Arachnids (/əˈræknɪdz/) are a class (Arachnida) of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Almost all adult arachnids have eight legs, although the front pair of legs in some species has converted to a sensory function, while in other species, different appendages can grow large enough to take on the appearance of extra pairs of legs. The term is derived from the Greek word ἀράχνη (aráchnē), from the myth of the hubristic human weaver Arachne who was turned into a spider. Spiders are the largest order in the class, which also includes scorpions, ticks, mites, harvestmen, and solifuges. In 2019, a molecular phylogenetic study also placed horseshoe crabs in Arachnida.
Almost all extant arachnids are terrestrial, living mainly on land. However, some inhabit freshwater environments and, with the exception of the pelagic zone, marine environments as well. They comprise over 100,000 named species.
|"Arachnida" from Ernst Haeckel's Kunstformen der Natur, 1904|
Almost all adult arachnids have eight legs, and arachnids may be easily distinguished from insects 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 pedipalps, have been adapted for feeding, locomotion, and/or reproductive functions. In Solifugae, the palps are quite leg-like, so that these animals appear to have ten legs. The larvae of mites and Ricinulei have only six legs; a fourth pair usually appears when they moult into nymphs. However, mites are variable: as well as eight, there are adult mites with six or even four legs.
Arachnids are further distinguished from insects by the fact they do not have antennae or wings. Their body is organized into two tagmata, called the prosoma, or cephalothorax, and the opisthosoma, or abdomen. The cephalothorax is derived from the fusion of the cephalon (head) and the thorax, 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. A telson is present in scorpions, where it has been modified to a stinger, and in the Schizomida, whip scorpions and Palpigradi.
Like all arthropods, arachnids have an exoskeleton, and they also have an internal structure of cartilage-like tissue, called the endosternite, to which certain muscle groups are attached. The endosternite is even calcified in some Opiliones.
Most arachnids lack extensor muscles in the distal joints of their appendages. Spiders and whipscorpions extend their limbs hydraulically using the pressure of their hemolymph. Solifuges and some harvestmen extend their knees by the use of highly elastic thickenings in the joint cuticle. Scorpions, pseudoscorpions 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.
There are characteristics that are particularly important for the terrestrial lifestyle of arachnids, such as internal respiratory surfaces in the form of tracheae, or modification of the book gill into a book lung, an internal series of vascular lamellae used for gas exchange with the air. 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 appendages modified for more efficient locomotion on land, internal fertilisation, special sensory organs, and water conservation enhanced by efficient excretory structures as well as a waxy layer covering the cuticle.
The excretory glands of arachnids include up to four pairs of coxal glands along the side of the prosoma, and one or two pairs of Malpighian tubules, emptying into the gut. Many arachnids have only one or the other type of excretory gland, although several do have both. The primary nitrogenous waste product in arachnids is guanine.
Arachnid blood is variable in composition, depending on the mode of respiration. Arachnids with an efficient tracheal system do not need to transport oxygen in the blood, and may have a reduced circulatory system. In scorpions and some spiders, however, the blood contains haemocyanin, a copper-based pigment with a similar function to haemoglobin in vertebrates. The heart is located in the forward part of the abdomen, and may or may not be segmented. Some mites have no heart at all.
Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. Only in the harvestmen and among mites, such as the 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 from specialized glands to kill prey or enemies. Several mites and ticks are parasites, some of which are carriers of disease.
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, which acts as a pump, sucking the food through the mouth and on into the oesophagus and stomach. In some arachnids, the oesophagus also acts as an additional pump.
The stomach is tubular in shape, with multiple diverticula extending throughout the body. The stomach and its diverticula both produce digestive enzymes and absorb nutrients from the food. It extends through most of the body, and connects to a short sclerotised intestine and anus in the hind part of the abdomen.
Arachnids have two kinds of eyes: the lateral and median ocelli. The lateral ocelli evolved from compound eyes and may have a tapetum, which enhances the ability to collect light. With the exception of scorpions, which can have up to five pairs of lateral ocelli, there are never more than three pairs present. The median ocelli develop from a transverse fold of the ectoderm. The ancestors of modern arachnids probably had both types, but modern ones often lack one type or the other. The cornea of the eye also acts as a lens, and is continuous with the cuticle of the body. Beneath this is a transparent vitreous body, and then the retina and, if present, the tapetum. In most arachnids, the retina probably does not have enough light sensitive cells to allow the eyes to form a proper image.
