Biology 320 Invertebrate Zoology Fall 2005

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Biology 320Invertebrate ZoologyFall 2005

• Chapter 16 Introduction to Phylum Arthropoda

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Phylum Arthropoda

• Horseshoe crabs, crustaceans, arachnids, insects,
  and millipedes
• One million described species
• 80 of all known animals
• Another 30 million undescribed species of
  insects, estimated
• Only birds, bats, and insects have the ability to
  fly
• Amazing adaptive diversity

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• Thought to be closely related to annelids
• Recently thought to be more closely related to
  nematodes
• Two major extant taxa
• Chelicerata horseshoe crabs and arachnids
• Mandibulata
• Crustacea crabs, barnacles, and water fleas
• Tracheata insects and centipedes

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Segmentation

• Metamerism is the technical term for segmentation
• Individual segments are termed somites
• Certain body regions of arthropods display a
  fusion of several segments
• Arthropods display tagmosis (tagmatization)
• Segments are organized into regions called tagma
  (tagmata)
• Insects posses a head, thorax, and abdomen
• Arachnids and crustaceans (typically) possess a
  cephalothorax and abdomen

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Cephalization

• Highest degree of cephalization of any phylum we
  have discussed to this point
• Head possesses
• Concentration of ganglia (brain)
• Concentration of sensory structures
• Eyes and antennae
• Mouth, feeding appendages, and legs (in those
  with a cephalothorax)

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Appendages

• Segmented with articulations
• Specialized for specific functions
• Some posses more than 10 types
• Enormous variety
• May be simple, or have multiple branches

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Body Wall

• Cuticle (exoskeleton) is most superficial layer
• Made of chitin and protein
• Provides strength and protection
• Epidermis
• Monolayered
• Secretes cuticle
• Thin connective tissue layer
• Specialized individual muscles, rather than sheets

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Exoskeleton

• Cuticle has hard areas and soft areas
• Allows for movement at articulations
• Some areas are sclerotized
• Proteins crosslink causing cuticle to harden and
  darken
• Typical of arachnids and insects
• Some areas are mineralized
• Calcium salts are added to cuticle
• Crustaceans supplement sclerotization with
  mineralization

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• Two main layers of exoskeleton
• Epicuticle thin outer layer that is water / gas
  resistant
• Waterproofed by different types of lipids
• Useful in dry environments
• Important for osmoregulation in FW environments
• Procuticle thick portion

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• Must be periodically molted in order for growth
  to occur
• Rigid exoskeleton cannot stretch
• Process known as ecdysis
• Regulated by the steroid hormone ecdysone
• Even the lining of the foregut, hindgut,
  tracheae, etc. are shed

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Ecdysis

• Has been thoroughly studied in decapods
• When animal prepares to molt
• Old cuticle begins to detach
• New cuticle begins to form
• Layer of molting fluid (containing proteases and
  chitinases) forms between the two cuticles
• Epicuticle of new cuticle protects it from
  enzymes
• Calcium from old cuticle is reclaimed
• During ecdysis
• Animal is very vulnerable
• Usually molts inside burrow, shelter, etc.
• Molting related complications / deaths are common

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• After molting
• Animal takes up water or air, and swells as a
  result
• May ingest old cuticle to reclaim calcium
• Sclerotization and mineralization (crustaceans)
  occur
• Excess water or air is eliminated
• Animal now has room for growth
• Some molt periodically
• Others have a fixed number of instars (life
  stages) separated by molts

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Musculature

• Lack sheets of muscles
• Small individual muscles that are specialized
• Flexors
• Extensors
• Often arranged antagonistically
• Control one articulation or one sclerite
• Striated, for rapid contraction

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Locomotion

• Arthropods posses a large variety of appendages,
  that serve different purposes
• Walking legs
• Paddles swimming
• Wings - flight

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Coelom

• Arthropods have an exoskeleton, and therefore do
  not rely heavily on a hydrostat
• Coelom is reduced compared to that of the
  annelids
• Hemocoel is the functional body cavity
• No segmental divisions

