast milk, often into the bloodstream or into cavities inside the body or its outer surface .- Types :...
s to kill prey or enemies. Several mites are parasites, some of which are carriers of
diseaseA disease or medical condition isan abnormal condition of an organism that impairs bodily functions, associated with specific symptoms and signs...
.
Arachnids pour digestive juices produced in their stomachs over their prey after killing it with their pedipalps and chelicerae. 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
pharynxThe pharynx is the part of the neck and throat situated immediately posterior to the mouth and nasal cavity, and cranial, or superior, to the esophagus, larynx, and trachea.-Functions:...
, which acts as a pump, sucking the food through the mouth and on into the oesophagus and
stomachIn most mammals, the stomach is a hollow, muscular organ of the gastrointestinal tract , between the esophagus and the small intestine. It is involved in the second phase of digestion, following mastication . The word stomach is derived from the Latin stomachus, which derives from the Greek word...
. 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
intestineIn anatomy, the intestine is the segment of the alimentary canal extending from the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
and
anusThe anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to expel feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest, such as bones; food...
in the hind part of the abdomen.
Senses
Arachnids have two kinds of eyes, the lateral and median ocelli. The lateral ocelli evolved from compound eyes and may have a
tapetumThe tapetum lucidum is a layer of tissue in the eye of many vertebrate animals, that lies immediately behind or sometimes within the retina. It reflects visible light back through the retina, increasing the light available to the photoreceptors...
, which enhances the ability to collect light. The median ocelli develop from a transverse fold of the
ectodermThe ectoderm is the start of a tissue that covers the body surfaces. It emerges first and forms from the outermost of the germ layers.Generally speaking, the ectoderm differentiates to form the nervous system, and the epidermis .In vertebrates, the ectoderm has three parts: external ectoderm , the ...
. The ancestors of modern arachnids probably had both types, but modern ones often lack one type or the other. The
corneaThe cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43...
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
retinaThe vertebrate retina is a light sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
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
trichobothriaTrichobothria are elongate setae, present in the arachnids, and various orders of insects, that function in the detection of airborne vibrations and currents.-Morphology:...
.
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
proprioceptionProprioception is the sense of the relative position of neighbouring parts of the body...
, and possibly also hearing.
Reproduction
Arachnids may have one or two
gonadThe gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
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
spermatophoreA spermatophore is a capsule or mass created by males of various animal species, containing spermatozoa and transferred in entirety to the female's ovipore during copulation...
. Complex courtship rituals have evolved in many arachnids to ensure the safe delivery of the sperm to the female.
Arachnids usually lay yolky
eggsIn most birds and reptiles, an egg is the zygote, resulting from fertilization of the ovum. To enable incubation the egg is usually kept within a favourable temperature range as it nourishes and protects the growing embryo. When the embryo is adequately developed it breaks out of the egg in the...
, which hatch into immatures that resemble adults. Scorpions, however, are either ovoviviparous or viviparous, depending on species, and bear live young.
Systematics
- † Trigonotarbida
The Order Trigonotarbida is an extinct group of 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...
— extinct
- Amblypygi
Amblypygi is an order of invertebrate animals belonging to the class Arachnida, in the subphylum Chelicerata of the phylum Arthropoda.Amblypygids are also known as whip spiders and tailless whip scorpions...
— "blunt rump" tailless whip scorpions with front legs modified into whipThe word whip describes two basic types of tools:A long stick-like device, usually slightly flexible, with a small bit of leather or cord, called a "popper", on the end. Depending on length and flexibility, this type is often called a riding whip, riding crop or "bat"...
-like sensory structures as long as 25 cm or more (140 species)
Phylogeny of the Chelicerata
(after Giribet et al. 2002)
- Araneae — spiders (40,000 species)
- Mesothelae
The Mesothelae are a suborder of spiders that includes the extinct families Arthrolycosidae and Arthromygalidae and the only extant family Liphistiidae....
— very rare, basalIn phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
spiders, with abdomen segmented and spinnerets median
- Opisthothelae — spiders with abdomen unsegmented and spinnerets located posteriorly
- Araneomorphae
The Araneomorphae are a suborder of spiders. They are distinguished by having chelicerae that point diagonally forward and cross in a pinching action, in contrast to the Mygalomorphae , where they point straight down...
— most common spiders
- Mygalomorphae
The Mygalomorphae, , are an infraorder of spiders. The latter name comes from the orientation of the fangs which point straight down and do not cross each other .-Description:...
— tarantulaTarantulas comprise a group of hairy and often very large spiders belonging mainly to the family Theraphosidae, of which approximately 900 species have been identified. Historically tarantulas were the bigger genera from the family Lycosidae...
s and tarantula-like spiders
- † Phalangiotarbida — extinct
- Opiliones
Harvestmen are eight-legged invertebrate animals belonging to the order Opiliones in the class Arachnida, in the subphylum Chelicerata of the phylum Arthropoda. , over 6,400 species of harvestmen have been discovered worldwide, although the real number of extant species may exceed 10,000...
— phalangids, harvestmen or daddy-long-legs (6,300 species)
- Palpigradi — microwhip scorpions (80 species)
- Pseudoscorpionida — pseudoscorpions (3,000 species)
- Ricinulei
The Order Ricinulei is a group of arachnids known as hooded tickspiders. In older works they are sometimes referred to as Podogona.As of 2007, approximately 60 species of ricinuleids have been described worldwide, all in the single family Ricinoididae They occur today in west-central Africa and...
— ricinuleids, hooded tickspiders (60 species)
- Schizomid
Schizomida is an order of arachnids, superficially resembling spiders and generally less than in length....
a — "split middle" whip scorpions with divided exoskeletons (220 species)
- Scorpiones — scorpions (2,000 species)
- Solifugae
Solifugae is an order of Arachnida, containing more than 1,000 described species in about 140 genera. The name derives from Latin, and means those that flee from the sun. The order is also known by the names Solpugida, Solpugides, Solpugae, Galeodea and Mycetophorae...
— solpugids, windscorpions, sun spiders or camel spiders (900 species)
- † Haptopoda
Haptopoda is an extinct arachnid order known exclusively from only eight specimens from the Upper Carboniferous of Coseley, Staffordshire, UK. It is monotypic, i.e. has only one species, Plesiosiro madeleyi described by Reginald Pocock in his important 1911 monograph on British Carboniferous...
— extinct
- Thelyphonida — vinegarroons or whip scorpions (formerly uropygida) forelegs modified into sensory appendages and a long tail on abdomen tip (100 species)
- Acarina
Acarina or Acari are a taxon of arachnids that contains mites and ticks. The diversity of the Acari is extraordinary and its fossil history goes back to at least the early Devonian era; possibly even the Ordovician. As a result, acarologists have proposed a complex set of taxonomic ranks to...
