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From Wikipedia, the free encyclopedia

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.[1] Spiders are the largest order in the class, which also includes scorpions, ticks, mites, harvestmen, and solifuges.[2]

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.

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Transcription

You’ve got your spiders, scorpions, harvestmen, ticks, and mites… Arachnids aren’t the most popular group of animals – lots of people think they’re scary or just plain old pests. But they are fascinating, and important parts of ecosystems all around the world. Whether they’re dancing, hunting, or being a pain in the nose, there have been lots of amazing new arachnid species discovered or officially described in just the last five years. We’ll start with a new species of peacock spider, which are basically the birds-of-paradise of the arachnid world. This little guy – just a few millimeters long – is called Maratus bubo [ma-RAH-tus BOO-boe] since his abdomen kinda happens to look like a horned owl. He’s one of seven new peacock spiders officially announced in May 2016, and was discovered in 2015 by Jürgen Otto [Yur-gen] and David Knowles, who were out spider-hunting in southwest Australia. These brightly colored males strut their stuff to find a mate, and their dances are pretty impressive: Lifting two middle legs to frame his bright abdomen, he shimmies from side-to-side and jiggles his booty – eyes locked on his audience-of-one. This is supposed to show off how healthy he is, since he’s hoping to pass on his genes to the next generation of spiders. Otto and his colleague David Hill have helped discover and categorize dozens of peacock spiders, and more movers-and-shakers could be on the way soon. Jürgen Otto found this other new species in 2015 – a jumping spider that was just sitting on his luggage as he unpacked from a camping trip! This critter doesn’t have the mad dancing flair of peacock spiders, and takes a more cautious approach to wooing the ladies… almost like playing “peek-a-boo.” The males have large hairy paddles on two of their middle legs, which are important in finding a mate. See, he’ll hide just out of his potential mate’s view – like, on the other side of a leaf – then stick out one of these paddles and wave. Now, female spiders of lots of species generally attack, kill, or even eat the males. And Otto noticed that most of the females were lunging at the male spiders’ waving leg paddles. So, at first, Otto thought the male spider was trying to tire the female into submission... but eventually the males just gave up and scurried away. More research suggested it might have to do with finding out the female’s personality, or even whether she’s mated before. Certain spider species will only mate once, so if she’s aggressive, it might mean she can’t mate anymore. But if she sits still and tolerates his coy waving long enough, he’ll consider it an invitation to make a more... personal introduction. This next spider may not have any fancy decorations, but it’s got its own signature move: the flic-flac or back handspring. It seems to move like a normal spider at first. But when it’s startled, it turns into a leggy tumbleweed and flings itself away from danger, or straight at whatever disturbed it – y’know, to act all macho and intimidating. It can flip forwards or backwards. But usually, the spider /pushes off/ of its front legs and front-handsprings across the sand or even up slopes! It’s the only spider known to move this way, and could even keep up with a human jogger. But such a high-energy move needs to be saved for emergency situations, or this spider would be exhausted. The flic-flac spider was discovered in 2009 by Ingo Rechenberg [REHH-en-berg], a bionics professor from Berlin who was visiting the Erg Chebbi [urg cheh-bee] desert in Morocco. Rechenberg was so impressed by the spider’s tumbling that he built some rolling robots that mimic its movement – especially to help the robots move across sand, a notoriously challenging terrain. He also showed the spider to arachnologist Peter Jäger [YAY-ger] from the Senckenberg [ZEN-ken-berg] Research Institute in Frankfurt, who officially described the new species in 2014, and named it after its discoverer. How many eyes do spiders have? You might be thinking eight... but not always! This new species of huntsman spider, discovered by Peter Jäger in a cave in Laos [lao], has /zero/. If you spend your life in the pitch black, it’s better to use energy for other senses like smell or touch, because vision isn’t gonna help you get around. So it’s not unusual for animals living deep underground, underwater, or in caves to lose their eyesight over evolutionary time. But this spider’s not just blind – it’s completely eyeless. No lenses, no light detecting pigments, just a smooth, featureless face above those menacing fangs. Jäger found other new huntsmen spider species in the Laos caves, but none of them had completely gotten rid of their eyes! That being said, some of the species’ eyes were more developed than others, ranging from a complete-looking set of eight, to two tiny remnants that probably didn’t do much. We’ve got a lot to learn before we understand why these spiders live in such similar environments, but apparently see the world so differently. Lots of people have heard Spiderman’s origin story… over and over again, thanks to all the reboots… but the origin of /spiders/ is much more mysterious to scientists. However new research published in March 2016 on a proto-spider, or almost-spider, fossil from France tells us more of this ancient story. In fact, they even named this proto-spider after a Greek myth – Arachne [uh-rock-knee or uh-rack-knee] who was turned into a spider by the goddess Athena for her pride, and her father Idmon [id-mahn]. The 305-million-year-old fossil is stunningly well-preserved – it’s even in 3D! And the team of researchers, headed