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Chapter 14. The Arthropods: Blueprint for Success. Evolutionary Perspective. Metamerism modified by tagmatization Chitinous exoskeleton Paired, jointed appendages Ecdysis Ventral nervous system Coelom reduced to cavity around gonads Open circulatory system Complete digestive tract

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chapter 14

Chapter 14

The Arthropods: Blueprint for Success

evolutionary perspective
Evolutionary Perspective
  • Metamerism modified by tagmatization
  • Chitinous exoskeleton
  • Paired, jointed appendages
  • Ecdysis
  • Ventral nervous system
  • Coelom reduced to cavity around gonads
  • Open circulatory system
  • Complete digestive tract
  • Metamorphosis often present
classification and relationships to other animals
Classification and Relationships to other Animals
  • Ecdysozoans
    • Cuticle, ecdysis, loss of epidermal cilia (figure 14.2)
  • Monophyletic with five subphyla (table 14.1)
    • Chelicerata, Crustacea, Hexapoda, Myriapoda, Trilobitomorpha (entirely extinct)
metamerism and tagmatization
Metamerism and Tagmatization
  • Metamerism evident externally
    • Segmental body wall
    • Segmental appendages
  • Metamerism reduced internally
    • No septa
    • Most organs are not metameric
  • Tagmatization obvious
    • Specializations for feeding, sensory perception, locomotion, and visceral functions
learning outcomes section 14 3
Learning Outcomes: Section 14.3
  • Describe the structure of the arthropod exoskeleton or cuticle.
  • Assess the influence the exoskeleton has had on the evolution of the arthropods.
the exoskeleton
The Exoskeleton
  • Exoskeleton or cuticle
    • External jointed skeleton
  • Functions
    • Structural support
    • Protection
    • Prevents water loss
    • Levers for muscle attachment and movement
  • Covers all body surfaces and invaginations
  • Secreted by epidermis (hypodermis)
the exoskeleton1
The Exoskeleton
  • Epicuticle (figure 14.3)
    • Lipoprotein
    • Impermeable to water
    • Barrier to microorganisms and pesticides
  • Procuticle
    • Chitin
      • polysaccharide
    • Outer procuticle hardened by sclerotization or deposition of calcium carbonate
    • Inner procuticle less hardened and flexible
      • Articular membranes at joints (figure 14.4)
  • Modifications include sensory receptors
    • Sensilla
the exoskeleton2
The Exoskeleton
  • Growth accompanied by ecdysis (figure 14.5)
    • Enzymes from hypodermal glands begin digesting old procuticle (a, b).
    • New procuticle and epicuticle secreted (c, d).
    • Old exoskeleton splits (e)
    • Calcium carbonate deposition and/or sclerotization hardens new exoskeleton (f).
the hemocoel
The Hemocoel
  • Embryonic blastocoel
  • Internal cavity for open circulatory system
    • Fluids bathe internal organs.
    • Exchange of nutrients, wastes, and sometimes gases
  • Not a hydrostatic compartment
  • Radical change in body form and physiology as an immature (larva) becomes an adult.
    • Reduces competition between adult and immature stages
subphylum trilobitomorpha
Subphylum Trilobitomorpha
  • Dominant life form from Cambrian period (600 mya) to Carboniferous period (345 mya)
  • Substrate feeders
  • Three tagmata: head, thorax, and pygidium
  • Three longitudinal sections
  • Biramous appendages
subphylum chelicerata
Subphylum Chelicerata
  • Spiders, mites, ticks, horseshoe crabs
  • Two tagmata
    • Prosoma
      • Eyes
      • Chelicerae
        • Often chelate
        • Usually feeding appendages
      • Pedipalps
        • Sensory, feeding, locomotion, reproduction
      • Walking legs
    • Opisthosoma
      • Digestive, reproductive, excretory, and respiratory organs
class meristomata
Class Meristomata

Figure 14.7 A eurypterid, Euripterus remipes.

  • Subclasses
    • Eurypterida
      • Extinct giant water scorpions

(figure 14.7)

class meristomata1
Class Meristomata

Figure 14.8a Limulus polyphemus.