— miteMites, along with ticks, belong to the subclass Acarina and the class Arachnida. Mites are among the most diverse and successful of all the invertebrate groups. They have exploited an incredible array of habitats, and because of their small size most go totally unnoticed...
s and tickTick is the common name for the small arachnids in superfamily Ixodoidea that, along with other mites, constitute the Acarina. Ticks are ectoparasites , living by hematophagy on the blood of mammals, birds, and occasionally reptiles and amphibians...
s (30,000 species)
- Acariformes
The Acariformes are the most diverse of the two superorders of mites. There are over 32,000 described species in 351 families, and a total estimates of 440,000 to 929,000.-Systematics and taxonomy:...
- Sarcoptiformes
- Trombidiformes
- Opilioacariformes
- Parasitiformes
The Parasitiformes are a superorder of Acari . Many species are parasitic , but not all; for example, about half of the 10,000 known species in the suborder Mesostigmata are predatory and cryptozoan, living in the soil-litter, rotting wood, dung, carrion, nests or house dust...
— holothyrans, ticks and mesostigmatic mites
It is estimated that a total of 98,000 arachnid species have been described, and that there may be up to 600,000 in total, including undescribed species.
Acarina
Acarina or
Acari are a
taxon|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement. Defining what belongs or does not belong to such a...
of arachnids that contains
miteMites, along with ticks, belong to the subclass Acarina and the class Arachnida. Mites are among the most diverse and successful of all the invertebrate groups. They have exploited an incredible array of habitats, and because of their small size most go totally unnoticed...
s and
tickTick is the common name for the small arachnids in superfamily Ixodoidea that, along with other mites, constitute the Acarina. Ticks are ectoparasites , living by hematophagy on the blood of mammals, birds, and occasionally reptiles and amphibians...
s. Its fossil history goes back to the
DevonianThe Devonian is a geologic period and system of the Paleozoic era spanning from . It is named after Devon, England, where rocks from this period were first studied....
era, although there is also a questionable
OrdovicianThe Ordovician is a geologic period and system, the second of six of the Paleozoic era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian period and is followed by the Silurian period...
record. The Devonian era was the time frame in which certain species of animals developed legs. In most modern treatments, the Acari is considered a
subclassSubclass may refer to:* Class , a taxonomic rank intermediate between class and superorder* Subclass , a class that is derived from another class or classes...
of Arachnida and is composed of 2–3 orders or superorders:
AcariformesThe Acariformes are the most diverse of the two superorders of mites. There are over 32,000 described species in 351 families, and a total estimates of 440,000 to 929,000.-Systematics and taxonomy:...
,
ParasitiformesThe Parasitiformes are a superorder of Acari . Many species are parasitic , but not all; for example, about half of the 10,000 known species in the suborder Mesostigmata are predatory and cryptozoan, living in the soil-litter, rotting wood, dung, carrion, nests or house dust...
, and Opilioacariformes. Most acarines are minute to small (e.g. 0.080–1.00 mm), but the giants of the Acari (some ticks and red velvet mites) may reach lengths of 10–20 mm. It is estimated that over 50,000 species have been described (as of 1999) and that a million or more species are currently living. The study of mites and ticks is called acarology.
Only the faintest traces of primary segmentation remain in mites, the prosoma and opisthosoma being insensibly fused, and a region of flexible cuticle (the cirumcapitular furrow) separates the chelicerae and pedipalps from the rest of the body. This anterior body region is called the
capitulumThe term Capitulum can refer to several things:*In botany, a type of flower head where the bracts are located under the basis, such as a Daisy's*In anatomy, the capitulum of the humerus*The Latin word for a Chapter...
or gnathosoma and is also found in the
RicinuleiThe Order Ricinulei is a group of arachnids known as hooded tickspiders. In older works they are sometimes referred to as Podogona.As of 2007, approximately 60 species of ricinuleids have been described worldwide, all in the single family Ricinoididae They occur today in west-central Africa and...
. The remainder of the body is called the
idiosomaIdiosoma is a genus of trapdoor spiders in the family Idiopidae with three species, found only in Western Australia.The genus was transferred from Ctenizidae to Idiopidae in 1985....
and is unique to mites. Most adult mites have four pairs of legs, like other arachnids, but some have fewer. For example, gall mites like
Phyllocoptes variabilis (superfamily Eriophyioidea) have a wormlike body with only two pairs of legs; some parasitic mites have only one or three pairs of legs in the adult stage. Larval and prelarval stages have a maximum of three pairs of legs; adult mites with only three pairs of legs may be called 'larviform'.
Acarine
ontogenyOntogeny describes the origin and the development of an organism from the fertilized egg to its mature form...
consists of an egg, a prelarval stage (often absent), a larval stage (hexapod except in Eriophyoidea, which have only 2 pairs of legs), and a series of nymphal stages. Larvae (and prelarvae) have a maximum of 3 pairs of legs (legs are often reduced to stubs or absent in prelarvae); legs IV are added at the first nymphal stage.
Acarines live in practically every
habitatAn ecoregion , sometimes called a bioregion, is an ecologically and geographically defined area smaller than a "realm" or "ecozone". Ecoregions cover relatively large areas of land or water, and contain characteristic, geographically distinct assemblages of natural communities and species...
, and include aquatic (freshwater and sea water) and terrestrial species. They outnumber other
arthropodAn arthropod is an invertebrate that has an exoskeleton , a segmented body, and jointed attachments called appendages. Arthropods are animals belonging to the Phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s in the soil organic matter and
detritusIn biology, detritus is non-living particulate organic material . It typically includes the bodies or fragments of dead organisms as well as fecal material...
. Many are
parasiticParasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the host....
, and they affect both
vertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
s and
invertebrateAn invertebrate is an animal without a vertebral column. The group includes 95% of all animal species — all animals except those in the Chordate subphylum Vertebrata ....
s. Most parasitic forms are external parasites, while the free living forms are generally
predaceousIn ecology, predation describes a biological interaction where a predator feeds on its prey, . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of the prey...
and may even be used to control undesirable arthropods. Others are
detritivoreDetritivores, also known as detritus feeders or saprophages, are heterotrophs that obtain nutrients by consuming detritus . By doing so, they contribute to decomposition and the nutrient cycles....
s that help to break down forest
litterPlant litter is dead plant material, such as leaves, bark, and twigs, that has fallen to the ground. Litter provides habitat for small animals, fungi, and plants, and the material may be used to construct nests. As litter decomposes, nutrients are released to the environment...
and dead organic matter such as
skinThe skin is the outer covering of the body. In humans, it is the largest organ of the integumentary system made up of multiple layers of mesodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs. Skin of a different nature exists in amphibians, reptiles, birds...
cells. Others still are plant feeders and may damage
cropsA crop is the annual or season's yield of any plant that is grown in significant quantities to be harvested as food, as livestock fodder, fuel, or for any other economic purpose. This category includes crop species as well as agricultural techniques related to cropping.There are many types of crops...
. Damage to crops is perhaps the most costly economic effect of mites, especially by the spider mites and their relatives (Tetranychoidea), earth mites (
PenthaleidaePenthaleidae, also referred to as earth mites, are a family of mites that are major winter pests of a variety of crops and pastures in southern Australia....
