Chelicerates
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- Created by: Becky_Berry
- Created on: 13-11-23 14:50
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
- Araneae: spiders, 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
- internal fertilisation is either direct or indirect
- 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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