91-598: Mites are small arachnids (eight-legged arthropods ). Mites span two large orders of arachnids, the Acariformes and the Parasitiformes , which were historically grouped together in the subclass Acari . However, most recent genetic analyses do not recover the two as each other's closest relative within Arachnida, rendering the group non- monophyletic . Most mites are tiny, less than 1 mm (0.04 in) in length, and have
182-424: A cephalothorax (with no separate head) or prosoma, and an opisthosoma or abdomen. Segmentation has almost entirely been lost and the prosoma and opisthosoma are fused, only the positioning of the limbs indicating the location of the segments. At the front of the body is the gnathosoma or capitulum. This is not a head and does not contain the eyes or the brain, but is a retractable feeding apparatus consisting of
273-407: A prion disease of sheep. The mite Varroa destructor is a serious pest of honey bees , contributing to colony collapse disorder in commercial hives . This organism is an obligate external parasite, able to reproduce only in bee colonies. It directly weakens its host by sucking up the bee's fat, and can spread RNA viruses including deformed wing virus . Heavy infestation causes the death of
364-461: 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
455-444: 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 ,
546-444: 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 ,
637-573: A colony, generally over the winter. Since 2006, more than 10 million beehives have been lost. Various mites prey on other invertebrates and can be used to control their populations. Phytoseiidae , especially members of Amblyseius , Metaseiulus , and Phytoseiulus , are used to control pests such as spider mites. Among the Laelapidae , Gaeolaelaps aculeifer and Stratiolaelaps scimitus are used to control fungus gnats , poultry red mites and various soil pests. Mites were first observed under
728-596: A few are associated with allergies or may transmit diseases. The scientific discipline devoted to the study of mites is called acarology . The mites are not a defined taxon, but is used for two distinct groups of arachnids, the Acariformes and the Parasitiformes . The phylogeny of the Acari has been relatively little studied, but molecular information from ribosomal DNA is being extensively used to understand relationships between groups. The 18 S rRNA gene provides information on relationships among phyla and superphyla, while
819-519: A few phylogenomic studies have found strong support for monophyly of Acari and a sister relationship between Acariformes and Parasitiformes, although this finding has been questioned, with other studies suggesting that this likely represents a long branch attraction artefact. Mites are tiny members of the class Arachnida ; most are in the size range 250 to 750 μm (0.01 to 0.03 in) but some are larger and some are no bigger than 100 μm (0.004 in) as adults. The body plan has two regions ,
910-470: 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
1001-436: A million or more species as yet undescribed. The tropical species Archegozetes longisetosus is one of the strongest animals in the world, relative to its mass (100 μg): It lifts up to 1,182 times its own weight, over five times more than would be expected of such a minute animal. A mite also holds a speed record: for its length, Paratarsotomus macropalpis is the fastest animal on Earth. The mites living in soil consist of
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#17330847873431092-762: A network of sinuses and most mites lacks a heart, movement of fluid being driven by the contraction of body muscles. But ticks, and some of the larger species of mites, have a dorsal, longitudinal heart. Gas exchange is carried out across the body surface, but many species additionally have between one and four pairs of tracheae . The excretory system includes a nephridium and one or two pairs of Malpighian tubules . Several families of mites, such as Tetranychidae, Eriophyidae, Camerobiidae, Cunaxidae, Trombidiidae, Trombiculidae, Erythraeidae and Bdellidae have silk glands used to produce silk for various purposes. Additionally, water mites (Hydrachnidia) produce long thin threads that may be silk. The sexes are separate in mites; males have
1183-418: A pair of testes in the mid-region of the body, each connected to the gonopore by a vas deferens , and in some species there is a chitinous penis ; females have a single ovary connected to the gonopore by an oviduct , as well as a seminal receptacle for the storage of sperm . In most mites, sperm is transferred to the female indirectly; the male either deposits a spermatophore on a surface from which it
1274-446: A range of taxa. Oribatida and Prostigmata are more numerous in soil than Mesostigmata, and have more soil-dwelling species. When soil is affected by an ecological disturbance such as agriculture, most mites (Astigmata, Mesostigmata and Prostigmata) recolonise it within a few months, whereas Oribatida take multiple years. Many mites are parasitic on plants and animals. One family of mites, Pyroglyphidae , or nest mites, live primarily in
1365-422: A simple, unsegmented body plan. The small size of most species makes them easily overlooked; some species live in water, many live in soil as decomposers , others live on plants, sometimes creating galls , while others are predators or parasites . This last type includes the commercially destructive Varroa parasite of honey bees , as well as scabies mites of humans. Most species are harmless to humans, but
1456-541: A small scale, walking is used to access other suitable locations in the immediate vicinity. Some species mount to a high point and adopt a dispersal posture and get carried away by the wind, while others waft a thread of silk aloft to balloon to a new position. Parasitic mites use their hosts to disperse, and spread from host to host by direct contact. Another strategy is phoresy ; the mite, often equipped with suitable claspers or suckers, grips onto an insect or other animal, and gets transported to another place. A phoretic mite
1547-413: 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
1638-413: 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
1729-427: 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
1820-435: 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
1911-594: 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
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#17330847873432002-505: 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,