), thread-footed mites (
TarsonemidaeTarsonemidae is a family of mites, also called thread-footed mites or white mites.Only a limited number of tarsonemid genera are known to feed on higher plants while most species in this family feed on the thin-walled mycelia of fungi or possibly algal bodies...
) and the gall and rust mites (Eriophyoidea). Some parasitic forms affect humans and other
mammalMammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...
s, causing damage by their feeding, and can even be vectors of diseases such as
scrub typhusScrub typhus is a form of typhus caused by Orientia tsutsugamushi.Although it is similar in presentation to other forms of typhus, it is caused by an agent in a different Genus, and is frequently classified separately from the other typhi....
and
rickettsiaRickettsia is a genus of motile, Gram-negative, non-sporeforming, highly pleomorphic bacteria that can present as cocci , rods or thread-like . Obligate intracellular parasites, the Rickettsia survival depends on entry, growth, and replication within the cytoplasm of eukaryotic host cells...
l pox. A well-known effect of mites on humans is their role as an
allergenAn allergen is a nonparasitic antigen capable of stimulating a type-I hypersensitivity reaction in atopic individuals.Most humans mount significant Immunoglobulin E responses only as a defense against parasitic infections. However, some individuals mount an IgE response against common...
and the stimulation of
asthmaAsthma is a predisposition to chronic inflammation of the lungs in which the airways are reversibly narrowed. Asthma affects 7% of the population of the United States, and 300 million worldwide...
in people affected by the respiratory disease. The use of predatory mites (e.g.
PhytoseiidaePhytoseiidae is a family of mites which feed on thrips and other mite species. They are often used as a biological control agent for managing mite pests.-Amblyseiinae:Amblyseiinae Muma, 1961...
) in
pest controlPest control refers to the regulation or management of a species defined as a pest, usually because it is perceived to be detrimental to a person's health, the ecology or the economy....
and herbivorous mites that attack
weedA weed in a general sense is a plant that is considered by the user of the term to be a nuisance, and normally applied to unwanted plants in human-made settings such as gardens, lawns or agricultural areas, but also in parks, woods and other natural areas. More specifically, the term is often...
s are also of importance. An unquantified, but major positive contribution of the Acari is their normal functioning in
ecosystemAn ecosystem is a system of interdependent organisms which share the same habitat, in an area functioning together with all of the physical factors of the environment. Ecosystems can be permanent or temporary. Ecosystems usually form a number of food webs...
s, especially their roles in the decomposer subsystem.
Amblypygi
Amblypygids are also known as
tailless whip scorpions or
cave spiders. Approximately 5 families, 17 genera and 136 species have been described. They are found in
tropicalThe tropics,the tropics are very hot. hi everyone! peace out!seated in the equatorial regions of the world, are limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately 23°26' N latitude and the Tropic of Capricorn in the southern hemisphere at 23°26' S latitude...
and
subtropicalThe subtropics are the geographical zone of the Earth immediately north and south of the tropical zone, which is bounded by the Tropic of Cancer and the Tropic of Capricorn, at latitudes 23.5°N and 23.5°S...
regions worldwide. Some species are subterranean; many are
nocturnalNocturnality is an animal behavior characterized by being active during the night and sleeping during the day. The opposite, diurnality, is associated with human lifestyle and most familiar animal species. The intermediate crepuscular schedule is also common. Some species are active both during...
. During the day, they may hide under logs, bark, stones, or leaves. They prefer a humid environment. Amblypygids may range from 5 to 40 mm. Their bodies are broad and highly flattened and the first pair of legs (the first walking legs in most arachnid orders) are modified to act as sensory organs. (Compare solifugids,
uropygidThelyphonida is an arachnid order comprising invertebrates commonly known as vinegarroons. They are often called uropygids in the scientific community after the former order Uropygi...
s, and
schizomidSchizomida is an order of arachnids, superficially resembling spiders and generally less than in length....
s.) These very thin modified legs can extend several times the length of body. They have no
silkSilk is a natural protein fiber, some forms of which can be woven into textiles. The best-known type of silk is obtained from cocoons made by the larvae of the mulberry silkworm Bombyx mori reared in captivity...
glands or
venomVenom is any of a variety of toxins used by certain types of animals. Generally, venom is injected by such means as a bite or a sting.-The distinction between venom and poison:...
ous fangs, but can have prominent pincer-like
pedipalpPedipalps , are the second pair of appendages of the prosoma in the subphylum Chelicerata. They are traditionally thought to be homologous with mandibles in Crustacea and insects, although more recent studies Pedipalps (commonly shortened to palps or palpi), are the second pair of appendages of the...
s. Amblypygids often move about sideways on their six walking legs, with one "whip" pointed in the direction of travel while the other probes on either side of them. Prey are located with these "whips", captured with pedipalps, then torn to pieces with
cheliceraeThe chelicerae are mouth parts of the Chelicerata, an arthropod subphylum that includes arachnids, Merostomata , and Pycnogonida . Chelicerae are pointed appendages which are used to grasp food, and are found in place of the chewing mandibles most other arthropods have...
.
FossilFossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous rock formations and sedimentary layers is known as the fossil record...
ised amblypygids have been found dating back to the
Carboniferous periodThe Carboniferous is a geologic period and system that extends from the end of the Devonian period, about 359.2 ± 2.5 Ma , to the beginning of the Permian period, about 299.0 ± 0.8 Ma ....
.
Amblypygids, particularly the species
Phrynus marginemaculatusPhrynus marginemaculatus , also known as the Tailless whip-scorpion, is neither fully scorpion or fully a spider. it resembles a cross between both. They can also be called amblypygids which means they lack a tail that many other similar species carry. They are found all over the world, mostly in...
and
Damon diademaDamon diadema is the species imported as the Tanzanian Giant Tailless Whipscorpion.- Description :These arachnids are from 4 to 28 mm long and resemble spiders. They are dull brown, with a flat body. Although they have four pairs of legs, they use only the three hind pairs for moving around...
, are thought to be one of the few species of arachnids that show signs of
social behaviorIn biology, psychology and sociology social behavior is behavior directed towards society, or taking place between, members of the same species. Behavior such as predation which involves members of different species is not social...
. Research conducted at
Cornell UniversityCornell University is a private university located in Ithaca, New York, USA, that is a member of the Ivy League.Cornell counts more than 255,000 living alumni, 28 Rhodes Scholars and 41 Nobel laureates affiliated with the university as faculty or students...
by entomologists suggests that mother amblypygids comfort their young by gently caressing the offspring with her feelers. Further, when two or more siblings were placed in an unfamiliar environment, such as a cage, they would seek each other out and gather back in a group.
Araneae
Araneae, or spiders, are the most familiar of the arachnids, and the most numerous, if only described species are counted. All spiders produce
silkSpider silk, also known as gossamer, is a protein fiber spun by spiders. Spiders use their silk to make webs or other structures, which function as nets to catch other animals, or as nests or cocoons for protection for their offspring...
