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40-399: Parasitiformes are a superorder of Arachnids , constituting one of the two major groups of mites , alongside Acariformes . Parasitiformes has, at times, been classified at the rank of order or suborder. It is uncertain whether Parasitiformes and Acariformes are closely related, and in many analyses they are recovered more closely related to other arachnids. Amongst the best known members of

80-461: 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

120-444: A clade called Arachnopulmonata was also well supported. Pseudoscorpiones may also belong here, as all six orders share the same ancient whole genome duplication , and analyses support pseudoscorpions as the sister group of scorpions. Genetic analysis has not yet been done for Ricinulei, Palpigradi, or Solifugae, but horseshoe crabs have gone through two whole genome duplications, which gives them five Hox clusters with 34 Hox genes ,

160-476: A fourth pair usually appears when they moult into nymphs . However, mites are variable: as well as eight, there are adult mites with six or, like in Eriophyoidea , even four legs. While the adult males in some members of Podapolipidae have six legs, the adult females have only a single pair. Arachnids are further distinguished from insects by the fact they do not have antennae or wings . Their body

200-413: 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

240-427: 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

280-435: 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

320-409: Is a stub . You can help Misplaced Pages by expanding it . Arachnid Arachnids are arthropods in the class Arachnida ( / ə ˈ r æ k n ɪ d ə / ) of the subphylum Chelicerata . Arachnida includes, among others, spiders , scorpions , ticks , mites , pseudoscorpions , harvestmen , camel spiders , whip spiders and vinegaroons . Adult arachnids have eight legs attached to

360-509: Is derived from the Greek word ἀράχνη ( aráchnē , 'spider'), from the myth of the hubristic human weaver Arachne , who was turned into a spider. Almost all adult arachnids have eight legs, unlike adult insects which all have six legs. However, arachnids also have two further pairs of appendages that have become adapted for feeding, defense, and sensory perception. The first pair, the chelicerae , serve in feeding and defense. The next pair,

400-403: Is organized into two tagmata , called the prosoma and opisthosoma , also referred to as the cephalothorax and abdomen . However, there are questions about the validity of the latter terms. While the term cephalothorax implies a fused cephalon (head) and thorax , there is currently neither fossil nor embryological evidence that arachnids ever had a separate thorax-like division. Likewise,

440-507: The cephalothorax . In some species the frontmost pair of legs has converted to a sensory function, while in others, different appendages can grow large enough to take on the appearance of extra pairs of legs. Almost all extant arachnids are terrestrial , living mainly on land. However, some inhabit freshwater environments and, with the exception of the pelagic zone , marine environments as well. They comprise over 110,000 named species , of which 51,000 are species of spiders. The term

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480-509: The cladogram below. Including fossil taxa does not fundamentally alter this view, although it introduces some additional basal groups. Chelicerata (sea spiders, horseshoe crabs and arachnids ) [REDACTED] [REDACTED] [REDACTED] Myriapoda (centipedes, millipedes, and allies) [REDACTED] [REDACTED] Pancrustacea (crustaceans and hexapods) [REDACTED] [REDACTED] The extant chelicerates comprise two marine groups: Sea spiders and horseshoe crabs, and

520-424: The house dust mite , are also the only arachnids able to ingest solid food, which exposes them to internal parasites, although it is not unusual for spiders to eat their own silk. And one species of spider is mostly herbivorous. Scorpions, spiders and pseudoscorpions secrete venom from specialized glands to kill prey or defend themselves. Their venom also contains pre-digestive enzymes that helps breaking down

560-453: The pedipalps , have been adapted for feeding, locomotion, and/or reproductive functions. In scorpions, pseudoscorpions, and ricinuleids the pedipalps end in a pair of pinchers, while in whip scorpions, Schizomida , Amblypygi , and most harvestmen, they are raptorial and used for prey capture. 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;

600-453: The 'abdomen' of many arachnids contains organs atypical of an abdomen, such as a heart and respiratory organs. The cephalothorax is usually covered by a single, unsegmented carapace. The abdomen is segmented in the more primitive forms, but varying degrees of fusion between the segments occur in many groups. It is typically divided into a preabdomen and postabdomen, although this is only clearly visible in scorpions, and in some orders, such as

640-436: The abdomen has no appendages. 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 whip scorpions extend their limbs hydraulically using

680-409: 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

720-624: The chelae in most of these groups may not be homologous with those found in Xiphosura . The pedipalps are distinctly raptorial (i.e., modified for seizing prey) in the Amblypygi , Uropygi, Schizomida , and some Opiliones belonging to the laniatorid group . Pedipalps of spiders have the same segmentation as the legs, but the tarsus is undivided, and the pretarsus has no lateral claws. Pedipalps contain sensitive chemical detectors and function as taste and smell organs, supplementing those on

760-920: 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. Acariformes [REDACTED] Opiliones [REDACTED] Ricinulei [REDACTED] Solifugae [REDACTED] Parasitiformes [REDACTED] Pseudoscorpiones [REDACTED] Scorpiones [REDACTED] Araneae [REDACTED] Amblypygi [REDACTED] Uropygi (Thelyphonida s.s. ) [REDACTED] Tetrapulmonata , here consisting of Araneae , Amblypygi and Uropygi (Thelyphonida s.s. ) ( Schizomida