In addition to the eyes, almost all arachnids have two other types of sensory organs. The most important to most arachnids are the fine sensory hairs that cover the body and give the animal its sense of touch. These can be relatively simple, but many arachnids also possess more complex structures, called trichobothria.
Finally, slit sense organs are slit-like pits covered with a thin membrane. Inside the pit, a small hair touches the underside of the membrane, and detects its motion. Slit sense organs are believed to be involved in proprioception, and possibly also hearing.
Arachnids may have one or two gonads, which are located in the abdomen. The genital opening is usually located on the underside of the second abdominal segment. In most species, the male transfers sperm to the female in a package, or spermatophore. Complex courtship rituals have evolved in many arachnids to ensure the safe delivery of the sperm to the female.
Arachnids usually lay yolky eggs, which hatch into immatures that resemble adults. Scorpions, however, are either ovoviviparous or viviparous, depending on species, and bear live young. In most arachnids only the females provide parental care, with harvestmen being one of the few exceptions.
The phylogenetic relationships among the main subdivisions of arthropods have been the subject of considerable research and dispute for many years. A consensus emerged from about 2010 onwards, based on both morphological and molecular evidence. Extant (living) arthropods are a monophyletic group and are divided into three main clades: chelicerates (including arachnids), pancrustaceans (the paraphyletic crustaceans plus insects and their allies), and myriapods (centipedes, millipedes and allies). The three groups are related as shown in the 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 as of March 2016, with successive studies producing different results. A study in 2014, based on the largest set of molecular data to date, concluded that there were systematic conflicts in the phylogenetic information, particularly affecting the orders Acariformes, Parasitiformes and Pseudoscorpiones, which have had much faster evolutionary rates. Analyses of the data using sets of genes with different evolutionary rates produced mutually incompatible phylogenetic trees. The authors favoured relationships shown by more slowly evolving genes, which demonstrated the monophyly of Chelicerata, Euchelicerata and Arachnida, as well as of some clades within the arachnids. The diagram below summarizes their conclusions, based largely on the 200 most slowly evolving genes; dashed lines represent uncertain placements.
Tetrapulmonata, here consisting of Araneae, Amblypygi and Thelyphonida (Schizomida 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, Ricinulei and Solifugae, a combination not found in most other studies.
In early 2019, a phylogenetic analysis placed the horseshoe crabs, Xiphosura, as the sister group to Ricinulei. It also grouped pseudoscorpions with mites and ticks, which the authors considered may be due to long branch attraction.
A fossil arachnid in 100 million year old (mya) amber from Myanmar, Chimerarachne yingi, has spinnerets (to produce silk); it also has a tail, like the Palaeozoic Uraraneida, some 200 million years after other known fossils with tails. The fossil resembles the most primitive living spiders, the mesotheles.
The subdivisions of the arachnids are usually treated as orders. Historically, mites and ticks were treated as a single order, Acari. However, molecular phylogenetic studies suggest that the two groups do not form a single clade, with morphological similarities being due to convergence. They are now usually treated as two separate taxa – Acariformes, mites, and Parasitiformes, ticks – which may be ranked as orders or superorders. The arachnid subdivisions are listed below alphabetically; numbers of species are approximate.
It is estimated that 98,000 arachnid species have been described, and that there may be up to 600,000 in total.
Amblypygi is an ancient order of arachnid chelicerate arthropods also known as whip spiders and tailless whip scorpions (not to be confused with whip scorpions and vinegaroons that belong to the related order Thelyphonida). The name "amblypygid" means "blunt tail", a reference to a lack of the flagellum that is otherwise seen in whip scorpions. They are harmless to humans. Amblypygids possess no silk glands or venomous fangs. They rarely bite if threatened, but can grab fingers with their pedipalps, resulting in thorn-like puncture injuries.