, a thin, strong
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
strand extruded by the spider from spinnerets most commonly found on the end of the abdomen. Many species use it to trap insects in
websA spider web, spiderweb, spider's web or cobweb is a device built by a spider out of proteinaceous spider silk extruded from its spinnerets.Insects can get trapped in spider webs, providing nutrition to the spider; however, not all spiders build...
, although there are many species that hunt freely. Silk can be used to aid in climbing, form smooth walls for burrows, build egg sacs, wrap prey, temporarily hold sperm, and even
flyBallooning is a term used for the mechanical kiting that many, especially smaller species of spiders, as well as certain mites and some caterpillars use to disperse through the air. Many small spiders use silk to lift themselves off a surface or use the silk as an anchor in mid air...
, among other applications.
All spiders except those in the families Uloboridae and
HolarchaeidaeThe Holarchaeidae are a spider family with only two described species in one genus.They are only up to 1.5mm in size and shiny black to beige in color...
, and in the suborder
MesothelaeThe Mesothelae are a suborder of spiders that includes the extinct families Arthrolycosidae and Arthromygalidae and the only extant family Liphistiidae....
(together about 350 species) can inject
venomVenom is any of a variety of toxins used by certain types of animals. Generally, venom is injected by such means as a bite or a sting.-The distinction between venom and poison:...
to protect themselves or to kill and liquefy prey. Only about 200 species, however, have bites that can pose health problems to humans. Many larger species' bites may be painful, but will not produce lasting health concerns.
Spiders are found all over the world, from the tropics to the Arctic, with some extreme species even living underwater in silken domes they supply with air, and on the tops of the highest mountains.
Haptopoda
Haptopoda is an extinct order known exclusively from a few specimens from the Upper
CarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian period, about 359.2 ± 2.5 Ma , to the beginning of the Permian period, about 299.0 ± 0.8 Ma ....
of the
United KingdomThe United Kingdom of Great Britain and Northern Ireland is a sovereign state located off the northwestern coast of continental Europe. It is an island country, spanning an archipelago including Great Britain, the northeastern part of Ireland, and many small islands...
. It is monotypic, i.e. has only one species:
Plesiosiro madeleyi Pocock 1911. Relationships with other arachnids are obscure, but closest relatives may be the Amblypygi, Thelyphonida and Schizomida of the tetrapulmonate clade.
Opiliones
Opiliones (better known as "
harvestmen" or "
daddy longlegs") are arachnids that are harmless to people and are known for their exceptionally long walking legs, compared to their body size. , over 6,300 species of Phalangids have been discovered worldwide. The order Opiliones can be divided in four suborders:
CyphophthalmiThe Cyphophthalmi are a suborder of harvestmen, with about 36 genera, and more than hundred described species.The six families are currently grouped into two infraorders, the Tropicophthalmi and the Temperophthalmi; however, these are not supported by modern phylogenetic analysis.They are smaller...
,
EupnoiThe Eupnoi are a suborder of harvestmen, with more than 200 genera, and about 1,700 described species.They consist of two superfamilies, the Phalangioidea with many long-legged species common to northern temperate regions, and the small group Caddoidea, which have prominent eyes and spiny...
,
DyspnoiThe Dyspnoi are a suborder of harvestmen, with about 32 genera, and about 320 described species.Several fossil species are known, including two extinct families.The superfamilies Ischyropsalidoidea and Troguloidea are monophyletic...
and
LaniatoresLaniatores is the largest suborder of the arachnid order Opiliones with over 4,000 described species worldwide. The majority of the species are highly dependent on humid environments and usually correlated with tropical and temperate forest habitats....
. Well-preserved fossils have been found in the 400-million year old
Rhynie chertThe Rhynie chert is an Early Devonian Lagerstättefound near the village of Rhynie, Aberdeenshire, Scotland, containing exceptionally preserved plant, fungus, lichen and animal material petrified in three dimensions by covering with fast-setting volcanic minerals...
s of Scotland, which look surprisingly modern, indicating that the basic structure of the harvestmen hasn't changed much since then.
The difference between harvestmen and spiders is that in harvestmen the two main body sections (the
abdomenIn vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
with ten segments and
cephalothoraxThe cephalothorax is an anatomical term used in arachnids and malacostracan crustaceans for the first major body section. The remainder of the body is the abdomen , which may also bear lateral appendages as well as the tail, if present...
, or
prosoma and
opisthosomaThe opisthosoma is the posterior portion of the arachnids body behind the prosoma . The number of segments and appendages on the opisthosoma vary. Scorpions have 13, but the first is only seen during its embryological development. Other arachnids have twelve or less...
) are nearly joined, so that they appear to be one
ovalAn oval is any curve resembling an egg or an ellipse but may also refer to:* A sporting arena of oval shape** a cricket field** an Australian rules football field** a Oval track used mainly in American auto racing...
structure. In more advanced species, the first five abdominal segments are often fused into a dorsal shield called the
scutumA scute or scutum is a bony external plate or scale, as on the shell of a turtle, the skin of crocodilians, or the feet of some birds.-Properties:Scutes are similar to scales and serve the same function...
, which is normally fused with the
carapaceA carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids as well as vertebrates such as chelonians, order Testudines, turtles and tortoises.-Crustaceans:In crustaceans, the carapace is a part of the exoskeleton...
. Sometimes this shield is only present in males. The two most posterior abdominal segments can be reduced or separated in the middle on the surface to form two plates lying next to each other. The second pair of legs are longer than the others and works as antennae. They have a single pair of eyes in the middle of their heads, orientated sideways. They have a pair of prosomatic
scent glandScent glands are found in the genital area of most mammals and in various other parts of the body, such as the underarms of humans and the preorbital glands of deer. They produce a semi-viscous fluid which contains pheromones. These odor-messengers indicate information such as status, territory...
s that secrete a peculiar smelling fluid when disturbed. Harvestmen do not have silk glands and do not possess poison glands, posing absolutely no danger to humans. They breathe through
tracheaeTerrestrial arthropods have evolved an open respiratory system composed of spiracles, tracheae, and tracheoles to transport metabolic gasses to and from tissue. Some terrestrial woodlice have evolved pseudotrachea, a system which is also called corpus alatum, and is made up of air tubes that...
. Between the base of the fourth pair of legs and the abdomen a pair of
spiracleSpiracles are small openings on the surface of some animals that usually lead to respiratory systems.In elasmobranchs , a spiracle is found behind each eye, and is often used to pump water through the gills while the animal is at rest .-Spiracles in insects:Insects and some more advanced spiders...
s are located, one opening on each side. In more active species, spiracles are also found upon the
tibiaThe tibia, shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates and connects the knee with the ankle bones.The tibia is named for the greek aulos flute, also known as a tibia.-In humans:...
of the legs. They have a
gonoporeA gonopore is a genital pore in some invertebrates and especially some insects.For insects, it is the opening of the genital duct. More specifically, in the unmodified female it is the opening of the common oviduct, and in the male, it is the opening of the ejaculatory duct....
on the ventral
cephalothoraxThe cephalothorax is an anatomical term used in arachnids and malacostracan crustaceans for the first major body section. The remainder of the body is the abdomen , which may also bear lateral appendages as well as the tail, if present...