by Russell Garwood from the University of Manchester, used high-res scanning techniques to create a detailed “virtual fossil.” That way, they could study how it compares to modern spiders. It /does/ look a lot like a spider, which suggests that this body plan is pretty ancient. But it doesn’t have spinnerets, those silk-spinning organs that all modern spiders have. The researchers think this proto-spider /did/ have a simple way to make silk. But without spinnerets it wouldn’t have had enough control to make intricate webs – the silk would just kinda spurt out. So they think the fossil is an ancient cousin, not a direct ancestor of the modern spider. And spinnerets must’ve appeared in a later, separate part of the spider’s history. In the forests around the southwest Oregon mountains, there lives a creature known as Cryptomaster behemoth [be-HE-moth]. It might sound like something out of a conspiracy theory, but this little monster is real. But it’s /not/ a spider. It’s a harvestman, which some people call daddy-long-legs. In 1969, the Cryptomaster leviathan [Le-VYE-uh-thun] was discovered, and named for its secretive behavior and large body size compared to other daddy-long-legs. For decades, it was thought to be one-of-a-kind in the genus… until January this year. A team from San Diego University collected 77 Cryptomaster daddy-long-legs from 14 different regions of southern Oregon. And they /weren’t/ all similar. Careful measurements of body parts, mapping their habitats, and genetic analyses all confirmed that the Cryptomaster genus was really two species, not just one. So, they had to pick a name for this sister species. And what’s worthy enough to match the biblical leviathan? Well, a behemoth of course! Mites are one of the smallest and most diverse group of arachnids – including the things that live on your face, or the dust mites in your bed. This cool worm-like mite species, called the buckeye dragon mite, was discovered by Samuel Bolton in the /exotic/ soil of the Ohio State University campus and described in 2014. It /might/ [pun emphasis up to Aranda] not look like much at first, but electron microscopy reveals a whole new beast. It’s a microbivore, or something that feeds on single-celled organisms like yeast and bacteria – but only the juices inside. Bolton’s team studied this mite’s intricate mouthparts and think its feeding habits probably resemble something between a hamster and a trash compactor. Here’s their hypothesis: as the mite travels through the soil, special cup-shaped hairs near its mouth attract microbes through intermolecular forces. The microbes get stored in a little pouch above its mouth. And when the time is right, the researchers think a pincer stabs into the pouch, crushing the cells until they burst and release all those delicious juices. The team hasn’t observed the buckeye dragon mite feeding to test their theory, but they think this technique would extract lots of nutritious microbial goop – great for living in poor quality soil where food is hard to come by. To discover a new arachnid, sometimes you just need to follow your nose. Not that researcher Tony Goldberg had much of a choice. After a research trip to Kibale [ki-BALL-ay], Uganda in 2012, he returned to his University of Wisconsin-Madison lab and felt a sharp pain up his right nostril. And he discovered... a tick. These bloodsuckers are well known for latching onto skin, but the nose thing isn’t well understood. And this tick’s DNA wasn’t a match for any known species, so it /could/ be a new one. To know for sure, the team needs to do some more research. But the tick did inspire a different kind of study: Goldberg researches diseases that are transmitted between humans and animals. And he wanted to study whether /chimpanzees/ in Kibale also had these nose ticks – especially because ticks can spread some pretty nasty diseases. He called up some colleagues who had hundreds of photos of young chimp faces for their own research on facial development, and at least 20% of them had nostril stowaways! It seems really unlikely that so many ticks would get randomly get lost and end up in their noses, so it could be a survival strategy – to avoid being caught by social grooming. Brazil is one of the most biodiverse places on Earth, and can be a great place to find new species. So two researchers from Rio de Janeiro and Copenhagen thought that the number of Brazilian whip spiders was suspiciously low compared to nearby countries. And they wanted to test if this was really true, or just a gap in the research. Whip spiders are not actually spiders, despite their name and looks. They don’t have silk or venom glands like most spiders, and use spiny claw-like pedipalps to catch insect and small vertebrate prey. The “whips” are modified front legs that work as touch and chemical sensors, which help them navigate the caves and forest floors where they live. The researchers scoured Brazilian museum collections for whip spiders from the Amazon, focusing on one genus called Charinus [CHAIR-in-us or maybe CARE-in-us because Latin?]. Taking painstaking measurements of specimens’ legs, eyes, and genitals, they uncovered 8 new whip spiders native to Brazil – almost doubling the known species in the Charinus genus as of February 2016. Work like this helps understand the area’s full biodiversity while it’s still there, since these whip spiders’ habitats are threatened by dam building, deforestation, and mining. So even though they get a bad rep sometimes, all these new arachnid species have their own, awesome stories and niche on Earth. Thanks to Jürgen Otto, Ingo Rechenberg, Peter Jäger, Russell Garwood, James Starrett, Samuel Bolton, Tony Goldberg and Gustavo Silva de Miranda for their help with this episode, and thanks to all of our patrons on Patreon who make this show possible. If you want to help us keep making videos like this, just go to patreon.com/scishow And don’t forget to go to youtube.com/scishow and subscribe!

