Adult arachnids have eightlegs attached to thecephalothorax. In some species the frontmost pair of legs has converted to a sensory function, while in others, differentappendages can grow large enough to take on the appearance of extra pairs of legs.
Almost allextant arachnids areterrestrial, living mainly on land. However, some inhabit freshwater environments and, with the exception of thepelagic zone, marine environments as well. They comprise over 110,000 namedspecies, of which 51,000 are species of spiders.[3][4]
The term is derived from theGreek wordἀράχνη (aráchnē, 'spider'), from the myth of the hubristic human weaverArachne, who was turned into a spider.[5]
Basic characteristics of arachnids include four pairs oflegs (1) and a body divided into twotagmata: the cephalothorax (2) and the abdomen (3)
Almost all adult arachnids have eight legs, unlike adultinsects 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, thechelicerae, serve in feeding and defense. The next pair, thepedipalps, have been adapted for feeding, locomotion, and/orreproductive functions. In scorpions, pseudoscorpions, andricinuleids the pedipalps end in a pair of pinchers, while in whip scorpions,Schizomida,Amblypygi, and most harvestmen, they areraptorial and used for prey capture.[6] InSolifugae, the palps are quite leg-like, so that these animals appear to have ten legs. Thelarvae of mites and Ricinulei have only six legs; a fourth pair usually appears when theymoult intonymphs. However, mites are variable: as well as eight, there are adult mites with six or, like inEriophyoidea, even four legs.[7][8] While the adult males in some members ofPodapolipidae have six legs, the adult females have only a single pair.[9]
Arachnids are further distinguished from insects by the fact they do not haveantennae orwings. Their body is organized into twotagmata, called theprosoma andopisthosoma, also referred to as thecephalothorax andabdomen. However, there are questions about the validity of the latter terms. While the termcephalothorax implies a fusedcephalon (head) andthorax, 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.[10]
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 the mites, the abdominal sections are completely fused.[11] Atelson is present in scorpions, where it has been modified to a stinger, and into a flagellum in thePalpigradi, Schizomida (very short) andwhip scorpions.[12] At the base of the flagellum in the two latter groups there are glands which produce acetic acid as a chemical defense.[13] Except for a pair ofpectines in scorpions,[14] and thespinnerets in spiders, the abdomen has no appendages.[15]
Like all arthropods, arachnids have anexoskeleton, and they also have an internal structure ofcartilage-like tissue, called theendosternite, to which certain muscle groups are attached. The endosternite is even calcified in someOpiliones.[16]
Most arachnids lackextensor muscles in thedistal joints of their appendages. Spiders and whip scorpions extend their limbs hydraulically using the pressure of theirhemolymph.[17]Solifuges and someharvestmen extend their knees by the use of highly elastic thickenings in the joint cuticle.[17] Scorpions, pseudoscorpions and some harvestmen have evolved muscles that extend two leg joints (the femur-patella and patella-tibia joints) at once.[18][19] The equivalent joints of the pedipalps of scorpions though, are extended by elastic recoil.[20]
There are characteristics that are particularly important for the terrestrial lifestyle of arachnids, such as internal respiratory surfaces in the form oftracheae, or modification of thebook gill into abook lung, an internal series ofvascularlamellae used forgas exchange with the air.[21] 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 thespiracle. This type of tracheal system has almost certainly evolved from the book lungs, and indicates that the tracheae of arachnids are nothomologous with those of insects.[22]
Further adaptations to terrestrial life areappendages modified for more efficient locomotion on land,internal fertilisation, special sensory organs, and water conservation enhanced by efficientexcretory structures as well as a waxy layer covering the cuticle.[citation needed]
The excretory glands of arachnids include up to four pairs ofcoxal glands along the side of the prosoma, and one or two pairs ofMalpighian 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 isguanine.[22]
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 containshaemocyanin, a copper-based pigment with a similar function tohaemoglobin in vertebrates. Theheart is located in the forward part of the abdomen, and may or may not be segmented. Some mites have no heart at all.[22]
Arachnids are mostlycarnivorous, 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 ofmites also include tiny animals, fungi, plant juices and decomposing matter.[23] Almost as varied is the diet ofharvestmen, where we will find predators, decomposers and omnivores feeding on decaying plant and animal matter, droppings, animals and mushrooms.[24][25][26] Theharvestmen and some mites, such as thehouse dust mite, are also the only arachnids able to ingest solid food, which exposes them to internal parasites,[27] although it is not unusual for spiders to eat their own silk. Andone species of spider is mostly herbivorous.[28] Scorpions, spiders and pseudoscorpions secretevenom from specializedglands to kill prey or defend themselves.[29] Their venom also contains pre-digestive enzymes that helps breaking down the prey.[30][31][32] The saliva of ticks contains anticoagulants and anticomplements, and several species produce aneurotoxin.[33][34]
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 the mouth. Behind the mouth is a muscular, sclerotisedpharynx, which acts as a pump, sucking the food through the mouth and on into theoesophagus andstomach. In some arachnids, the oesophagus also acts as an additional pump.[citation needed]
The stomach is tubular in shape, with multiplediverticula 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 sclerotisedintestine andanus in the hind part of the abdomen.[22]