, and the copulation is direct as the male has a
penisThe penis is an external sexual organ of certain biologically male organisms, in both vertebrates and invertebrates....
(while the female has an
ovipositorThe ovipositor is an organ used by some animals for oviposition, i.e. the laying of eggs. It consists of a maximum of three pairs of appendages formed to transmit the egg, to prepare a place for it, and to place it properly...
).
Typical body length does not exceed 7 mm (about ¼ in) even in the largest species. However, leg span is much larger and can exceed 160 mm (over 6 in). Most species live for a year. Many species are omnivorous, eating primarily small insects and all kinds of plant material and
fungiA fungus is any member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. The Fungi are classified as a kingdom that is separate from plants, animals and bacteria...
; some are
scavengerScavenging, or necrophagy, is a carnivorous feeding behaviour in which a predator consumes corpses or carrion that were not killed to be eaten by the predator or others of its species. Scavengers play an important role in the ecosystem by contributing to the decomposition of dead animal remains...
s of the decays of any dead animal, bird dung and other fecal material. They are mostly
nocturnalNocturnality is an animal behavior characterized by being active during the night and sleeping during the day. The opposite, diurnality, is associated with human lifestyle and most familiar animal species. The intermediate crepuscular schedule is also common. Some species are active both during...
and coloured in hues of brown, although there are a number of
diurnalDiurnality is an animal behavior characterized by being active during the day and sleeping at night. Animals that are not diurnal might be nocturnal or crepuscular . Many animal species are diurnal, including many mammals, insects and birds...
species that have vivid patterns in yellow, green and black with varied reddish and blackish mottling and reticulation.
Palpigradi
Palpigradi, commonly known as "microwhip scorpions", are tiny cousins of the
uropygidThelyphonida is an arachnid order comprising invertebrates commonly known as vinegarroons. They are often called uropygids in the scientific community after the former order Uropygi...
, or whip scorpion, no more than 3 mm in length. They have a thin, pale, segmented carapace that terminates in a whip-like flagellum, made up of 15 segments. The carapace is divided into two plates between the third and fourth leg set. They have no eyes. Some species have three pairs of
book lungA book lung is a type of respiration organ used for atmospheric gas exchange and is found in arachnids, such as scorpions and spiders. Each of these organs is found inside a ventral abdominal cavity and connects with the surroundings through a small opening. Book lungs are not related to the lungs...
s, while others have no lungs at all. Approximately 80 species of Palpigradi have been described worldwide, all in the family
Eukoeneniidae, which contains four genera.
They are believed to be predators like their larger relatives, feeding on minuscule insects in their habitat. Their mating habits are unknown, except that they lay only a few relatively large eggs at a time. Microwhip scorpions need a damp environment to survive, and they always hide from light, so they are commonly found in the moist earth under buried stones and rocks. They can be found on every continent, except in Arctic and Antarctic regions.
Phalangiotarbida
Phalangiotarbi (
HaaseHaase may refer to:*Haase , a Veteran Era car make*Barry Haase , Australian politician*Curt Haase , German soldier*Ernie Haase, American tenor*Friedrich Haase , German actor...
, 1890) is an extinct arachnid order known exclusively from the Palaeozoic (Devonian to Permian) of Europe and North America.
The affinities of phalangiotarbids are obscure, with most authors favouring affinities with Opiliones (harvestmen) and/or Acari (mites and ticks). Phalangiotarbida has been recently proposed to be sister group to (Palpigradi+Tetrapulmonata): the taxon Megoperculata sensu Shultz (1990). (Pollitt et al., 2004).
Pseudoscorpions
Pseudoscorpions are small arthropods with a flat, pear-shaped body and pincers that resemble those of
scorpionScorpions are predatory arthropod animals of the order Scorpiones within the class Arachnida. There are about 2,000 species of scorpions, found widely distributed south of about 49° N, except New Zealand and Antarctica...
s. They range from 2 to 8 mm ( to 1/3 inch) in length. The opisthosoma is made up of twelve segments, each guarded by plate-like tergites above and sternites below. The abdomen is short and rounded at the rear, rather than extending into a segmented tail and stinger like true scorpions. The colour of the body can be yellowish-tan to dark-brown, with the paired claws often a contrasting colour. They may have two, four or no eyes. They have two very long
palpal chelae (
pedipalpPedipalps , are the second pair of appendages of the prosoma in the subphylum Chelicerata. They are traditionally thought to be homologous with mandibles in Crustacea and insects, although more recent studies Pedipalps (commonly shortened to palps or palpi), are the second pair of appendages of the...
s or pincers) that strongly resemble the pincers found on a scorpion. The pedipalps generally consist of an immobile "hand" and "finger", with a separate movable finger controlled by an
adductor muscleThe term adductor muscle can have several meanings:* in a broad sense, any muscle that causes adduction can be called an adductor muscle. For example in biology, the muscle or muscles in the interior of a bivalve mollusk which serve to close the valves are called adductor muscles, and the large...
. A venom gland and duct are usually located in the mobile finger; the poison is used to capture and immobilise the pseudoscorpion's prey. During digestion, pseudoscorpions pour a mildly corrosive fluid over the prey, then ingest the liquefied remains. Pseudoscorpions spin silk from a gland in their jaws to make disk-shaped cocoons for mating, molting, or waiting out cold weather. Another trait they share with their closest relatives, the
spiderSpiders are air-breathing chelicerate arthropods that have eight legs, and chelicerae modified into fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
s, is breathing through
spiracleSpiracles are small openings on the surface of some animals that usually lead to respiratory systems.In elasmobranchs , a spiracle is found behind each eye, and is often used to pump water through the gills while the animal is at rest .-Spiracles in insects:Insects and some more advanced spiders...
s. Most spiders have one pair of spiracles, and one of book lungs, but pseudoscorpions do not have book lungs.
There are more than 2,000 species of pseudoscorpions recorded. They range worldwide, even in temperate to cold regions, but have their most dense and diverse populations in the
tropicsThe tropics,the tropics are very hot. hi everyone! peace out!seated in the equatorial regions of the world, are limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately 23°26' N latitude and the Tropic of Capricorn in the southern hemisphere at 23°26' S latitude...
and
subtropicsThe subtropics are the geographical zone of the Earth immediately north and south of the tropical zone, which is bounded by the Tropic of Cancer and the Tropic of Capricorn, at latitudes 23.5°N and 23.5°S...
. The fossil record of pseudoscorpions dates back over 380 million years, to the
DevonianThe Devonian is a geologic period and system of the Paleozoic era spanning from . It is named after Devon, England, where rocks from this period were first studied....
period, near the time when the first land-animal fossils appear.
