Arthropods

IV. Arthropods (CH14-CH15)

• Key Features of Arthropods:

  • Characterized by segmented body parts and joined appendages 

  • most numerous and largest of all animal phyla

  • jointed, external Exoskeleton

    • made of chitin

    • for structural support, protection, prevents loss of water, muscle attachment, and movement

    • calcium carbonate deposition hardens new exoskeleton

  • ventral nervous system

  • undergo ecdysis (molting)

• Evolutionary Success Factors:

  • metamerism modified by tagmatization

  • chitinous exoskeleton

    • Protection against dehydration

    • provide Structural support

    • Segmentation that aids in function.

  • paired, jointed appendages

    • for locomotive and sensory functions.

  • exhibit ecdysis (shedding or molting the outer cuticle or exoskeleton)

  • ventral nervous system with brain and nervous innervation around digestive tract

  • reduced coelom to cavity around gonads

  • open circuclatory system that opens into hemocoel

  • complete digestive tract

  • metamorphosis often present

• Metamerism and Tagmatization:

  • Exhibit metamerization: Repeating embryonic segments called metamers.

    • metamers: each body segment

    • metamerism is evident externally - have segmented body wall and appendages

    • Metamerism is highly modified and specialized — segments are not uniform

    • Segments are not separated by septa — the coelom is reduced and replaced by a hemocoel (blood-filled cavity), which is part of why they have an open circulatory system

    • Segmentation is fixed — the number of segments is determined during development and does not change

    • most organs are not metameric

    • Each segment bears highly specialized appendages adapted for specific functions (walking, feeding, sensing, reproduction)

  • Through tagmatization, these segments fuse to form distinct body parts.

    • tagmata: fused metamers forming body parts

    • Example of tagma formations include head, thorax, and abdomen.

    • Segments are grouped into distinct tagmata (body regions) with very different functions

      • feeding

      • sensory perception

      • locomotion

      • visceral functions

    • Tagmatization is far more pronounced than in annelids — segments within each tagma are heavily fused and modified

• Metamorphosis:

  • radical change in body form and physiology as an immature, larva stage becomes an adult

    • reduces competition between adult and immature stages

      • reduces competition for resources between the different life stages

      • allowing larvae and adults to occupy distinct ecological niches and utilize different resources

      • increases survival rates and efficiency in resource use.

      • By diverging in their nutritional habits, larvae can thrive on available food resources without competing with adult forms, leading to better survival and reproductive success in both forms.

• Major Classses and Taxa of Athropods:

  • Subphylum Chelicerata: (spiders, mites, tick, horseshoe crabs, sea spiders)

    • Key Features:

      • two tagma: prosoma (cephalothorax) and opisthosoma (abdomen)

        • have prosoma (cephalothorax): Structure that merges the head and thorax.

          • consists of:

            • eyes

            • chelicerae (specialized, two- or three-segmented, claw-like mouthparts (jaws))

              • pincher-like

              • feeding appendages

            • pepipalps (second pair of appendages)

              • foot-hands

              • used for sensory, feeding, locomotion, reproduction

              • walking legs

        • abdomen: Distinct region located posterior to the cephalothorax.

          • consists of:

            • digestive system

            • reproductive organs

            • excretory organs

            • respiratory organs

      • Six Pairs of Appendages: The prosoma generally features six pairs of appendages: one pair of chelicerae, one pair of pedipalps, and four pairs of walking legs.

        • no antennae

      • Gas exchange via book lungs and/or tracheae

      • Excretion via coxal glands and/or Malpighian tubules; primary waste is uric acid (minimizes water loss)

      • Ventral nervous system with fused ganglia

      • Mostly terrestrial; exoskeleton was a key preadaptation for water conservation

  • 3 Classes of Chelicerata:

    • class Arachnida: consists of spiders, scorpions, ticks, mites, harvestman (spider-like aquatic spiders)

      • Book lungs for gas exchange between blood and water

      • 8 legs

      • no antennae

      • chelicerae (fangs) and pedipalps

    • Class Merostomata: consist of horse shoe crabs, limulus

      • known for appendages around mouth region

      • marine

    • Class Pycnogonida: consists of sea spiders

      • reduced abdomen

      • no special respiratory or excretory structures

      • 4-6 long walking legs

  • Subphylum Myriapoda: Consists of centipedes (Chilopoda) and millipedes (Diplopoda).

    • jaw like mandibles derived from third or fourth body segment

    • compound eyes with 8 retinula cells

    • 1-2 pairs of antennae

    • uniramous or biramous appendages

    • body divided into head and trunk

    • 4 pairs of head appendages

    • gas exchange through spiracles and tracheae

    • Segmented Body & Many Legs: Their name means "ten thousand feet."

      • Defined by number of legs per segment.

  • 2 Classes of Myriapoda:

    • Class Diplopoda: millipedes

      • 2 pairs of legs per segment

      • body usually round in cross section

    • Class Chilopoda: centipedes

      • one pair of legs per segment

      • body oval in cross section

      • poison claws

  • Subphylum Crustacea: lobster, crayfish, crabs, shrimp

    • mostly aquatic

    • head with two pairs of antennae

    • one pair of mandibles with two pairs of maxillae

    • biramous appendages

    • Molting (Ecdysis): Crustaceans must shed their hard exoskeleton to grow, leaving them vulnerable during the process.

    • Body Segmentation: The body is usually divided into three tagmata: head, thorax, and abdomen, often covered by a protective shield called a carapace.

    • Respiration: Most breathe through gills attached to their appendages, even in terrestrial forms which keep them moist.

    • Part of Pancrustacea (paraphyletic grouping)

  • Subphylum Hexapoda: Insects and relatives

    • monophyletic assemblage with pancrustacea

    • body divided into head, thorax, and abdomen

    • 5 pairs of head appendages

    • 3 pairs of uniramous appendages on thorax

    • Respiration via tracheae

    • Undergo either complete metamorphosis (egg → larva → pupa → adult) or incomplete metamorphosis (egg → nymph → adult)

  • 2 Classes of Hexapoda:

    • Class Insecta: insects

      • winged insects

    • Class entognatha:

      • wingless insects

  • Class Triplobitomorpha (extinct)

    • Key Features:

      • Dominant life form from Cambrian period (600 million years ago) to Carboniferous period (345 million years ago).

      • Substrate feeders

      • Three tagmata: head, thorax, and pygidium (rear body segment)

      • All body segments articulated so they could roll up into a ball & protect the soft ventral surface

      • Biramous appendages