Chelicerates

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Taxonomy

  • Chelicerata: a subphylum of arthropods in which the body comprises a cephalothorax/prosoma and an abdomen/opisthosoma
    • first pair of feeding structures are the chelicerae
    • Arachnida: a class of terrestrial chelicerate arthropods that includes spiders, scorpions, mites, and ticks
    • Xiphosura: an order of chelicerates containing horseshoe crabs, up to 60cm long, with a body covered in tough cuticle
    • Eurypterida: an order of extinct 'sea scorpions' that first appeared in the early Ordovician or late Cambrian
    • Pycnogonida: class of marine chelicerates, also known as sea spiders 
  • relatively old lineage with the oldest fossils dating to the Cambrian, at least ~510 million years ago
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Arachnida

  • contains 11 main extant lineages
  • Acari: mites and ticks, with around 56,000 species
  • Araneaespiders, with around 48,000 species
  • Opiliones: harvestmen, with around 67,000 species found worldwide
  • Pseudoscorpiones: false scorpions, around 3,500 species
  • Scorpiones: scorpions, around 24,000 species
  • Solifugae: camel spiders, around 1,100 species
  • Schizomida: short-tailed whip scorpions, around 300 species
  • Amblypygi: whip spiders, around 220 species
  • Thelyphonida: also known as Uropygi, the whip scorpions and vinegaroons, around 110 species
  • Palpigradi: microwhip scorpions, around 92 species
  • Ricinulei: hooded tickspiders, around 76 species
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Xiphosura

  • currently contains only four living species, in the family Limulidae
    • Limulidae: a family of chelicerates, in which the body, divided into prosoma and opisthosoma with a spine-like telson, is protected by a hard carapace
  • prosoma bears five pairs of legs, all but the last with claws
  • come onto land to reproduce
  • found on the Pacific coast of North America
  • originally in the group Merostomata
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Pycnogonida

  • very old marine chelicerates also known as sea spiders, containing over 1,300 known species
  • leg spans ranging from 1mm to over 70cm
  • the exoskeleton is highly primitive
  • four simple eyes, but some species have lost these eyes completely
  • have two extra appendages on the prosoma which are used by males for brooding
    • oviger: one of a pair of legs, located behind the palps and in front of the walking legs, used for grooming and in the male, carrying eggs, may be reduced or absent in females 
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Anatomy

  • no antennae
  • tagmosis is in two parts: prosoma and opisthosoma
  • united by possessing appendages, known as chelifores in Pycnogonida and chelicerae in Euchelicerata
    • primitive condition is three segments, but most arachnids only have two segments 
  • only have two parts to their brain: protocerebrum and tritocerebrum 
  • arachnids have median and lateral ocelli that vary in number and presence
  • digestive and excretory system consists of a preoral chamber where food is liquified with digestive enzymes or chewed, before entering the pharynx
  • many chelicerates use book lungs, book gills, or a tracheal system
  • possess an open circulatory system where the hemolymph is pumped through the hemocoel by a dorsal tube-shaped heart that collects hemolymph
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Physiology

  • most spiders have six spinnerets, which either move independently or in concert
    • silk glands in the opisthosoma produce the silk, which is highly variable depending on its use
    • silk is a complex and fibrous protein which is five times stronger than steel and twice as elastic as nylon
    • Biosteel is Nexia's recombinant dragline spider silk programme and is based on using Nexia's transgenic goat technology
    • in some groups, such as the Pseudoscorpiones, the Chelicerae bear openings of silk glands
    • Prostigmata are silk producers 
  • Phoneutria is a genus of spiders which present a significant threat to humans, but only inject venom 1/3 of the time
    • also known as the Brazilian wandering spiders
    • medically significant venom
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Evolution

  • thrive and diversify quite a lot in the Cambrian explosion, and the first potential chelicerate appeared around 545 million years ago
  • eurypterids first appeared on the boundary between the Cambrian and Ordovician
  • fossil pycnogonids appeared in the early Devonian
  • pseudoscorpions appeared during the Devonian
  • phalangotarbida and fossil Opiliones first appeared during the Carboniferous 
  • palpigrades appeared on the boundary between the Jurassic and Cretaceous
  • amber mines were found in the Oligocene
  • the very first chelicerae were scorpion-like and giant
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Ecology

  • Bagheera kiplingi is a neotropical jumping spider that exploits an ant-plant mutualism, and is predominantly herbivorous
  • the yellow-tailed scorpion is the only scorpion that can be found in the UK, on the Isle of Sheppy in Kent
  • Trombidiformes also contain follicular mites such as Demodex canis, Demodex folliculorum, and Demodex brevis
  • phoresy is present in clades associated with ephemeral habitats, which helps them colonise new places
  • mites can be found in soft furniture (Dermatophagoides farinae) and in cheese (Tyrophagus putrescentiae)
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Social Behaviour

  • most arachnids are solitary, but social behaviour is known as some taxa
  • Opiliones sometimes form dense aggregations, potentially numbering in the thousands on rocks, trees, or in caves 
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Reproduction

  • use both external and internal fertilisation
    • internal fertilisation is either direct or indirect
      • indirect fertilisation is known in arachnids
      • direct fertilisation is known to occur in some mites and phalangid Opiliones
    • external fertilisation occurs in Pcynogonida and Xiphosura
  • all chelicerates lay eggs except for scorpions and some mites
  • parthenogenic taxa have been recorded in several arachnid taxa including mites, scorpions, harvestmen, and amblypygids
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