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Published byHarvey Pope Modified over 9 years ago
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Chelicerata 1. Pycnogonida - marine, ‘pycnogonids'
2. Merostomata - the horseshoe crabs 3. Arachnida - contains all the terrestrial chelicerates … 18 orders (7 ‘major’) features: chelicerae, no evidence of antennae widely separated from other ‘arthropods’
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chelicerae should be contrasted with the antennae (arising on same embryonic segment) in other arthropod groups in many chelicerate groups food is processed by the modified bases of some anterior appendages (= ‘gnathobases’)
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Pycnogonida ‘sea spiders’ small and isolated group
may be ‘sister group’ to other arthropods characterised by ‘head’ structures: proboscis chelifores + chelae palp structure ovigers body extremely reduced, organs in leg base
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Merostomata horseshoe crabs (Xiphosura, Limulus)
western side of Nth Atlantic and Nth Pacific Oceans characterised by: carapace chelicerae book gills relationships with the other chelicerates are based more on tradition than other evidence
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Arachnida Body organised as prosoma/opisthosoma
Prosoma appendages of arachnids: 1 pair of chelicerae 1 pair of pedipalps 4 pairs of legs chelicerae, pedipalps & legs I often highly modified Opisthosomal appendages vary 18 orders (7 ‘major’)
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Scorpion - a generalised arachnid
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Arachnida Eurypterida (extinct) Uropygi Palpigradi Solifugae
Scorpiones Araneae Schizomida Amblypygi Ricinulei Opiliones Pseudoscorpiones Acari (7 Orders)
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Arachnida - Eurypterida, Scorpiones, Pseudoscorpiones, Araneae
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Eurypterida ‘sea scorpions’ marine/freshwater … 450+Ma - c 250 Ma
size range medium to gigantic Pterygotus rhenaniae (~ 380Ma) 1.8m long now generally regarded as basal stock from which other arachnids radiated
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Scorpiones most plesiomorphic of extant arachnids
prosoma is covered by a solid carapace dorsally; ventrally by the coxae of the legs. opisthosoma segmented (primitive) and divided into preabdomen and postabdomen. telson modified to form sting chelae short, strong, feeding appendages pedipalps large, chelate, grasp prey presence of cuticular UV fluorescent layer
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Scorpiones - ecology sting has a sharp point, penetrates integument of prey or enemy venom produced in adjacent poison glands, injected by voluntary muscular action mostly sit-and-wait predators, operate from burrows or refuges VERY fast reflexes vision very limited use vibrations to create 3-D world picture trichobothria and sensory setae/slit sensillae grasp and tear vs touch and sting reproduction - spermatophore … maternal care
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success of scorpions morphologically very conservative – have found a niche/niches and are exploiting it/them moderately speciose (1000s of species) can be VERY abundant extreme generalists in prey taken considerable diversity – many niches
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Pseudoscorpiones body and appendages are more highly modified than in scorpions. pedipalps chelate, used for prey capture, have poison glands in the finger or hand and opening at the tip chelicerae short, strong, used to open prey comb on the chelicera cleans buccal cavity silk glands on chelicerae used to make retreats reproduction - spermatophores, maternal care of young
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success of pseudoscorpions
very important component of ‘cryptozoa’ moderately speciose (1000s of species) can be VERY abundant (>106/ha found) hooked into small insects/mites/soft-bodies as a resource considerable diversity - many niches
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Spiders - Order Araneae
about sp described likely estimates indicate 2-3x this number spiders are the most highly derived group within the chelicerates ancestral chelicerates resembled scorpions defining synapomorphies chelicerae modified to fangs structure of silk glands and spinnerets
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Features: the great many uses of silk.
the utilization of venom and diversity of feeding habits. the well developed vision of some hunting spiders. evidence for a high degree of behavioural plasticity.