  • Subclass Xiphosura
    • Horseshoe crabs
      • Limulus (Atlantic Ocean and Gulf of Mexico)
      • Book gills
        • Gas exchange between blood and water
      • Reproduction
        • Dioecious
        • External fertilization
class arachnida
Class Arachnida
  • Spiders, mites, ticks, scorpions
  • Arose from ancient euryptrids
  • Very early terrestrial groups
    • 280-400 mya
    • Exoskeleton was preadaptation for water conservation.
form and function
Form and Function
  • Carnivores
    • Chelicerae to hold prey or as fangs
    • Gut
      • Foregut
        • Cuticular
        • Pumping stomach
      • Hindgut
        • Cuticular
        • Water reabsorption
      • Midgut
        • Noncuticular
        • Secretion and absorption
form and function1
Form and Function
  • Excretion
    • Coxal glands
      • Paired sacs bathed in blood of body sinuses
      • Homologous to nephridia
      • Excretory pores at base of posterior appendages
    • Malpighian tubules
      • Blind ending diverticula of gut tract
      • Empty via digestive tract
    • Uric acid
form and function2
Form and Function
  • Gas Exchange
    • Book lungs
      • Paired ventral invaginations of body wall
      • Gas exchange between air and blood across book lung lamellae
    • Tracheae
      • Branched, chitin-lined tubes
      • Open at spiracles along abdomen
form and function3
Form and Function
  • Circulation
    • Open with dorsal contractile vessel
    • Pumps blood into tissue spaces of hemocoel
    • Returns to dorsal vessel via ostia
  • Nervous system
    • Ventral with fusion of ganglia
form and function4
Form and Function
  • Senses
    • Mechanoreceptors
      • Modifications of exoskeleton
      • Sensilla respond to displacement.
    • Chemical sense
      • Pores in exoskeleton
    • Vision
      • Eyes detect movement and changes in light intensity.

Figure 14.10 An arthropod seta (a) and an eye (ocellus) (b).

form and function5
Form and Function
  • Reproduction
    • Dioecious
    • Indirect sperm transfer
      • Male deposits spermatophores, which are transferred to the female.
    • Courtship rituals common
    • Copulation occurs in spiders via modified pedipalp of male.
  • Development
    • Direct
order scorpionida
Order Scorpionida
  • Prosoma
    • Shieldlike carapace
  • Opisthosoma
    • Preabdomen
    • Postabdomen (“tail” with sting)
  • Courtship prior to mating
  • Oviparous, ovoviviparous, or viviparous
order araneae
Order Araneae
  • Spiders
  • Prosoma
    • Chelicerae with poison glands and fangs
    • Pedipalps leglike
      • Sperm transfer in males
    • 6-8 eyes
  • Opisthosoma
    • Connected to prosoma via pedicel
    • Swollen or elongate
    • Visceral functions and spinnerets
order araneae1
Order Araneae
  • Silk
    • Protein
    • Repeating sequence of glycine and alanine
    • Beta sheet
    • Stored as gel prior to spinning
    • Chemical modification when forced through spinnerets
  • Webs, line retreats, safety lines, wrapping eggs, dispersal of young (ballooning)
order araneae2
Order Araneae
  • Feeding
    • Insects and other arthropods
    • Hunt or capture in webs
    • Paralyze prey
      • May wrap in silk
    • Inject enzymes into prey body wall
  • Two spiders are venomous to humans.

Figure 14.15 (a) Black widow spiders (Lactrodectus mactans) has a neurotoxic venom. (b) Brown recluse spiders (Loxosceles reclusa) have a histolytic venom.



order araneae3
Order Araneae
  • Reproduction
    • Complex behaviors
      • Chemical, tactile, and visual signals
    • Male’s pedipalps enlarged into embolus
      • Male deposits sperm on web and collects with pedipalps.
      • Transfers sperm to female during mating
    • Female deposits eggs in silk case.
      • In webbing, a retreat, or carries with her
order opiliones
Order Opiliones

Figure 14.16 Order Opiliones (Leiobunum sp).

  • Harvestmen or daddy longlegs
  • Prosoma broadly joins opisthosoma
  • Legs long and slender
  • Omnivores
  • External and internal digestion
order acarina
Order Acarina
  • Mites
    • Prosoma and opisthosoma fused and covered by single carapace
    • 1mm or less
    • Free-living
      • Herbivores or scavengers
        • Many pest species
    • Ectoparasites
      • Chigger (Trombicula)
      • Follicle mite (Demodex)

Figure 14.17 Dermatophagoides farinae is common in homes and grain storage areas.

order acarina1
Order Acarina
  • Ticks
    • Ectoparasites in all life stages
    • Up to 3cm
    • Females lay eggs after engorging with blood.
    • Important in disease transmission
      • Rocky Mountain spotted fever
      • Lyme disease
class pycnogonida subphylum cheliceriformes
Class Pycnogonida (Subphylum Cheliceriformes?)
  • Sea spiders
  • Marine
  • Feed on cnidarian polyps
  • Dioecious
  • Molecular, developmental, and morphological characters are being used to reevaluate taxonomic status.