As of 2016, 5 families, 17 genera and around 155 species had been discovered and described. They are found in tropical and subtropical regions worldwide; they are mainly found in warm and humid environments and like to stay protected and hidden within leaf litter, caves, or underneath bark. Some species are subterranean; all are nocturnal. Fossilized amblypygids have been found dating back to the Carboniferous period, such as Graeophonus.Araneoidea
Araneoidea is a taxon of araneomorph spiders, termed "araneoids", treated as a superfamily. As with many such groups, its circumscription has varied; in particular some families at one time moved to the Palpimanoidea have more recently been restored to Araneoidea. A 2014 treatment includes 18 families, with the araneoids making up about 26% of the total number of known spider species; a 2016 treatment includes essentially the same taxa, but now divided into 17 families.Book lung
A book lung is a type of respiration organ used for atmospheric gas exchange that is found in many arachnids, such as scorpions and spiders. Each of these organs is found inside an open ventral abdominal, air-filled cavity (atrium) and connects with the surroundings through a small opening for the purpose of respiration.
Book lungs are not related to the lungs of modern land-dwelling vertebrates. Their name describes their structure. Stacks of alternating air pockets and tissue filled with hemolymph (the arthropod equivalent of blood) give them an appearance similar to a "folded" book.(Their number varies from just one pair in most spiders to four pairs in scorpions. The unfolded "pages" (plates) of the book lung are filled with hemolymph.) The folds maximize the surface exposed to air, and thereby maximize the amount of gas exchanged with the environment. In most species, no motion of the plates is required to facilitate this kind of respiration.
Sometimes, book lungs can be absent, and gas exchange is performed by the thin walls inside the cavity instead, with their surface area increased by branching into the body as thin tubes called tracheae. The tracheae possibly have evolved directly from the book lungs because, in some spiders, the tracheae have a small number of greatly elongated chambers. Many arachnids, such as mites and harvestmen (Opiliones), have no traces of book lungs and breathe through tracheae or through their body surfaces only. The absence or presence of book lungs divides the Arachnida into two main groups, the pulmonate arachnids (book lungs present; scorpions and the Tetrapulmonata; whip scorpions, Schizomida, Amblypygi, and spiders), and the apulmonate arachnids (book lungs absent; microwhip scorpions, harvestmen, Acarina, pseudoscorpions, Ricinulei and sunspiders). One of the long-running controversies in arachnid evolution is whether the book lung evolved from book gills just once in a common arachnid ancestor, or whether it evolved in multiple groups of arachnids in parallel as they came onto land.
The oldest book lungs have been recovered from extinct trigonotarbid arachnids preserved in the 410-million-year-old Rhynie chert of Scotland. These Devonian fossil lungs are almost indistinguishable from the lungs of modern arachnids, fully adapted to a terrestrial existence.Carapace
A carapace is a dorsal (upper) 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 turtles and tortoises. In turtles and tortoises, the underside is called the plastron.Cephalothorax
The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms prosoma and opisthosoma are equivalent to cephalothorax and abdomen in some groups.) The word cephalothorax is derived from the Greek words for head (κεφαλή, kephalé) and thorax (θώραξ, thórax). This fusion of the head and thorax is seen in chelicerates and crustaceans; in other groups, such as the Hexapoda (including insects), the head remains free of the thorax. In horseshoe crabs and many crustaceans, a hard shell called the carapace covers the cephalothorax.Cercus
Cerci (singular cercus) are paired appendages on the rear-most segments of many arthropods, including insects and symphylans. Many forms of cerci serve as sensory organs, but some serve as pinching weapons or as organs of copulation. In many insects, they simply may be functionless vestigial structures.
In basal arthropods, such as silverfish, the cerci originate from the eleventh abdominal segment. As segment eleven is reduced or absent in the majority of arthropods, in such cases, the cerci emerge from the tenth abdominal segment. It is not clear that other structures so named are homologous. In the Symphyla they are associated with spinnerets.Hemolymph
Hemolymph, or haemolymph, is a fluid, analogous to the blood in invertebrates, that circulates in the interior of the arthropod body remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which hemolymph cells called hemocytes are suspended. In addition to hemocytes, the plasma also contains many chemicals. It is the major tissue type of the open circulatory system characteristic of arthropods (e.g. arachnids, crustaceans and insects). In addition, some non-arthropods such as molluscs possess a hemolymphatic circulatory system.Holothyrida
The Holothyrida are a small order of mites in the superorder Parasitiformes. No fossils are known. With body lengths of more than 2 mm (3⁄32 in) they are relatively large mites, with a heavily sclerotized body. They mainly feed on the body fluids of dead arthropods. This was possibly the common way of feeding for ticks before they adapted for feeding on the blood of live animals.Although only 25 species are currently described, many others have been collected.