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Four major groups of spiders are recognised:
Mesothelae (= Liphistiomorpha) - segmented abdomens, mid-ventral spinnerets, 4 book lungs, paraxial chelicerae. (& fossils c 300Mya) Mygalomorphae - terminal spinnerets (segmented), 4 book lungs, paraxial chelicerae Hypochilomorphae - diaxial chelicerae, 4 book lungs (relicts - 1 USA, 1 China, several Australia & NZ - gradungulids etc) Araneomorphae - diaxial chelicerae, 2 or 0 book lungs
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Relationships: (Mesothelae(Mygalomorphae(Hypochilomorphae, Araneomorphae))) Mesothelae, Mygalomorphae grades?
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Mesothelae suborder of their own two genera: Liphistius, Heptathela
~ 40 species recognised Japan, East Asia to Indonesian region 4 booklungs, paraxial chelicerae, mid-ventral spinnerets abdomen has dorsal segmental sclerites! occupy silk-lined burrows with trapdoors
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Opisthothelae Mygalomorphae
‘primitive’ group ~ 2000 species 4 booklungs, paraxial chelicerae terminal spinnerets, often segmented ambush predators from silk-lined burrows (sometimes with trapdoors) or bivouac retreats
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Opisthothelae Araneomorphae
‘true spiders’ 2 divisions recognised Hypochilomorphae, Araneae terminal spinnerets, diaxial chelicerae
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Araneomorphae Hypochilomorphae
relicts … ~ 40 sp, 8 genera Hypochilidae (USA, China - 2 sp), Austrochilidae (Tasmania, S. Am.), Gradungulidae (southern NZ, Australia) 4 booklungs, semi-diaxial chelicerae web-builders and ‘snatchers’
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Araneomorphae Araneae
‘ordinary spiders’ most diverse and ecologically successful spiders - 30,000+ species 2 or 0 booklungs, tracheae web-spinners (diverse kinds), cursorial hunters
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Anatomy two body divisions: prosoma opisthosoma
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Prosoma chelicerae fangs (synapomorphy of spiders)
pedipalps - manipulation, mating (in ) 4 pairs of walking legs eyes - most 8, many 6, rare <6 (sometimes 0)
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Opisthosoma booklungs reproductive openings spinnerets
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Chelicerae (= fangs) derived from chelate feeding organs sub-chelate
paraxial vs diaxial organisation
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Pedipalps sensory manipulation mating embolus
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Walking legs spiders one of the arachnid groups lacking extensor muscles - run on hydraulics ... short, high-speed bursts. walking leaping
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Sensory structures eyes trichobothria slit sensillae
single lyriform organs chemosensory organs
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Eyes primitively 8, some 6, rarely 0 around anterior margin/on turret
general form special forms salticid (jumping spider) eyes
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Trichobothria long, thin setae attached to socket … extensively innervated at socket only on appendages (legs mainly) may be constrained to linear movement VERY sensitive to air movements may also be used to monitor vibrations in the web or along silk lines
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Slit sensillae single lyriform organs how they function
appearance, structure amplify flexing of exoskeleton lyriform organs fields of slit sensillae … very sensitive how they function
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Chemosensory organs well-developed on basis of bioassays where?
capabilities? certainly used in intra-specific activity probably used in navigation use in predatory activity probable
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Booklungs derived from bookgills by invagination structure operation
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Spinnerets silk spinning organs are located on the ventral surface just anterior to the terminal anus. in Mygalomorphae they are obviously segmented and may be long and very mobile. Mygalomorphs mostly have 4 spinnerets, Hexathelidae have 6.
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Silk! silk the key to understanding spiders
use of silk opened up new niches most important ecological aspect is silk allowed access to flying insects - a large resource mostly not available to other organisms a ‘new zone of adaptive radiation’
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Functions of silk include:
1. Draglines - common to most spiders (= lifeline) 2. Sperm webs 3. Nest building - retreats (overnight and overwintering) 4. Egg cocoons - wrapping eggs 5. Locomotion aids: Ballooning strands - dispersal of spiderlings Bridge building (adults) 6. Sensory extensions - trip-lines, ‘microphones’ 7. Web building - prey capture 8. Swathing prey - to immobilize or keep in web. 9. Mating aids (mating webs/constraining mate)
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Silk glands 9 kinds recognised
derived from excretory glands associated with appendages on ancestral ‘abdomen’ protein produced as aqueous solution solidifies on being stretched (thixotrophic)
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Nephila silk glands 1. piriform 2. aciniform 3. ampullate 4. aggregate 5. flagelliform 6. cylindrical and 3 other types that female Nephila don’t have!