Contents

Morphology

Basic characteristics of arachnids include four pairs of legs (1) and a body divided into two tagmata: the cephalothorax (2) and the abdomen (3)
Basic characteristics of arachnids include four pairs of legs (1) and a body divided into two tagmata: the cephalothorax (2) and the abdomen (3)

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.[3]

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.[4] A telson is present in scorpions, where it has been modified to a stinger, and in the Schizomida, whip scorpions and Palpigradi.[5]

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.[6]

Locomotion

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.[7] Solifuges and some harvestmen extend their knees by the use of highly elastic thickenings in the joint cuticle.[7] Scorpions, pseudoscorpions and some harvestmen have evolved muscles that extend two leg joints (the femur-patella and patella-tibia joints) at once.[8][9] The equivalent joints of the pedipalps of scorpions though, are extended by elastic recoil.[10]

Physiology

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.[11] 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.[12]

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.[12]

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.[12]

Diet and digestive system

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,[13] 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.[12]

Senses

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.[13] 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.[12]

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.[12]

Reproduction

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.[12]

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.[citation needed]

Taxonomy and evolution

Phylogeny

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).[14][15][16][17][18] The three groups are related as shown in the cladogram below.[16] Including fossil taxa does not fundamentally alter this view, although it introduces some additional basal groups.[19]

Arthropoda

Chelicerata (sea spiders, horseshoe crabs and arachnids)

Mandibulata

Pancrustacea (crustaceans and insects)

Myriapoda (centipedes, millipedes, and allies)

The extant chelicerates comprise two marine groups: sea spiders and horseshoe crabs, and the terrestrial arachnids. These are related as shown below.[15][18] (Pycnogonida (sea spiders) may be excluded from the chelicerates, which are then identified as the group labelled "Euchelicerata".[20])

Chelicerata

Pycnogonida (sea spiders)

Euchelicerata

Xiphosura (horseshoe crabs)

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.[18]

Arachnida

Acariformes

Trombidium holosericeum (aka).jpg

Opiliones

Harvestman opilio canestrinii male.jpg

Ricinulei

Cryptocellus goodnighti.jpg

Solifugae

Galeodes.jpg

Parasitiformes

Pseudoscorpiones

Ar 1.jpg

Scorpiones

SCORPIO MAURUS PALMATUS.jpg  

Tetrapulmonata

Araneae

Araneus diadematus (aka).jpg  

Amblypygi

Amblypigid.jpg  

Thelyphonida (Uropygi)

Whipscorpion.jpg  

Arachnopulmonata
Hubbardia pentapeltis (Schizomida)
Hubbardia pentapeltis (Schizomida)

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.[18]

Fossil history

Fossil Goniotarbus angulatus (Phalangiotarbi)
Fossil Goniotarbus angulatus (Phalangiotarbi)
Fossil of Kreischeria (Trigonotarbida)
Fossil of Kreischeria (Trigonotarbida)

The Uraraneida are an extinct order of spider-like arachnids from the Devonian and Permian.[21]

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.[22]

Taxonomy

Eukoenenia spelaea (Palpigradi)
Eukoenenia spelaea (Palpigradi)

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.

