Chelicerata

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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 generalisedarachnid
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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 450Ma - 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 highlymodified 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 (gt106/ha found)
• hooked into small insects/mites/soft-bodies as a
  resource
• considerable diversity - many niches

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Spiders - Order Araneae

• about 32 000 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 lt6 (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
• 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. aciniform3.
ampullate4. aggregate5. flagelliform6.
cylindricaland 3 other types that female
Nephila dont 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, reversableLV2 - insects, slow,
  irreversableLV3 - 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 1927-1980.
• Shortest 8 minutes! 5 lt 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 brieflyother kinds hold prey
  firmly
• cribellate silk (velcro)
• ecribellate sticky silk (glue)

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• cribellate 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 (gt106/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