During the elaborate mating dance, the male of some pseudoscorpion species pulls a female over a
spermatophoreA spermatophore is a capsule or mass created by males of various animal species, containing spermatozoa and transferred in entirety to the female's ovipore during copulation...
previously laid upon a surface. In other species, the male also pushes the sperm into the female genitals using the forelegs. The female carries the fertilised eggs in a brood pouch attached to her
abdomenIn vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
, and the young ride on the mother for a short time after they hatch. Up to two dozen young are hatched in a single
broodBrood may refer to:* Brood, a collective term for offspring* Brooding, the incubation of bird eggs by their parents* Brood , the young of a beehive*Brood: to think deeply about something, often in a dark or melancholy manner....
; there may be more than one brood per year. The young go through three
moltsEcdysis is the moulting of the cuticula in arthropods and related groups . Since the cuticula of these animals is also the skeletal support of the body and is inelastic, it is shed during growth and a new, larger covering is formed...
over the course of several years before reaching adulthood. Adult pseudoscorpions live 2 to 3 years. They are active in the warm months of the year, overwintering in silken coccoons when the weather grows cold.
Pseudoscorpions are generally beneficial to humans since they prey on
clothes mothA moth is an insect closely related to the butterfly, both being of the order Lepidoptera. The differences between butterflies and moths are more than just taxonomy. Sometimes the names "Rhopalocera" and "Heterocera" are used to formalize the popular distinction...
larvae, carpet beetle larvae,
booklicePsocoptera are an order of insects that are commonly known as booklice, barklice or barkflies. They first appeared in the Permian period, 295–248 million years ago. They are often regarded as the most primitive of the hemipteroids . Their name originates from the Greek word psokos meaning...
,
antAnts are social insects of the family Formicidae , and along with the related wasps and bees, they belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants...
s,
miteMites, along with ticks, belong to the subclass Acarina and the class Arachnida. Mites are among the most diverse and successful of all the invertebrate groups. They have exploited an incredible array of habitats, and because of their small size most go totally unnoticed...
s, and small flies. They are small and inoffensive, and are rarely seen due to their size. They usually enter the home by "riding along" with larger insects (known as phoresy), or are brought in with firewood. They are often observed in bathrooms or laundry rooms, since they seek humidity. They may sometimes be found feeding on mites under the wing covers of certain beetles.
Ricinulei
Riniculei (
hooded tickspiders) are 5–10 mm long. Their most notable feature is a "hood" that can be raised and lowered over the head; when lowered, it covers the mouth and the chelicerae. Ricinulei have no eyes. The pedipalps end in pincers that are small relative to their bodies, when compared to those of the related orders of
scorpionScorpions are predatory arthropod animals of the order Scorpiones within the class Arachnida. There are about 2,000 species of scorpions, found widely distributed south of about 49° N, except New Zealand and Antarctica...
s and
pseudoscorpionA pseudoscorpion, , is an arachnid belonging to the order Pseudoscorpionida, also known as Pseudoscorpiones or Chelonethida....
s. The heavy-bodied abdomen forms a narrow pedicel, or waist, where it attaches to the prosoma. In males, the third pair of legs are modified to form copulatory organs. Malpighian tubules and a pair of coxal glands make up the excretory system. They have no lungs, as gas exchange takes place through the trachea.
Ricinulei are predators, feeding on other small arthropods. Little is known about their mating habits; the males have been observed using their modified third leg to transfer a
spermatophoreA spermatophore is a capsule or mass created by males of various animal species, containing spermatozoa and transferred in entirety to the female's ovipore during copulation...
to the female. The eggs are carried under the mother's hood, until the young hatch into six-legged "larva", which later
moltEcdysis is the moulting of the cuticula in arthropods and related groups . Since the cuticula of these animals is also the skeletal support of the body and is inelastic, it is shed during growth and a new, larger covering is formed...
into their adult forms. Ricinulei require moisture to survive. Approximately 57 species of ricinuleids have been described worldwide, all in a single family that contains 3 genera.
Schizomida
Schizomida is an
orderIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
of arachnids that tend to live in the top layer of soils. Schizomids present the prosoma covered by a large protopeltidium and smaller, paired, mesopeltidia and metapeltidia. There are no eyes. The
opisthosomaThe opisthosoma is the posterior portion of the arachnids body behind the prosoma . The number of segments and appendages on the opisthosoma vary. Scorpions have 13, but the first is only seen during its embryological development. Other arachnids have twelve or less...
is a smooth oval of 12 recognisable somites. The first is reduced and forms the pedicel. The last three are much constricted, forming the
pygidiumThe pygidium is the posterior body part or shield of crustaceans and some arthropods, such as insects and the extinct trilobites. It contains the anus and, in females, the ovipositor...
. The last somite bears the flagellum, which in this order is short and consists of not more than four segments.
The name means "split or cleaved middle", referring to the way the
cephalothoraxThe cephalothorax is an anatomical term used in arachnids and malacostracan crustaceans for the first major body section. The remainder of the body is the abdomen , which may also bear lateral appendages as well as the tail, if present...
is divided into two separate plates. Like the related orders
UropygiThelyphonida is an arachnid order comprising invertebrates commonly known as vinegarroons. They are often called uropygids in the scientific community after the former order Uropygi...
,
AmblypygiAmblypygi is an order of invertebrate animals belonging to the class Arachnida, in the subphylum Chelicerata of the phylum Arthropoda.Amblypygids are also known as whip spiders and tailless whip scorpions...
, and Solpugida, the schizomids use only six legs for walking, having modified their first two legs to serve as sensory organs. They also have large well-developed pedipalps (pincers) just behind the sensory legs.
Scorpions
Scorpions are characterised by a metasoma (tail) comprising six segments, the last containing the scorpion's
anusThe anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to expel feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest, such as bones; food...
and bearing the
telsonThe telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...
(the sting). The telson, in turn, consists of the
vesicleA vesicle is a bubble of liquid within a cell. More technically, a vesicle is a small, intracellular, membrane-enclosed sac that stores or transports substances within a cell. Vesicles form naturally because of the properties of lipid membranes ...
, which holds a pair of
venomVenom is any of a variety of toxins used by certain types of animals. Generally, venom is injected by such means as a bite or a sting.-The distinction between venom and poison:...
glands and the hypodermic aculeus, the venom-injecting
barbBarb may refer to:* A backward-facing point on a fish hook or similar implement, rendering extraction from the victim's flesh more difficult* wind barbs for each station on a map of reported weather conditions...
. The abdomen's front half, the mesosoma, is made up of six segments. The first segment contains the
sexual organsA sex organ, or primary sexual characteristic, as narrowly defined, is any of the anatomical parts of the body which are involved in sexual reproduction and constitute the reproductive system in a complex organism; in mammals, these include:*Female...
as well as a pair of vestigial and modified appendages forming a structure called the genital operculum. The second segment bears a pair of featherlike sensory organs known as the
pectines; the final four segments each contain a pair of
book lungA book lung is a type of respiration organ used for atmospheric gas exchange and is found in arachnids, such as scorpions and spiders. Each of these organs is found inside a ventral abdominal cavity and connects with the surroundings through a small opening. Book lungs are not related to the lungs...
s. The mesosoma is armored with
chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world. It is the main component of the cell walls of fungi, the exoskeletons of arthropods, such as crustaceans Chitin...
ous plates, known as tergites on the upper surface and sternites on the lower surface.