2093-439: Is just a hitch-hiker and does not feed during the time it is carried by its temporary host. These travelling mites are mostly species that reproduce rapidly and are quick to colonise new habitats. Mites are tiny and apart from those that are of economic concern to humans, little studied. The majority are beneficial, living in the soil or aqueous environments and assisting in the decomposition of decaying organic material, as part of
2184-490: Is located on the ventral surface between the fourth pair of legs. Some species have one to five median or lateral eyes but many species are blind, and slit and pit sense organs are common. Both body and limbs bear setae (bristles) which may be simple, flattened, club-shaped or sensory. Mites are usually some shade of brown, but some species are red, orange, black or green, or some combination of these colours. Many mites have stigmata (openings used in respiration). In some mites,
2275-503: 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 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
2366-403: 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,
2457-403: Is picked up by the female, or he uses his chelicerae or third pair of legs to insert it into the female's gonopore. In some of the Acariformes , insemination is direct using the male's penis. The spermatophora in all mites are aflagellate. The eggs are laid in the substrate , or wherever the mite happens to live. They take up to six weeks to hatch, according to species, with the next stage being
2548-411: 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 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
2639-418: 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 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
2730-507: 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
2821-525: The ITS2 , and the 18S ribosomal RNA and 28S ribosomal RNA genes, provide clues at deeper levels. The mite fossil record is sparse, due to their small size and low preservation potential. The oldest fossils of acariform mites are from the Rhynie Chert , Scotland, which dates to the early Devonian , around 410 million years ago while the earliest fossils of Parasitiformes are known from amber specimens dating to
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2912-481: 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, the abdomen has no appendages. Like all arthropods, arachnids have an exoskeleton , and they also have an internal structure of cartilage -like tissue, called
3003-416: The carbon cycle . Two species live on humans, namely Demodex folliculorum and Demodex brevis ; both are frequently referred to as eyelash mites. The majority of mite species are harmless to humans and domestic animals , but a few species can colonize mammals directly, acting as vectors for disease transmission, and causing or contributing to allergenic diseases. Mites which colonize human skin are
3094-624: The chelicerae , the pedipalps and the oral cavity. It is covered above by an extension of the body carapace and is connected to the body by a flexible section of cuticle . Two-segmented chelicerae is the ancestral condition in Acariformes, but in more derived groups they are single-segmented. And three-segmented chelicerae is the ancestral condition in Parasitiformes, but has been reduced to just two segments in more derived groups. The pedipalps differ between taxa depending on diet; in some species
3185-509: 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
3276-509: 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
3367-477: 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 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
3458-405: 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 the pressure of their hemolymph . Solifuges and some harvestmen extend their knees by the use of highly elastic thickenings in
3549-424: 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
3640-509: The microscope by the English polymath Robert Hooke . In his 1665 book Micrographia , he stated that far from being spontaneously generated from dirt, they were "very prettily shap'd Insects". In 1898, Arthur Conan Doyle wrote a satirical poem, "A Parable", with the conceit of some cheese mites disputing the origin of the round cheddar cheese in which they all lived. The world's first science documentary featured cheese mites , seen under
3731-453: 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;
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3822-453: 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
3913-436: 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
4004-402: 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 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
4095-405: 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 is derived from the Greek word ἀράχνη ( aráchnē , 'spider'), from
4186-490: The appendages resemble legs while in others they are modified into chelicerae-like structures. The oral cavity connects posteriorly to the mouth and pharynx . Most mites have four pairs of legs (two pairs in Eriophyoidea ), each with six segments, which may be modified for swimming or other purposes. The dorsal surface of the body is clad in hardened tergites and the ventral surface by hardened sclerites ; sometimes these form transverse ridges. The gonopore (genital opening)
4277-409: 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
4368-409: 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
4459-632: 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. 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
4550-639: The body of honey bees , and Acarapis woodi (family Tarsonemidae ) lives in their tracheae . Hundreds of species are associated with other bees, mostly poorly described. They attach to bees in a variety of ways. For example, Trigona corvina workers have been found with mites attached to the outer face of their hind tibiae. Some are thought to be parasites, while others are beneficial symbionts . Mites also parasitize some ant species, such as Eciton burchellii . Most larvae of Parasitengona are ectoparasites of arthropods, while later life stages in this group tend to shift to being predators. Plant pests include
4641-431: 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
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#17330847873434732-459: 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
4823-401: The cause of several types of itchy skin rashes, such as gamasoidosis , rodent mite dermatitis , grain itch , grocer's itch , and scabies ; Sarcoptes scabiei is a parasitic mite responsible for scabies, which is one of the three most common skin disorders in children. Demodex mites, which are common cause of mange in dogs and other domesticated animals, have also been implicated in
4914-920: 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