800-419: 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

840-554: The female in a package, or spermatophore . The males in harvestmen and some mites have a penis. Complex courtship rituals have evolved in many arachnids to ensure the safe delivery of the sperm to the female. Members of many orders exhibit sexual dimorphism. Arachnids usually lay yolky eggs , which hatch into immatures that resemble adults. Scorpions, however, are either ovoviviparous or viviparous , depending on species, and bear live young. Also some mites are ovoviviparous and viviparous, even if most lay eggs. In most arachnids only

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880-640: 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

920-489: 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

960-401: The four modern groups, Ixodida, Mesostigmata, and Opilioacarida, are known from Cretaceous aged amber , dating to around 100 million years ago. They are suspected to have diversified substantially earlier. The genetic divergence between the groups is less than that of Acariform mites, suggesting a younger origin, likely dating to the late Paleozoic . This article about a mite or tick

1000-631: The group are the ticks , though the Mesostigmata is by far the most diverse group with over 8,000 described species, including economically important species such as the varroa mite . Many species are parasitic (most famous of which are ticks ), but not all. For example, about half of the 10,000 known species in the suborder Mesostigmata are predatory and cryptozoan , living in soil-litter, rotting wood, dung, carrion, nests or house dust. A few species have switched to grazing on fungi or ingesting spores or pollen . Phylogenetic relationships of

1040-429: The groups, after Klompen, 2010: Opilioacarida Mesostigmata Holothyrida Ixodida The phytoseiid mites, which account for about 15% of all described Mesostigmata are used with great success for biological control . There are over 12,000 described species of Parasitiformes, and the total estimate is between 100,000 and 200,000 species. The oldest known fossils of Parasitiformes, representing three out of

1080-600: The highest number found in any invertebrate, yet it is not clear if the oldest genome duplication is related to the one in Arachnopulmonata. Onychophora [REDACTED] Mandibulata [REDACTED] [REDACTED] [REDACTED] Pycnogonida [REDACTED] † Chasmataspidida [REDACTED] † Eurypterida [REDACTED] Parasitiformes [REDACTED] Acariformes [REDACTED] Pseudoscorpiones [REDACTED] Pedipalp Pedipalps are composed of six segments or articles. From

1120-413: The legs, as in spiders , or chelate weapons ( pincers ) of great size, as in scorpions . The pedipalps of Solifugae are covered in setae , but have not been studied in detail. Comparative studies of pedipalpal morphology may suggest that leg-like pedipalps are primitive in arachnids. At present, the only reasonable alternative to this view is to assume that xiphosurans reflect the morphology of

1160-557: The legs. In sexually-mature male spiders, the final segment of the pedipalp, the tarsus, develops a complicated structure (sometimes called the palpal bulb or palpal organ) that is used to transfer sperm to the female seminal receptacles during mating. The details of this structure vary considerably between different groups of spiders and are useful for identifying species. The pedipalps are also used by male spiders in courtship displays, contributing to vibratory patterns in web-shaking, acoustic signals, or visual displays. The cymbium

1200-481: The mites, the abdominal sections are completely fused. A telson is present in scorpions, where it has been modified to a stinger, and into a flagellum in the Palpigradi , Schizomida (very short) and whip scorpions . At the base of the flagellum in the two latter groups there are glands which produce acetic acid as a chemical defense. Except for a pair of pectines in scorpions, and the spinnerets in spiders,

1240-486: 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

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1280-416: 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

1320-457: The pre-digested bodies of insects and other small animals. But ticks, and many mites, are parasites, some of which are carriers of disease. The diet of mites also include tiny animals, fungi, plant juices and decomposing matter. Almost as varied is the diet of harvestmen , where we will find predators, decomposers and omnivores feeding on decaying plant and animal matter, droppings, animals and mushrooms. The harvestmen and some mites, such as

1360-460: 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

1400-407: The prey. The saliva of ticks contains anticoagulants and anticomplements, and several species produce a neurotoxin . Arachnids produce digestive enzymes in their stomachs, and use their pedipalps and chelicerae to pour them over their dead prey. The digestive juices rapidly turn the prey into a broth of nutrients, which the arachnid sucks into a pre-buccal cavity located immediately in front of

1440-473: The primitive arachnid pedipalp and to conclude that this appendage is primitively chelate. Pedipalps are traditionally thought to be homologous with mandibles in crustaceans and insects , although more recent studies (e.g. using Hox genes ) suggest they are probably homologous with the crustacean second antennae . Chelate or sub-chelate (pincer-like) pedipalps are found in several arachnid groups ( Ricinulei , Uropygi , scorpions and pseudoscorpions ) but

1480-431: The proximal end (where they are attached to the spider) to the distal, they are: the coxa, the trochanter , the femur , the short patella , the tibia , and the tarsus . In spiders, the coxae frequently have extensions called maxillae or gnathobases, which function as mouth parts with or without some contribution from the coxae of the anterior legs . The limbs themselves may be simple tactile organs outwardly resembling

1520-492: 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. Pycnogonida (sea spiders) [REDACTED] Xiphosura (horseshoe crabs) [REDACTED] Arachnida [REDACTED] Discovering relationships within

1560-432: 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

1600-498: Was not included in the study), received strong support. Somewhat unexpectedly, there was support for a clade comprising Opiliones , Ricinulei and Solifugae , a combination not found in most other studies. In early 2019, a molecular phylogenetic analysis placed the horseshoe crabs, Xiphosura , as the sister group to Ricinulei. It also grouped pseudoscorpions with mites and ticks, which the authors considered may be due to long branch attraction . The addition of Scorpiones to produce

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