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Hemal System

• Transports nutrients, hormones, wastes, and
  gasses (in some)
• Open system in most
• System consists of
• Large dorsal heart
• Tubular and contractile
• Possesses ostia
• Arteries that branch off of heart
• Sinuses are separated by perforated diaphragms
• Pericardial dorsal
• Perivisceral middle
• Perineural (in some) - ventral
• Blood
• Crustaceans and chelicerates have hemocyanin
• Hemocoel
• Muscles and organs are housed here

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• Generalized blood flow
• Oxygenated blood leaves heart via arteries
• Travels to sinuses and hemocoel where tissues and
  organs are bathed in blood
• Deoxygenated / waste laden blood travels to
  respiratory structures (in all but tracheates)
  and gas exchange occurs
• Blood enters heart via ostia

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Excretion

• Two main excretory structures
• Saccate nephridia
• Malpighian tubules
• Saccate nephridia
• Mainly found in aquatic species
• Not actually used for excretion, as wastes
  (ammonia) diffuse across gills
• Maintain ion balance and fluid volume
• Go by many name, such as green glands in
  crayfish
• Consists of a tube and an end sac which is bathed
  in blood

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• Malpighian tubules
• Primary excretory organ of terrestrial species
• Stringy tubules located at the junction of the
  midgut and hindgut
• Create uric acid and guanine and transfer to the
  gut lumen
• Wastes form a precipitate in this low pH
  environment
• Nitrogenous wastes are removed with feces
• Adaptation to terrestrial existence

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Gas Exchange

• Small arthropods conduct gas exchange across body
  surfaces
• Larger varieties make use of specialized
  structures that must be moist
• Interestingly, the epicuticle of respiratory
  structures is not water / gas proof
• Many types of respiratory structures
• Particular structures are mainly dependent on the
  type of habitat the animal inhabits

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• Aquatic invertebrates
• Gills
• Already discussed
• Book gills
• Flat sheets on ventral surface
• Possessed by horseshoe crabs
• Terrestrial species
• Tracheae
• Tubular invaginations that branch and lead
  directly to cells, not to the hemal system
• Spiracle diameter regulated by a sphincter
• Opening / closing of spiracles is useful for
  ventilation and reducing water loss
• Book lungs
• Common in arachnids
• Invagination of exoskeleton which contains many
  smaller invaginations (large SA)
• Exchange occurs between surfaces of book lungs
  and blood

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Digestive System

• Complex, but somewhat similar to annelid system
• Epithelium at junction of foregut-midgut secretes
  a thin peritrophic membrane around bolus
• Protects delicate walls of midgut from abrasion
• Gut lumen reabsorbs water

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Nervous System

• Similar to that of annelids
• CNS
• Brain
• Ventral nerve cord
• PNS
• Segmental nerves connecting to muscle and sensory
  organs
• Numerous types of sensory structures
• Exoskeleton is modified to accommodate sensory
  receptors

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• Mechanoreceptors
• Setae
• Hollow extensions of cuticle w/ sensory neurons
  inside
• Trichobothria
• Sensitive hairs that detect weak air currents
• Chemoreceptors
• Sensilla
• Similar to setae
• Equilibrium
• Statocysts
• Possessed by some
• Ocelli
• Most use light as a point of reference for
  orientation

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• Sound detection
• Tympanic membrane
• Responds to air vibrations
• Can also produce sound in some
• Useful for locating / attracting mates
• Moths use to hear bat echolocation
• Temperature humidity detectors
• Thermo-hygroreceptive sensillia
• Usually found on antennae of insects

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Eyes

• Most have ocelli
• Many also have compound eyes
• Composed of many subunits called ommatidia
• 15 thousands per eye
• Each unit has own lens and own field of vision
• Fields of vision of adjacent ommatidia overlap
• Composite image formed, similar to vertebrate
  eyes
• Are usually motion detectors that have a range of
  about 20cm
• Many have eyestalks
• Some have color vision and respond to colored
  flowers

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Reproduction

• Mostly gonochoric
• Fertilization is primarily internal in
  terrestrial species
• Can be internal or external in aquatic species
• Most copulate and there is often selectivity on
  the part of the female
• Females often store sperm in a seminal receptacle
• Sperm transfer is direct in some
• Penis may deliver sperm directly, or serve as an
  intromittant organ
• Spermatophores are frequently used for indirect
  transfer
• Many brood and display parental care