Arachnids have two kinds of eyes: the lateral and medianocelli. The lateral ocelli evolved fromcompound eyes and may have atapetum, 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 theectoderm. The ancestors of modern arachnids probably had both types, but modern ones often lack one type or the other.[27] Thecornea 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 theretina 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.[22]
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, calledtrichobothria.[citation needed]
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 inproprioception, and possibly also hearing.[22]
Arachnids may have one or twogonads, 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, orspermatophore. The males in harvestmen and some mites have a penis.[35] Complex courtship rituals have evolved in many arachnids to ensure the safe delivery of the sperm to the female.[22] Members of many orders exhibit sexual dimorphism.[36]
Arachnids usually lay yolkyeggs, which hatch into immatures that resemble adults. Scorpions, however, are eitherovoviviparous orviviparous, depending on species, and bear live young. Also some mites are ovoviviparous and viviparous, even if most lay eggs.[37] In most arachnids only the females provide parental care, with harvestmen being one of the few exceptions.[38][39]
Thephylogenetic 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 amonophyletic group and are divided into three main clades: chelicerates (including arachnids), pancrustaceans (theparaphyletic crustaceans plus insects and their allies), and myriapods (centipedes, millipedes and allies).[40][41][42][43][44] The three groups are related as shown in thecladogram below.[42] Including fossil taxa does not fundamentally alter this view, although it introduces some additional basal groups.[45]
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.[41][44] (Pycnogonida (sea spiders) may be excluded from the chelicerates, which are then identified as the group labelled "Euchelicerata".[46]) A 2019 analysis nests Xiphosura deeply within Arachnida.[47]
Discovering relationships within the arachnids has proven difficult as of March 2016[update], 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 ordersAcariformes,Parasitiformes andPseudoscorpiones, which have had much faster evolutionary rates. Analyses of the data using sets of genes with different evolutionary rates produced mutually incompatiblephylogenetic 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.[44]
Tetrapulmonata, here consisting ofAraneae,Amblypygi andUropygi (Thelyphonidas.s.) (Schizomida was not included in the study), received strong support. Somewhat unexpectedly, there was support for a clade comprisingOpiliones,Ricinulei andSolifugae, a combination not found in most other studies.[44] 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 tolong branch attraction.[47] The addition ofScorpiones to produce a clade called Arachnopulmonata was also well supported. Pseudoscorpiones may also belong here, as all six orders share the same ancient wholegenome duplication,[48][49] and analyses support pseudoscorpions as the sister group of scorpions.[50][51] 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 34Hox genes, the highest number found in any invertebrate, yet it is not clear if the oldest genome duplication is related to the one in Arachnopulmonata.[52]
More recent phylogenomic analyses that have densely sampled both genomic datasets and morphology have supported horseshoe crabs as nested inside Arachnida, suggesting a complex history of terrestrialization.[53][54] Morphological analyses including fossils tend to recover the Tetrapulmonata, including the extinct group theHaptopoda,[55][56][57][58][59] but recover other ordinal relationships with low support.
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 thePalaeozoic Uraraneida, some 200 million years after other known fossils with tails. The fossil resembles the most primitive living spiders, themesotheles.[61][55]
The subdivisions of the arachnids are usually treated asorders. Historically,mites andticks 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.[citation needed]
Solifugae – solpugids, windscorpions, sun spiders or camel spiders (1,200 species)
Uropygi (also called Thelyphonida) – whip scorpions or vinegaroons, forelegs modified into sensory appendages and a long tail on abdomen tip (120 species)
Extinct forms
†Haptopoda – extinct arachnids apparently part of theTetrapulmonata, the group including spiders and whip scorpions (1 species)
†Phalangiotarbida – extinct arachnids of uncertain affinity (30 species)
^Schmidt, Günther (1993).Giftige und gefährliche Spinnentiere [Poisonous and dangerous arachnids] (in German). Westarp Wissenschaften. p. 75.ISBN978-3-89432-405-6.
^Quesada-Hidalgo, Rosannette; Solano-Brenes, Diego; Requena, Gustavo S.; Machado, Glauco (April 2019). "The good fathers: efficiency of male care and the protective role of foster parents in a Neotropical arachnid".Animal Behaviour.150:147–155.doi:10.1016/j.anbehav.2019.02.007.S2CID73728615.
^Nazareth, Tais M.; Machado, Glauco (March 2010). "Mating system and exclusive postzygotic paternal care in a Neotropical harvestman (Arachnida: Opiliones)".Animal Behaviour.79 (3):547–554.doi:10.1016/j.anbehav.2009.11.026.S2CID53166528.
^Meusemann, Karen; Reumont, Björn M. von; Simon, Sabrina; Roeding, Falko; Strauss, Sascha; Kück, Patrick; Ebersberger, Ingo; Walzl, Manfred; Pass, Günther; Breuers, Sebastian; Achter, Viktor; Haeseler, Arndt von; Burmester, Thorsten; Hadrys, Heike; Wägele, J. Wolfgang & Misof, Bernhard (2010)."A Phylogenomic Approach to Resolve the Arthropod Tree of Life".Molecular Biology and Evolution.27 (11):2451–2464.doi:10.1093/molbev/msq130.PMID20534705.
^abRegier, Jerome C.; Shultz, Jeffrey W.; Zwick, Andreas; Hussey, April; Ball, Bernard; Wetzer, Regina; Martin, Joel W. & Cunningham, Clifford W. (2010). "Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences".Nature.463 (7284):1079–1083.Bibcode:2010Natur.463.1079R.doi:10.1038/nature08742.PMID20147900.S2CID4427443.
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