5005-920: 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
5096-425: The disease rickettsialpox . House dust mites , found in warm and humid places such as beds, cause several forms of allergic diseases, including hay fever , asthma and eczema , and are known to aggravate atopic dermatitis . Among domestic animals , sheep are affected by the mite Psoroptes ovis which lives on the skin, causing hypersensitivity and inflammation. Hay mites are a suspected reservoir for scrapie ,
5187-409: 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
5278-419: 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
5369-554: 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
5460-554: 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
5551-640: 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
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#17330847873435642-640: 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
5733-489: 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
5824-473: 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
5915-567: 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] Arachnid Arachnids are arthropods in
6006-413: The human skin disease rosacea , although the mechanism by which demodex contributes to the disease is unclear. Ticks are well known for carrying diseases, such as Lyme disease and Rocky Mountain spotted fever . Chiggers are known primarily for their itchy bite , but they can also spread disease in some limited circumstances, such as scrub typhus . The house-mouse mite is the only known vector of
6097-408: 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
6188-424: 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, 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
6279-539: The microscope; the short film was shown in London's Alhambra music hall in 1903, causing a boom in the sales of simple microscopes. 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
6370-489: The mid- Cretaceous , around 100 million years ago. Most fossil acarids are no older than the Tertiary (up to 65 mya ). Members of the superorders Opilioacariformes and Acariformes (sometimes known as Actinotrichida) are mites, as well as some of the Parasitiformes (sometimes known as Anactinotrichida). Recent genetic research has suggested that Acari is polyphyletic (of multiple origins). A study using molecular data from
6461-423: 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,
6552-738: The mitochondria and nucleus recovered Acariformes as sister to the Solifugae (camel spiders) and Parasitiformes as sister to the Pseudoscorpionida , with other arachnid orders separating these two groupings on the phylogenetic tree, as shown below. Palpigradi False scorpions [REDACTED] Ixodida (ticks) [REDACTED] Parasitic mites, inc. Varroa [REDACTED] Trombidiformes (chiggers, velvet mites, etc) [REDACTED] Sarcoptiformes (dust & fur mites, etc) [REDACTED] Camel spiders [REDACTED] other Arachnids including spiders and scorpions However,
6643-486: 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
6734-608: The most diverse and successful of all invertebrate groups. They have exploited a wide array of habitats , and because of their small size go largely unnoticed. They are found in freshwater (e.g. the water mites or Hydrachnidia ) and saltwater (most Halacaridae ), in the soil, in forests, pastures, agricultural crops, ornamental plants, thermal springs and caves. They inhabit organic debris of all kinds and are extremely numerous in leaf litter. They feed on animals, plants and fungi and some are parasites of plants and animals. Some 48,200 species of mites have been described, but there may be
6825-416: 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
6916-526: 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, the pedipalps , have been adapted for feeding, locomotion, and/or reproductive functions. In scorpions, pseudoscorpions, and ricinuleids
7007-504: The nests of birds and other animals. These mites are largely parasitic and consume blood , skin and keratin . Dust mites , which feed mostly on dead skin and hair shed from humans instead of consuming them from the organism directly, evolved from these parasitic ancestors. Ticks are a prominent group of mites that are parasitic on vertebrates, mostly mammal and birds, feeding on blood with specialised mouthparts. Parasitic mites sometimes infest insects. Varroa destructor attaches to
7098-504: 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; 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
7189-457: 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
7280-460: 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
7371-435: 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
7462-407: 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
7553-419: 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
7644-561: The six-legged larvae. After three moults, the larvae become nymphs, with eight legs, and after a further three moults, they become adults. Longevity varies between species, but the lifespan of mites is short compared to many other arachnids. Mites occupy a wide range of ecological niches . For example, Oribatida mites are important decomposers in many habitats. They eat a wide variety of material including living and dead plant and fungal material, lichens and carrion; some are predatory , though no oribatid mites are parasitic . Mites are among
7735-475: The so-called spider mites (family Tetranychidae ), thread-footed mites (family Tarsonemidae ), and the gall mites (family Eriophyidae ). Among the species that attack animals are members of the sarcoptic mange mites (family Sarcoptidae ), which burrow under the skin. Demodex mites (family Demodecidae ) are parasites that live in or near the hair follicles of mammals , including humans. Being unable to fly, mites need some other means of dispersal. On
7826-500: The stigmata are associated with peritremes: paired, tubular, elaborated extensions of the tracheal system. The higher taxa of mites are defined by these structures: Mite digestive systems have salivary glands that open into the preoral space rather than the foregut. Most species carry two to six pairs of salivary glands that empty at various points into the subcheliceral space. A few mite species lack an anus: they do not defecate during their short lives. The circulatory system consists of
7917-435: 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
8008-435: 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
8099-432: 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
8190-498: 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
8281-498: 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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