Arthropod

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Arthropod
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Phylogeny of Arthropods
Arthropoda
Annelids (worms)
Onychophorans (worms w/legs)
Chelicerates (spiders)
Crustaceans (lobsters)
Insects (butterflies)
Trilobites (extinct)
Worm-like Ancestor
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• General Characteristics
• metamerism (segmented body)
• jointed appendages
• bilateral symmetry
• secreted exoskeleton
• ecdysis (molting)
• open circulatory system
• ventral nerve cord and brain

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• DIVERSITY
• Numbers
• about 80 of all animals are arthropods
• estimated to be over 30 million arthropods
• far more of them than all other metazoan species
  combined
• in virtually every conceivable environment
• marine, terrestrial, freshwater, and aerial
  habitats.

Phylum Arthropoda Subphylum Trilobita
(extinct) Subphylum Chelicerata Subphylum
Crustacea Subphylum Uniramia (insects)
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• Chelicerata
• comprises the xiphosurans (horseshoe crabs),
  arachnids, sea spiders
• mostly terrestrial
• cephalothorax-abdomen
• 6 pairs cephalothoracic appendages
• chelicerae, pedipalps, 4 pairs legs
• no antennae

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Subphylum Chelicerata

• a. Xiphosura (horseshoe crab, Limulus polyphemus)
  
• b. Arachnida
• most chelicerates are arachnids
• several taxa (orders), many are common and
  familiar
• SCORPIONES (scorpions )
• ARANAE (spiders )
• OPILIONES (harvestmen, daddy longlegs )
• PSEUDOSCORPIONES (pseudoscorpions)
• ACARINI (mites and ticks )
• c. Pycnogonida (the giant sea spider,
• Colossendeis australis)

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Horseshoe Crab
Limulus polyphemus, Carcinoscorpius rotunda, and
Tachypleus gigas.
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Horseshoe Crab

• Mates during spring and summer full and new
  moons, onto ocean beaches.
• Females lay up to 30 thousand eggs, which males
  fertilize before burying them in the sand
• These eggs provide a major food source for
  migrating birds along the Atlantic coast. Those
  that are not eaten hatch during the next high
  tide, and the tiny larvae are carried away to
  sea.

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Subphylum Crustacea

• a heterogeneous taxon with many subtaxa
• two pairs antennae
• marine, freshwater, and terrestrial
• Primarily aquatic
• Free-floating larval stage
• 26,000 known species

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Subphylum Crustacea

• ANOSTRACA
• fairy shrimp and brine shrimp
• PHYLLOPODA
• tadpole shrimps, ostracod, water fleas
• MALACOSTRACA
• the largest, most heterogeneous, and diverse
  crustacean taxon
• shrimps, crabs, lobsters, mantis shrimps,
  pillbugs, crayfishes
• COPEPODA
• copepods
• enormous ecological importance as planktonic
  herbivores
• CIRRIPEDIA
• barnacles

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Respiration

• Gills
• a. General
• gills are evaginations
• typical of aquatic animals
• blood is oxygenated by gills
• oxygen delivered to tissues by blood
• b. Crustacean Gills
• crustacean gills are usually associated with
  appendages
• blood circulates through the gill and is
  oxygenated
• c. Book Lungs
• present in many arachnid orders
• similar to book gills of horseshoe crabs
• oxygen transport from the lungs to the tissues is
  by blood

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Closed Circulatory System
Open Circulatory System
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• NERVOUS SYSTEM
• 1. General
• nervous system is adapted for the needs of a
  segmented animal
• as is that of annelids
• annelid and arthropod nervous systems are similar
  
• the similarities may be convergent
• 2. Arthropod Ground Plan
• ladder like nervous system
• dorsal brain in the head
• a pair of circumenteric connectives that encircle
  the gut
• a paired, ventral, longitudinal nerve cord
• paired segmental ganglia
• transverse commissures between ganglia
• longitudinal connectives
• segmental sensory and motor nerves