The order has a Gondwanan distribution. They are likely the sister group to Ixodida (ticks).List of natural horror films
Natural horror (also known as creature features) is a subgenre of horror films that features natural forces, typically in the form of animals or plants, that pose a threat to human characters.
Though killer animals in film have existed since the release of The Lost World in 1925, two of the first motion pictures to garner mainstream success with a "nature run amok" premise were The Birds, directed by Alfred Hitchcock and released in 1963; and Jaws, directed by Steven Spielberg and released in 1975. Following Jaws, numerous horror films of a similar narrative were produced, including Grizzly (1976), Piranha (1978), and Alligator (1980). Today, natural horror films are still produced, with varying tones, such as Birdemic: Shock and Terror (2008) and The Shallows (2016).Mesosoma
The mesosoma is the middle part of the body, or tagma, of arthropods whose body is composed of three parts, the other two being the prosoma and the metasoma. It bears the legs, and, in the case of winged insects, the wings.
In hymenopterans of the suborder Apocrita (wasps, bees and ants), it consists of the three thoracic segments and the first abdominal segment (the propodeum). For historical reasons, in ants it is commonly referred to by the alternative name alitrunk.In scorpions, it is composed of six segments and forms the first part of the abdomen, containing all of the major organs. The first segment contains the sexual organs 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 lungs. The mesosoma is armoured with chitinous plates, on the upper surface by the tergites and on the lower surface by the sternites.
In other arachnids such as spiders, the mesosoma is fused with the metasoma to form the opisthosoma.Mesostigmata
Mesostigmata is an order of mites belonging to the Parasitiformes. Unlike most members of that group, many of these mites are not parasitic but free-living and predatory. They can be recognized by the single pair of spiracles positioned laterally on the body.
The family with the most described species is Phytoseiidae. Other families of note are Diplogyniidae, Macrochelidae, Pachylaelapidae, Uropodidae and Veigaiidae.Opilioacariformes
Opilioacariformes is the smallest order (or superorder) of mites, containing a single family, and around 10 genera. They are rare, large mites, and are widely considered primitive, as they retain six pairs of eyes, and abdominal segmentation. Opilioacariformes may be the sister group to the Parasitiformes.The first member of the Opilioacariformes to be discovered was the Algerian species Opilioacarus segmentatus, which was described by Carl Johannes With in 1902, followed by the Sicilian Eucarus italicus and Eucarus arabicus from Aden, both in 1904. Two fossil specimens are known, one of which was discovered in Baltic amber from the Eocene, while the other one was discovered in the Burmese amber from the Late Cretaceous (Cenomanian).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).Pedipalp
Pedipalps (commonly shortened to palps or palpi) are the second pair of appendages of chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to the chelicerae ("jaws") and anterior to the first pair of walking legs.Sarcoptiformes
The Sarcoptiformes are an order of Acari comprising over 15,000 described species in around 230 families. Previously it was divided into two suborders, Oribatida and Astigmatina, but Oribatida has been promoted to an order, and Astigmatina is now an unranked taxon.Spider (solitaire)
Spider is a type of patience game. It is one of the more popular two-deck solitaire games.Thelyphonida
Thelyphonida is an arachnid order comprising invertebrates commonly known as whip scorpions or vinegaroons (also spelled vinegarroons and vinegarones). They are often called uropygids in the scientific community based on an alternative name for the order, Uropygi (which may then also include the order Schizomida). The name "whip scorpion" refers to their resemblance to true scorpions and possession of a whiplike tail. "Vinegaroon" is based on their ability when attacked to discharge an offensive liquid which contains acetic acid, producing a vinegar-like smell.Tick (comics)
The Tick is a monster superhero created by cartoonist Ben Edlund in 1986 as a newsletter mascot for the New England Comics chain of Boston area comic book stores. The character is a parody of American comic book superheroes.
After its creation, the character spun off into an independent comic book series in 1988, and gained mainstream popularity through an animated TV series on Fox in 1994. Two live-action TV series, a video game and various merchandise have also been based on the character. IGN's list of the Top 100 Comic Book Heroes of All Time ranked The Tick as #57.Trombidiformes
The Trombidiformes are a large, diverse order of mites.
Extant Arthropoda classes by subphylum