Chelicerata

1. Pycnogonida - marine, ‘pycnogonids'

2. Merostomata - the horseshoe crabs

3. Arachnida - contains all the terrestrialchelicerates … 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• ‘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 generalisedarachnid

Arachnida

• Eurypterida (extinct)

• Uropygi

• Palpigradi

• Solifugae

• Scorpiones

• Araneae

• Schizomida

• Amblypygi

• Ricinulei

• Opiliones

• Pseudoscorpiones

• 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

• Eurypterida (extinct)

• Uropygi

• Palpigradi

• Solifugae

• Scorpiones

• Araneae

• 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

•

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

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

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(Hypochilomorphae, Araneomorphae)))

– Mesothelae, Mygalomorphae grades?

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

OpisthothelaeMygalomorphae

• ‘primitive’ group

• ~ 2000 species

• 4 booklungs, paraxial chelicerae

• terminal spinnerets, often segmented

• ambush predators from silk-lined burrows (sometimes with trapdoors) or bivouac retreats

OpisthothelaeAraneomorphae

• ‘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’

AraneomorphaeAraneae

• ‘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

• 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. aciniform3. ampullate4. aggregate5. flagelliform6. cylindrical

and 3 other typesthat 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

• 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, 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

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 brieflyother 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

• 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