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NERVOUS SYSTEM
Side view of body showing relative position of
circulatory (yellow), digestive (green), and
nervous (blue) systems.
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• SENSE ORGANS
• 1. General
• receptors for light, vibrations, chemicals,
  equilibrium, gravity
• cuticle modified to form a vast array of
  receptors (sensilla)
• 2. Sensilla
• specialized for various environmental events
• consist of
• ltgtmodified cuticle, usually a seta
• ltgt a modified cilium
• ltgt sensory neurons
• ltgt support cells

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SENSE ORGANS

• 3. Mechanoreceptors
• a. Trichobothria
• extremely sensitive mechanoreceptors detect weak,
  low velocity air currents
• a long, slender, solid seta
• especially common and important in arachnids
• a blinded spider can capture flies using its
  trichobothria
• 4. Chemoreceptors
• detect chemicals
• modified hollow setae with substrate-specific
  chemosensory neurons
• chemoreceptive sensilla resemble mechanoreceptive
  setae

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SENSE ORGANS

• 5. Equilibrium and Gravity
• a. Statocysts
• several malacostracan crustaceans have statocysts
  
• analogous to the vestibule of the vertebrate
  inner ear
• detects gravitational field and acceleration
• 6. Tympanal Organs
• some arthropods can hear, i.e. detect vibrations
  in air
• tympanal organs are present in several insect
  groups
• cicadas, crickets, grasshoppers, moths
• a thin sheet of cuticle (eardrum) over an opening

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SENSE ORGANS

• 7. Hygroreceptors
• humidity receptors
• very important to terrestrial arthropods
• 8. Photoreceptors
• a. General
• widespread, often well-developed and
  sophisticated
• two types
• ltgt"median" pigment-cup ocelli
• ltgt"lateral" compound eyes
• ltgtboth often present in the same individual

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SENSE ORGANS

• 2. Pigment-cup Ocelli
• a. General
• highly variable within the arthropods
• all the receptor units (retina cells) share
• a common lens
• b. Structure
• a cup-shaped layer of pigment-containing cells
• encloses a cup-shaped layer of photoreceptor
  cells known as the retina
• concavity of the cup is oriented toward the light
  source
• pigment layer prevents the entry of light from
  any other direction
• a cuticular lens (cornea) usually present

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SENSE ORGANS

• 3. Compound Eyes
• General
• multiple light receiving units, known as
  ommatidia
• each ommatidium usually has its own lens (or
  cornea)
• most have one pair
• b. Ommatidia
• ommatidium is the individual, self-contained,
  independent light-detecting unit
• 15-several thousand in a single compound eye
• each includes
• gtfocusing system (cuticular cornea and
  crystalline cone)
• light-transmitting structure (cone stalk)
• gtlight sensitive cells
• gtscreening pigment to isolate (pigment cells)

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Compound Eye
Lens
Crystalline cone
Pigment cells
Facet
Visual cells
Nerve fibers from visual cells
Optic nerve
Ommatidia
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• REPRODUCTION
• 1. General
• most arthropods are gonochoric
• fertilization may be external or internal in
  aquatic arthropods
• always internal in terrestrial species
• 2. Sperm Transfer
• sperm transfer usually by mating or copulation
• females usually store the sperm in a seminal
  receptacle

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• 3. Spermatophores
• a. General
• many arthropods transfer sperm in packets
• an adaptation for sperm transfer on land
• male abandons spermatophore for female to find or
  may place it in the female gonopore
• most insects use copulation
• 4. Copulation
• present in many aquatic arthropods (crustaceans),
  also in derived terrestrial forms
• often with courtship and behavior patterns
• anatomical equipment for internal fertilization
  by copulation
• male inserts an intromittent organ into the
  reproductive tract of the female
• deposits sperm or spermatophores into her seminal
  receptacle

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Arthropod Development
Zoea
Megalops
Nauplius
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Symbiosis
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Camouflage
Decorator Crab
Sponge crab
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Camouflage anemone crab
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coral guard crabs and pocilloporid corals
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gall crabs and pocilloporid corals
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scarlet cleaner shrimp
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sea cucumber crab and sea cucumbers
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pompom crabs and sea anemones
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crabs and sea anemones