The cuticle of scorpions is covered with hairs in some places that act like balance organs. An outer layer that makes them fluorescent green under ultraviolet light is called the hyaline layer. Newly molted scorpions do not glow until after their cuticle has hardened. The fluorescent hyaline layer can be intact in fossil rocks that are hundreds of millions of years old.
Scorpions are opportunistic predators of small arthropods and insects. They use their chela (pincers) to catch the prey initially. Depending on the toxicity of their venom and size of their claws, they will then either crush the prey or inject it with neurotoxic venom. The neurotoxins consist of a variety of small
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
s as well as sodium and potassium cations, which serve to interfere with neurotransmission in the victim. Scorpions use their venom to kill or paralyze their prey so that it can be eaten; in general it is fast acting, allowing for effective prey capture. Scorpion venoms are optimised for action upon other
arthropodAn arthropod is an invertebrate that has an exoskeleton , a segmented body, and jointed attachments called appendages. Arthropods are animals belonging to the Phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s and therefore most scorpions are relatively harmless to humans; stings produce only local effects (such as pain, numbness or swelling). A few scorpion species, however, mostly in the family
ButhidaeButhidae is the largest family of scorpions, containing about 80 genera and over 800 species as of mid-2008. Its members are known as, for example, thick-tailed scorpions and bark scorpions. There are a few very large genera , but also a high number of species-poor or monotypic ones...
, can be dangerous to humans. The scorpion that is responsible for the most human deaths is the
Androctonus australisFattail scorpion or fat-tailed scorpion is the common name given to scorpions of the genus Androctonus, which is one of the most dangerous groups of scorpions species in the world. They are found throughout the semi-arid and arid regions of the Middle-East and Africa. They are a moderate sized...
, or fat-tailed scorpion of
North AfricaNorth Africa or Northern Africa is the northernmost region of the African continent, linked by the Sahara to Sub-Saharan Africa.Geopolitically, the UN definition of Northern Africa includes the following seven countries or territories; Algeria, Egypt, Libya, Morocco, Sudan, Tunisia,Mauritania, and...
. The toxicity of
A. australis's venom is roughly half that of
L. quinquestriatus, but since
A. australis injects quite a bit more venom into its prey, it is the most deadly to humans. Human deaths normally occur in the young, elderly, or infirm; scorpions are generally unable to deliver enough venom to kill healthy adults. Some people, however may be allergic to the venom of some species, in which case the scorpion's sting can more likely kill. A primary symptom of a scorpion sting is numbing at the injection site, sometimes lasting for several days. It has been found that scorpions have two types of venom: a translucent, weaker venom designed to stun only, and an opaque, more potent venom designed to kill heavier threats.
Unlike the majority of Arachnida species, scorpions are viviparous. The young are born one by one, and the brood is carried about on its mother's back until the young have undergone at least one
moultEcdysis is the moulting of the cuticula in arthropods and related groups . Since the cuticula of these animals is also the skeletal support of the body and is inelastic, it is shed during growth and a new, larger covering is formed...
. The young generally resemble their parents, requiring between five and seven moults to reach maturity. Scorpions have quite variable lifespans and the lifespan of most species is not known. The age range appears to be approximately 4–25 years (25 years being the maximum reported life span in the species
H. arizonensis). They are nocturnal and
fossorialA fossorial organism is one that is adapted to digging and life underground such as the badger, the naked mole rat, and the mole salamanders Ambystomatidae...
, finding shelter during the day in the relative cool of underground holes or undersides of rocks and coming out at night to hunt and feed. Scorpions prefer to live in areas where the temperatures range from 20
°CCelsius is a temperature scale that is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
to 37 °C (68
°FFahrenheit is the temperature scale proposed in 1724 by, and named after, the physicist Daniel Gabriel Fahrenheit . Today, the scale has been replaced by the Celsius scale in most countries; it is still in use for non-scientific purposes in the United States and a few other nations, such as...
to 99 °F), but may survive in the temperature range of 14 °C to 45 °C (57 °F to 113 °F).
Scorpions have been found in many fossil records, including coal deposits from the Carboniferous Period and in marine
SilurianThe Silurian is a geologic period and system that extends from the end of the Ordovician period, about 443.7 ± 1.5 Ma , to the beginning of the Devonian period, about 416.0 ± 2.8 Ma . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
deposits. They are thought to have existed in some form since about 425–450 million years ago. They are believed to have an oceanic origin, with gills and a claw like appendage that enabled them to hold onto rocky shores or seaweed.
Solifugae
Solifugae is a group of 900 species of arachnids, commonly known as
camel spiders,
wind scorpions, and
sun spiders. The name derives from
LatinLatin is an Italic language originally spoken in Latium and Ancient Rome. Through the Roman conquest, Latin spread throughout the Mediterranean and a large part of Europe...
, and means
those that flee from the sun. Most Solifugae live in tropical or semitropical regions where they inhabit warm and arid habitats, but some species have been known to live in grassland or forest habitats. The most distinctive feature of Solifugae is their large
cheliceraeThe chelicerae are mouth parts of the Chelicerata, an arthropod subphylum that includes arachnids, Merostomata , and Pycnogonida . Chelicerae are pointed appendages which are used to grasp food, and are found in place of the chewing mandibles most other arthropods have...
. Each of the two chelicerae are composed of two articles forming a powerful pincer; each article bears a variable number of teeth. Males in all families but
EremobatidaeEremobates is a genus of arachnids of the order Solifugae. About 2 inches long, this fast-moving creature has the largest jaw size to body ratio of any creature. It is not venomous, but has a remarkably powerful bite...
possess a
flagellumA flagellum is a tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and functions in locomotion. There are some notable differences between prokaryotic and eukaryotic flagella, such as protein composition, structure, and mechanism of propulsion...
on the basal article of the chelicera. Solifugae also have long
pedipalpPedipalps , are the second pair of appendages of the prosoma in the subphylum Chelicerata. They are traditionally thought to be homologous with mandibles in Crustacea and insects, although more recent studies Pedipalps (commonly shortened to palps or palpi), are the second pair of appendages of the...
s, which function as sense organs similar to insects'
antennaeAntennae in biology have historically been paired appendages used for sensing in arthropods and crustaceans. More recently, the term has also been applied to cilium structures present in many cell types of eukaryotes....
and give the appearance of the two extra legs. Pedipalps terminate in reversible adhesive organs.
Solifugae are
carnivorousA carnivore , meaning 'meat eater' , is an animal that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of vertebrate and/or invertebrate animal tissue, whether through predation or scavenging...
or
omnivorousOmnivores are species that eat both plants and animals as their primary food source...
