Transcript Chelicerata - James Cook University

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’
• 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’)
Pycnogonida
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‘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
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
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’)
Scorpion - a generalised
arachnid
Arachnida
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Eurypterida (extinct) • Schizomida
Uropygi
• Amblypygi
Palpigradi
• Ricinulei
Solifugae
• Opiliones
Scorpiones
• Pseudoscorpiones
Araneae
• Acari (7 Orders)
1. Scorpiones,
2. Amblypygi,
3. Schizomida,
4. Thelyphonida
(aka Uropygi,)
5. Ricinulei,
6. Palpigradi,
7. Pseudoscorpions,
8. Opiliones,
9. Solifugae,
10. Acari,
11. Araneae
Relationships among Arachnida … ?
Arachnida
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Eurypterida (extinct)
Uropygi
Palpigradi
Solifugae
Scorpiones
Araneae
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Schizomida
Amblypygi
Ricinulei
Opiliones
Pseudoscorpiones
Acari (7 Orders)
Arachnida Eurypterida,
Scorpiones,
Pseudoscorpiones,
Araneae
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
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
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
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
success of pseudoscorpions
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very important component of ‘cryptozoa’
moderately speciose (1000s of species)
can be VERY abundant (>106/ha found)
hooked into small insects/mites/softbodies as a resource
• considerable diversity - many niches
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
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.
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
Relationships:
• (Mesothelae(Mygalomorphae(Hypochilo
morphae, Araneomorphae)))
– Mesothelae, Mygalomorphae grades?
Mesothelae
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suborder of their own
two genera: Liphistius, Heptathela
~ 40 species recognised
Japan, East Asia to Indonesian region
4 booklungs, paraxial chelicerae, midventral spinnerets
• abdomen has dorsal segmental sclerites!
• occupy silk-lined burrows with trapdoors
Opisthothelae
Mygalomorphae
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‘primitive’ group
~ 2000 species
4 booklungs, paraxial chelicerae
terminal spinnerets, often segmented
ambush predators from silk-lined
burrows (sometimes with trapdoors) or
bivouac retreats
Opisthothelae
Araneomorphae
• ‘true spiders’
• 2 divisions recognised
– Hypochilomorphae, Araneae
• terminal spinnerets, diaxial chelicerae
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’
Araneomorphae
Araneae
• ‘ordinary spiders’
• most diverse and ecologically
successful spiders - 30,000+ species
• 2 or 0 booklungs, tracheae
• web-spinners (diverse kinds),
cursorial hunters
Anatomy
• two body divisions:
– prosoma
– opisthosoma
Prosoma
• chelicerae  fangs (synapomorphy of
spiders)
• pedipalps - manipulation, mating (in  )
• 4 pairs of walking legs
• eyes - most 8, many 6, rare <6
(sometimes 0)
Opisthosoma
• booklungs
• reproductive openings
• spinnerets
Chelicerae (= fangs)
• derived from chelate feeding organs
• sub-chelate
• paraxial vs diaxial organisation
Pedipalps
• sensory
• manipulation
• mating
– embolus
Walking legs
• spiders one of the arachnid groups
lacking extensor muscles - run on
hydraulics ... short, high-speed bursts.
• walking
• leaping
Sensory structures
• eyes
• trichobothria
• slit sensillae
– single
– lyriform organs
• chemosensory organs
Eyes
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primitively 8, some 6, rarely 0
around anterior margin/on turret
general form
special forms
– salticid (jumping spider) eyes
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
Slit sensillae
• single
– appearance, structure
– amplify flexing of exoskeleton
• lyriform organs
– fields of slit sensillae … very sensitive
• how they function
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
Booklungs
• derived from bookgills by invagination
• structure
• operation
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.
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’
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)
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)
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!
Silks
• many kinds of silk
• all kinds not present in every spider
• structural silks
– draglines, retreats, egg-cocoons, ...
• prey-catching silks
– cribellate
– ecribellate
Physiology
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haemolymph circulation
respiration
digestion
excretion
nervous system
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
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
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
Excretion
• Malpighian tubules
• guanine main nitrogenous waste
– also uric acid
Nervous system
• concentrated on prosoma
• exceptionally large for size of animal
(general feature of arachnids)
Reproduction
• mating - indirect insemination
• females store sperm
• mating systems
– courtship
– copulation
– cannibalism …
– other features
• maternal care
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
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
Toxicity!
• spider venoms are complex and often
contain rapid acting short term 'knock
out' components and/or proteolytic
enzymes.
• 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
Humans ...
• Atrax robustus male - usually dry-bites!
14 deaths 1927-1980.
– Shortest 8 minutes! 5 < 2h.
• antivenom developed 1980.
• some Hadronyche species probably
considerably more poisonous!
• Latrodectus, Loxosceles … others
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’)
Prey
• some spider species are prey
specialists,
• others are extremely general, attacking
as opportunity arises.
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’)
• 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
Web structures
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sensory lines
sheet webs
orb webs
cobwebs
other webs
special webs - bolus spider, net-casting
spider etc.
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
Salticidae
• largest (most speciose) family
• characterised by eye structure and
vision
• stalk then leap onto prey
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
web exercise
• find out about the morphology and likely
ecology of triginotarbid arachnids