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Silks many kinds of silk all kinds not present in every spider
structural silks draglines, retreats, egg-cocoons, ... prey-catching silks cribellate ecribellate
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Physiology haemolymph circulation respiration digestion excretion
nervous system
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Heart and circulation tube in dorsal opisthosoma … ‘ostia’ (valves) enables haemolymph to enter from pericardiac sinus, pumps forward to prosoma haemolymph provides working fluid for hydraulic system haemolymph clots very quickly to seal any leaks
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Respiration oxygen is dissolved in the haemolymph as it passes through the booklungs most (not all) spiders have haemocyanin which binds oxygen oxygen transported by blood flow very low resting metabolic rate
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Digestion extra-oral predigestion sucking pharynx
digestion in midgut and diverticulae (= caecae) secretions of coxal glands run forward in channels and contribute to extra-oral processing
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Excretion Malpighian tubules guanine main nitrogenous waste
also uric acid
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Nervous system concentrated on prosoma
exceptionally large for size of animal (general feature of arachnids)
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Reproduction mating - indirect insemination females store sperm
mating systems courtship copulation cannibalism … other features maternal care
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Prey handling varies, often specialised
species with paraxial fangs run over their prey and stab downwards, often pinning the prey to the ground in the process.**** Bjorn challenges **** silk-throwing ambush
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Venom poison glands in the basal segments of the chelicerae and head open at the tip of the fang poison is released from the gland by contraction of muscle surrounding the gland. spider can control which components are injected
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Toxicity! spider venoms are complex and often contain rapid acting short term 'knock out' components and/or proteolytic enzymes.
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Latrodectus - black-widow, red-back etc, etc (species complex - world wide) LV1 - insects, knockdown, reversable LV2 - insects, slow, irreversable LV3 - vertebrates only, causes pain Atrax - intensively investigated … composition complex - within a species varies with age, sex, locality, season and hunger level! Presume same in other mygalomorphs
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Humans ... Atrax robustus male - usually dry-bites! 14 deaths Shortest 8 minutes! 5 < 2h. antivenom developed 1980. some Hadronyche species probably considerably more poisonous! Latrodectus, Loxosceles … others
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Feeding normal method: crush, slobber and suck – extra-oral digestion, suck in fluids. vs Thomisidae liquids and small particles only - via ‘pumping pharynx’ (‘pumping stomach’)
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Prey some spider species are prey specialists,
others are extremely general, attacking as opportunity arises.
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Webs ? derived from safety lines or sensory lines many different kinds
some detain prey briefly other kinds hold prey firmly cribellate silk (‘velcro’) ecribellate sticky silk (‘glue’)
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cribellate silk ecribellate silk
tufts of very fine silk added to main thread tangle setae, protuberances - gets insects remains ‘sticky’ for a long time has evolved in several lineages ecribellate silk sticks through proteinaceous glue glue added as silk spins glue droplets produced by ‘strumming’ stickiness deteriorates rapidly
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Web structures sensory lines sheet webs orb webs cobwebs other webs
special webs - bolus spider, net-casting spider etc.
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Vagrant and cursorial hunters
‘option’ occurs in many spider lineages Salticidae (jumping spiders), Lycosidae (wolf spiders) best studied very common, but mostly nocturnal so not widely observed
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Salticidae largest (most speciose) family
characterised by eye structure and vision stalk then leap onto prey
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success of spiders possibly most successful of the arachnids (mites challenge ...) moderately speciose (substantially more species than the vertebrates) can be VERY abundant (>106/ha found) hooked into insects as a resource considerable diversity - many niches
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web exercise find out about the morphology and likely ecology of triginotarbid arachnids
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