, with most species feeding on
termiteThe termites are a group of eusocial insects usually classified at the taxonomic rank of order Isoptera . Along with ants and some bees and wasps which are all placed in the separate order Hymenoptera, termites divide labour among gender lines, produce overlapping generations and take care of...
s,
darkling beetleDarkling beetles are a family of beetles found worldwide, estimated at more than 20,000 species. Many of the beetles have black elytra. Darkling beetles eat both fresh and decaying vegetation. Major predators include birds, rodents, sunspiders, and lizards...
s, and other small
arthropodAn arthropod is an invertebrate that has an exoskeleton , a segmented body, and jointed attachments called appendages. Arthropods are animals belonging to the Phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s; however, solifugae have been videotaped consuming larger prey such as lizards. Prey is located with the pedipalps and killed and cut into pieces by the chelicerae. The prey is then liquefied and the liquid ingested through the pharynx. Reproduction can involve direct or indirect
spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
transfer; when indirect, the male emits a
spermatophoreA spermatophore is a capsule or mass created by males of various animal species, containing spermatozoa and transferred in entirety to the female's ovipore during copulation...
on the ground and then inserts it with his chelicerae in the female's genital pore.
Trigonotarbida
The
OrderIn scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Trigonotarbida is an extinct group of arachnids whose
fossilFossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous rock formations and sedimentary layers is known as the fossil record...
record extends from the
SilurianThe Silurian is a geologic period and system that extends from the end of the Ordovician period, about 443.7 ± 1.5 Ma , to the beginning of the Devonian period, about 416.0 ± 2.8 Ma . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
to the Lower
PermianThe Permian
[The term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named...]
and are known from several localities in
EuropeEurope is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally divided from Asia to its east by the water divide of the Ural Mountains, the Ural River, the Caspian Sea, the Caucasus Mountains , and the Black Sea to the southeast...
,
North AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
and
ArgentinaArgentina, officially the Argentine Republic , is the second largest country in South America, constituted as a federation of 23 provinces and an autonomous city, Buenos Aires. It is the eighth largest country in the world by land area and the largest among Spanish-speaking nations, though Mexico,...
. They superficially resemble
spiderSpiders are air-breathing chelicerate arthropods that have eight legs, and chelicerae modified into fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
s, to which they were clearly related.
These early arachnids seem to have been adapted to stalking prey on the ground. They have been found within the very structure of ground-dwellings plants, possibly where they hid to await their prey. Trigonotarbids are currently among the oldest known land arthropods. They lack silk glands on the
opisthosomaThe opisthosoma is the posterior portion of the arachnids body behind the prosoma . The number of segments and appendages on the opisthosoma vary. Scorpions have 13, but the first is only seen during its embryological development. Other arachnids have twelve or less...
and cheliceral poison glands, and most likely represented independent offshoots of the Arachnida.
Thelyphonida
The
Thelyphonida (formerly
Uropygida), commonly known as
vinegarroons or
whip scorpions, range from 25 to 85 mm in length; the largest species, of the genus
MastigoproctusThe King Whip is arguably the largest whip scorpion in the world. Native to the rain forest regions of northern South America, these whip scorpions can reach a length of up to 9 centimetres and can weigh over 30 grams. Despite popular belief, they are not venomous as like all other whip scorpions,...
, reaches 85 mm. Like the related orders
SchizomidSchizomida is an order of arachnids, superficially resembling spiders and generally less than in length....
a,
AmblypygiAmblypygi is an order of invertebrate animals belonging to the class Arachnida, in the subphylum Chelicerata of the phylum Arthropoda.Amblypygids are also known as whip spiders and tailless whip scorpions...
, and
SolifugaeSolifugae is an order of Arachnida, containing more than 1,000 described species in about 140 genera. The name derives from Latin, and means those that flee from the sun. The order is also known by the names Solpugida, Solpugides, Solpugae, Galeodea and Mycetophorae...
, the vinegarroons use only six legs for walking, having modified their first two legs to serve as antennae-like sensory organs. Many species also have very large
scorpionScorpions are predatory arthropod animals of the order Scorpiones within the class Arachnida. There are about 2,000 species of scorpions, found widely distributed south of about 49° N, except New Zealand and Antarctica...
-like pedipalps (pincers). They have one pair of eyes at the front of the
cephalothoraxThe cephalothorax is an anatomical term used in arachnids and malacostracan crustaceans for the first major body section. The remainder of the body is the abdomen , which may also bear lateral appendages as well as the tail, if present...
and three on each side of the head. Whip scorpions have no poison glands, but they do have glands near the rear of their abdomen that can spray a combination of
acetic acidAcetic acid, CH
3COOH, also known as ethanoic acid, is an organic acid which gives vinegar its sour taste and pungent smell. It is a weak acid, in that it is only partially dissociated acid in aqueous solution...
and octanoic acid when they are bothered. Other species spray
formic acidFormic acid is the simplest carboxylic acid. Its formula is HCOOH or CH2O2. It is an important intermediate in chemical synthesis and occurs naturally, most notably in the venom of bee and ant stings.In nature, it is found in the stings and bites of many insects of the order...
or
chlorineChlorine Chlorine Chlorine ( , from the Greek word 'χλωρóς' (khlôros, meaning 'pale green'), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17 (formerly VII, VIIa, or VIIb). As the chloride ion, which is part of common salt and...
. As of 2006, over 100 species have been described worldwide.
Whip scorpions are
carnivorousA carnivore , meaning 'meat eater' , is an animal that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of vertebrate and/or invertebrate animal tissue, whether through predation or scavenging...
, nocturnal hunters feeding mostly on insects but sometimes on worms and slugs. The prey is crushed between special teeth on the inside of the trochanters (the second segment of the leg) of the front legs. They are valuable in controlling the population of roaches and crickets.
Males secrete a sperm sac, which is transferred to the female. Up to 35 eggs are laid in a burrow, within a
mucous membraneThe mucous membranes are linings of mostly endodermal origin, covered in epithelium, which are involved in absorption and secretion. They line various body cavities that are exposed to the external environment and internal organs. It is at several places continuous with skin: at the nostrils, the...
that preserves moisture. Mothers stay with the eggs and do not eat. The white young that hatch from the eggs climb onto their mother's back and attach themselves there with special suckers. After the first molt they look like miniature whip scorpions, and leave the burrow; the mother dies soon after. The young grow slowly, going through three molts in about three years before reaching adulthood.
Vinegarroons are found in tropical and
subtropicalThe subtropics are the geographical zone of the Earth immediately north and south of the tropical zone, which is bounded by the Tropic of Cancer and the Tropic of Capricorn, at latitudes 23.5°N and 23.5°S...
areas worldwide, usually in underground burrows that they dig with their pedipalps. They may also burrow under logs, rotting wood, rocks, and other natural debris. They enjoy humid, dark places and avoid the light.
See also
- Endangered spiders
An endangered species is “a species at risk of extinction because of human activity, changes in climate, changes in predator-prey ratios, etc., especially when officially designated as such by a governmental agency such as the U.S. Fish and Wildlife Service”...
- Arachnids - Arachnidae Insect Life Forms