ARTHROPODS

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ARTHROPODS

• Phylum Arthropoda

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Phylum Arthropoda (jointed feet )

• Huge group, gt 1,000,000 species.
• estimate 1,000,000 spp. arthropods
  1,190,000 spp. animals
• 84 of all animal species are arthropods!!

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Phylum Arthropoda

• How can we explain the success of the arthropods?
  
• Exoskeleton!

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Phylum Arthropoda

• Exoskeleton of chitin and protein ( cuticle)
• structure
• epicuticle (oily, waxy)
• exocuticle (chitin protein)
• endocuticle (chitin only)
• epidermis secretes cuticle

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Problems associated with exoskeleton.

• Problem 1. MOVEMENT
• Solution Joints in exoskeleton.
• arthro-, joint
• -pod, leg, foot
• Exocuticle absent from joints may form hinges.
• Endocuticle alone allows flexibility.

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Problems associated with exoskeleton.

• MOVEMENT
• Solution Joints in exoskeleton.
• Exocuticle absent from joints may form hinges.
• Endocuticle alone allows flexibility.

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Problems associated with exoskeleton.

• Problem 2. GROWTH
• Solution Molting

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Molting (1)

• Secretion of "molting fluid" to dissolve old
  endocuticle.

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Molting (2)

• New cuticle formed under old exocuticle.
• Break out of old cuticle
• Old cuticle breaks at line of weakness

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Molting (3)

• Inflate with water/air to increase size while
  skeleton soft,
• but soft skeleton gravity limit size
• arthropods are mostly small.
• Hardening of new exocuticle.

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Growth stages

• Arthropod passes thru 3-20 growth stages in
  life cycle.
• Some stop molting as adults (insects, most
  spiders)
• Some continue to molt (crayfish, tarantulas)

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Problems associated with exoskeleton.

• Problem 3. SENSORY INPUT
• touch
• sensory setae connected to neurons
• smell taste
• hollow sensory setae w/ chemosensitive nerve
  endings

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Problems associated with exoskeleton.

• Problem 3. SENSORY INPUT
• vision
• clear cuticle over compound or simple eyes

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Problems associated with exoskeleton.

• Problem 3. SENSORY INPUT
• hearing
• tympanum endocuticle, vibrates like eardrum
• trichobothria (right ?)

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Benefits of Exoskeleton to individuals

• Support
• Locomotion
• lever system
• walk, swim, fly
• Mechanical protection (armor)
• Retards evaporation (in air) and/or osmosis (in
  water)
• water balance.

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Benefits of Exoskeleton to the phylum

• Reduction of coelom segmentation
• Abandoned hydrostatic system of annelid-like
  ancestor)
• Coelom reduced to pericardial cavity
• Segments fused tagma, tagmata
• Tagmosis
• Specialization of body regions ( tagmata)
• Specialization of appendages

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Tagmosis

• Head ( 4-6 segments)
• feeding, sensation
• Head appendages
• mandibles,
• maxillae,
• maxillipeds,
• chelicerae
• antennae

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Tagmosis

• Thorax ( 3-6 segments)
• locomotion, grasping.
• Thoracic appendages
• walking legs,
• wings
• chelipeds

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Tagmosis

• Abdomen (8- 30 segments)
• respiration, reproduction, etc.
• Abdominal appendages
• abdominal gills (aquatic insect larvae)
• swimmerets (crayfish)
• filtering legs (barnacles)
• gonopods (crayfish, etc.)
• spinnerets (spiders)

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Tagmosis

• Number of segments/legs in each tagma varies by
  subphylum, class.
• Cephalothorax of 6 segments in Chelicerata
• 1 pr. chelicerae
• 1 pr. pedipalps
• 4 pr. walking legs
• Cephalothorax of 13 segments in Crustacea
  (shrimps)
• 2 pr. antennae
• 1 pr. mandibles
• 2 pr. maxillae
• 3 pr. maxillipeds
• 5 pr. walking legs (1st pair modified as
  chelipeds)

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Other arthropod characters

• Open circulatory system
• Dorsal heart pumps hemolymph over brain
• Hemolymph moves through hemocoel back toward
  heart
• Ostia (holes) in sides of heart let hemolymph in
  to go around again.

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Other arthropod characters

• Respiratory systems
• Gills in aquatic/marine arthropods
• Tracheal systems in most terrestrial arthropods
• Book lungs (modified gills) in spiders scorpions

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Other arthropod characters

• Nervous system resembles that of annelid
• Dorsal brain with nerves around esophagus
• Paired ventral nerve cords
• Segmental ganglia
• Often fused into 1-2 ganglia in each tagma

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Distinguishing Characters of Ph. Arthropoda

• Jointed exoskeleton
• Tagmosis
• Compound eyes

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Classification of Arthropoda

• Subphylum Trilobita
• Subphylum Crustacea
• Subphylum Chelicerata
• Subphylum Uniramia

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Classification of Arthropoda

• Subphylum Trilobita
• Class trilobites
• Three-lobed head body (left, middle, right)
• Diverse in Paleozoic 540-240 MYA
• Extinct

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Classification of Arthropoda

• Subphylum Crustacea
• 2 pr. Antennae (antennules, antennae)
• Cephalothorax
• 13 segments appendage pairs
• Abdomen
• variable among Classes

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Classification of Arthropoda

• Subphylum Crustacea
• Class shrimps
• Class barnacles

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Classification of Arthropoda

• Subphylum Chelicerata
• Cephalothorax
• Jaws are chelicerae
• Pedipalps
• 4 pr. Walking legs
• Abdomen

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Classification of Arthropoda

• Subphylum Chelicerata
• Class Horseshoe crabs
• Horseshoe crabs
• Scorpions ??

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Classification of Arthropoda

• Subphylum Chelicerata
• Class Arachnids
• Lost compound eyes
• Spiders
• Daddy-long-legs
• Amblypygi
• Mites ticks
• more
• Scorpions ??

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Classification of Arthropoda

• Subphylum Uniramia
• Legs unbranched
• Class Centipedes
• 1st legs are fangs

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Classification of Arthropoda

• Subphylum Uniramia
• Class Millipedes
• Double segments (2 pr. legs per segment)

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Classification of Arthropoda

• Subphylum Uniramia
• Class Insects
• Head, thorax, abdomen
• 2 pr. Wings
• 800,000 species, majority of all arthropods

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Why are Arthropods so successful?

• Exoskeleton ? tagmosis ? evolution of flight ?
  speciation ? 106 species of insects.

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Why are Arthropods so successful?

• Exoskeleton ? protection from water loss ? early
  colonization of land ? head start.
• Arthropods were diverse and widespread on land
  before vertebrates!