  • Acariformes – mites (32,000 species)
  • Amblypygi – "blunt rump" tail-less whip scorpions with front legs modified into whip-like sensory structures as long as 25 cm or more (153 species)
  • Araneae – spiders (40,000 species)
  • Haptopoda – extinct arachnids apparently part of the Tetrapulmonata, the group including spiders and whip scorpions (1 species)
  • Opilioacariformes – harvestman-like mites (10 genera)
  • Opiliones – phalangids, harvestmen or daddy-long-legs (6,300 species)
  • Palpigradi – microwhip scorpions (80 species)
  • Parasitiformes – ticks (12,000 species)
  • Phalangiotarbi – extinct arachnids of uncertain affinity (30 species)
  • Pseudoscorpionida – pseudoscorpions (3,000 species)
  • Ricinulei – ricinuleids, hooded tickspiders (60 species)
  • Schizomida – "split middle" whip scorpions with divided exoskeletons (220 species)
  • Scorpiones – scorpions (2,000 species)
  • Solifugae – solpugids, windscorpions, sun spiders or camel spiders (900 species)
  • Thelyphonida (also called Uropygi) – whip scorpions or vinegaroons, forelegs modified into sensory appendages and a long tail on abdomen tip (100 species)
  • Trigonotarbida – extinct (late Silurian early Permian)
  • Uraraneida – extinct spider-like arachnids, but with a "tail" and no spinnerets (2 species)

It is estimated that 98,000 arachnid species have been described, and that there may be up to 600,000 in total.[23]

See also

References

  1. ^ "Arachnid". Oxford English Dictionary (2nd ed.). 1989.
  2. ^ Cracraft, Joel & Donoghue, Michael, eds. (2004). Assembling the Tree of Life. Oxford University Press. p. 297.
  3. ^ Schmidt, Günther (1993). Giftige und gefährliche Spinnentiere [Poisonous and dangerous arachnids] (in German). Westarp Wissenschaften. p. 75. ISBN 3-89432-405-8.
  4. ^ Ruppert, E.; Fox, R. & Barnes, R. (2007). Invertebrate Zoology: A Functional Evolutionary Approach (7th ed.). Thomson Learning. ISBN 0-03-025982-7.
  5. ^ The Colonisation of Land: Origins and Adaptations of Terrestrial Animals
  6. ^ Kovoor, J. (1978). "Natural calcification of the prosomatic endosternite in the Phalangiidae (Arachnida:Opiliones)". Calcified Tissue Research. 26 (3): 267–269. doi:10.1007/BF02013269. PMID 750069.
  7. ^ a b Sensenig, Andrew T. & Shultz, Jeffrey W. (February 15, 2003). "Mechanics of Cuticular Elastic Energy Storage in Leg Joints Lacking Extensor Muscles in Arachnids". Journal of Experimental Biology. 206 (4): 771–784. doi:10.1242/jeb.00182. ISSN 1477-9145. Retrieved 2012-05-18.
  8. ^ Shultz, Jeffrey W. (February 6, 2005). "Evolution of locomotion in arachnida: The hydraulic pressure pump of the giant whipscorpion, Mastigoproctus giganteus (Uropygi)". Journal of Morphology. 210 (1): 13–31. doi:10.1002/jmor.1052100103. ISSN 1097-4687.
  9. ^ Shultz, Jeffrey W. (January 1, 1992). "Muscle Firing Patterns in Two Arachnids Using Different Methods of Propulsive Leg Extension". Journal of Experimental Biology. 162 (1): 313–329. ISSN 1477-9145. Retrieved 2012-05-19.
  10. ^ Sensenig, Andrew T. & Shultz, Jeffrey W. (2004). "Elastic energy storage in the pedipedal joints of scorpions and sun-spiders (Arachnida, Scorpiones, Solifugae)". Journal of Arachnology. 32 (1): 1–10. doi:10.1636/S02-73. ISSN 0161-8202.
  11. ^ Garwood, Russell J. & Edgecombe, Gregory D. (September 2011). "Early Terrestrial Animals, Evolution, and Uncertainty". Evolution: Education and Outreach. New York: Springer Science+Business Media. 4 (3): 489–501. doi:10.1007/s12052-011-0357-y. ISSN 1936-6426. Retrieved 2015-07-21.
  12. ^ a b c d e f g Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 596–604. ISBN 0-03-056747-5.
  13. ^ a b Machado, Glauco; Pinto-da-Rocha, Ricardo & Giribet, Gonzalo (2007). Pinto-da-Rocha, Ricardo; Machado, Glauco & Giribet, Gonzalo, eds. Harvestmen: the Biology of Opiliones. Harvard University Press. ISBN 0-674-02343-9.
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