Figure 14.19 Class Pycnogonida

subphylum crustacea
Subphylum Crustacea
  • Crayfish, shrimp, lobsters, crabs, copepods cladocerans and others
  • Almost all are aquatic
    • Terrestrial isopods and crabs are exceptions.
  • Two pairs of antennae
  • Biramous appendages (figure 14.20)
class malacostraca
Class Malacostraca
  • Crabs, lobsters, crayfish, shrimp, krill, amphipods, isopods
  • Order Decapoda
    • Largest order
    • Shrimp, crayfish, lobsters, crabs
class malacostraca1
Class Malacostraca
  • Crayfish external structure
    • Cephalothorax
      • Fusion of head and thorax
      • Covered dorsally and laterally by carapace
      • Sensory, feeding, locomotion
    • Abdomen
      • Muscular “tail” in crayfish
      • Locomotor and visceral functions in others
    • Paired appendages
      • Serially homologous (derived from a common ancestral pattern)
class malacostraca2
Class Malacostraca
  • Crayfish internal structure
    • Digestive system
      • Complete with foregut, midgut, and hindgut
    • Respiratory system
      • Gills attach at base of cephalothoracic appendages.
      • Lie within gill chamber between carapace and lateral body wall
      • Second maxilla circulates water.
    • Circulation
      • Open
      • Dorsal heart and major arteries
      • Blood enters hemocoel, and gills before returning to pericardial sinus around heart.
class malacostraca3
Class Malacostraca
  • Ventral nervous system
    • Cephalization and centralization
    • Supraesophageal and subesophageal ganglia process sensory information and control head appendages.
    • Segmental ganglia
  • Sensory structures
    • Antennae
    • Compound eyes
    • Statocysts
    • Chemoreceptors
    • Proprioceptors
    • Tactile setae
class malacostraca4
Class Malacostraca
  • Endocrine system
    • Ecdysis, sex determination, color change
      • X-organs
        • Neurosecretory tissues in eyestalks
        • Molt-inhibiting hormone
          • Target Y-organ
      • Y-organs
        • Base of maxillae
        • Releases ecdysone when molt inhibiting hormone is not present and ecdysis occurs
    • Androgenic glands (males)
      • Promotes development of testes and male characteristics
class malacostraca5
Class Malacostraca
  • Excretion
    • Antennal (green) glands in crayfish
    • Maxillary glands in others
    • Homologous to coxal glands of arachnids
  • Reproduction
    • Dioecious
    • Mating after female molts
      • Fertilized eggs attach to female’s pleopods
      • Others have planktonic larvae

Figure 14.25

(a) Nauplius larva of a barnacle. (b) Zoea larvae of a crab.



order isopoda
Order Isopoda
  • “Pillbugs”
  • Aquatic and terrestrial
  • Dorsoventrally flattened

Figure 14.26a Order Isopoda.

order amphipoda
Order Amphipoda
  • Laterally compressed
  • Crawl or swim on sides
  • Beach-hoppers modified for jumping

Figure 14.26b Order Amphipoda.

class branchiopoda
Class Branchiopoda
  • Fairy shrimp
    • Temporary ponds
  • Brine shrimp
    • Great Salt Lake
  • Cladocera
    • Freshwater water fleas
    • Large carapace
    • Parthenogenesis common
  • Flattened, leaflike appendages

Figure 14.27 Order Cladocera.

class maxillopoda
Class Maxillopoda

Figure 14.1 Subclass Copepoda.

  • Subclass Copepoda
    • Most abundant crustaceans
    • Important in marine and freshwater food webs
    • First antennae modified for swimming
class maxillopoda1
Class Maxillopoda
  • Subclass Thecostracea, Infraclass Cirripedia
    • Barnacles
    • Marine
    • Monoecious
      • Nauplius and cypris larvae
      • Cypris larva settles and metamorphoses into sessile adult.
    • Some parasites

Figure 14.28 Class Maxillopoda, Infraclass Cirripedia.

(a) Internal structure of an acorn barnacle. (b) A stalked barnacle (Lepas).

further phylogenetic considerations
Further Phylogenetic Considerations
  • Diverse body forms and lifestyles of Arthropoda arose from single ancestor.
  • Crustaceans very successful in aquatic habitats
  • Chelicerata
    • First terrestrial arthropods
    • Account for evolution of many water conserving features of the phylum
      • Exoskeletal, excretory, and respiratory adaptations