Phylum Arthropoda

• Phylum Arthropoda includes chelicerates, myriapods, crustaceans, and insects

  • Bilaterally symmetrical and segmented body (with tagmata and jointed appendages)
  • Well-developed digestive, circulatory, respiratory, nervous, muscular and excretory systems
  • Covered with cuticular exoskeleton
  • Marine, freshwater and terrestrial habitats (many are capable of flight)
• Special features include:

  o   Highly modified segments

  o   Hard exoskeleton

  o   Jointed appendages
• The body wall has outer circular muscles and inner longitudinal muscles to move segmented body via peristalsis (alternating contractions of longitudinal muscles and circular muscles)
• Adaptive significance of segmentation

  • Segmentation creates potential for further specialisation and division of labour of different body parts to perform function more effectively for better survival
  • Tagmatisation: regional specialisation of segment by putting similar specialised segments in same region to organise them into one region
  • Tendency to organise segments into regions having similar structure, function and appendages
  • Each group of similar specialised segments is called tagma where segments can stay separate or fused within one tagma  
• Myriapods

  o   Head and trunk may contain many fused segments

  o   All segments have similar structure and function in the trunk
• Crustaceans

  • Cephalothorax also known as carapace
  • Cephalothorax and Abdomen are 2 tagmata
  • Difference in abdomens shows the gender difference
  • Males have a narrower, triangular abdomen
  • Females have a wider, rounded abdomen (bigger to store eggs)
• Chelicerates – Scorpions

  o   Telson (sting) is the last abdominal segment
• Chelicerates – Horseshoe crabs

  • Telson has a sharp end and cerated top
  • Telson is not venomous, for balance instead
  • Telson used to prevent slipping over and to right itself up when it is overturned
• Advantages of segmentation

  • Built-in redundancy – can afford to lose segments
  • If it loses/damages any segments, it can afford to lose the segments
  • There is more varied, precise, coordinated, synchronous movement when each segment is controlled individually
• The body of an arthropod is completely covered by the cuticle
• Cuticle


  A hard exoskeleton constructed from layers of chitin, proteins and waxes secreted by the epidermis
• Chitin


  A polysaccharide which is a polymer built for strength and flexibility due to polymerase run in long chains
• Sclerotin


  The main protein in mixture of cuticle that forms the scaffold of cuticle
• Sclerotin strengthening

  1. Cross-linking of protein chains via sclerotization to make matrix stronger and more rigid

  2. Can deposit minerals to harden exoskeleton
• Waxy lipids


  Incorporated to make exoskeleton waterproof
• Cuticle


  • Variable thickness to vary overall thickness and flexibility
• Hard exoskeleton contains less chitin and more sclerotin and minerals

  Soft exoskeleton contains more chitin and less sclerotin and minerals
• Function of cuticle in sea

  • Protection against predators
  • Protect against water pressure to maintain body shape
• Function of cuticle on land

  • Protection against predators and water loss due to dehydration
  • Support and maintain body shape against gravity
• Shedding of old cuticle at intervals to produce larger, new ones to allow for growth

  • New cuticle contains empty spaces for growth
  • Rigid, non-expansible exoskeletons prevent growth since it cannot stretch, therefore need to make new cuticle
• Challenges

  • Animal might get stuck in old cuticle while exiting
  • When shedding, there is limited locomotion so it cannot run from predators
  • Making entire cuticle requires time, energy and resources
  • New cuticle is soft, takes time to harden up so it is vulnerable to predators
  • Cannot camouflage as well due to less pigmentation of new cuticle at the beginning
  • The moment cuticle is soft, muscles can’t act as well as compared to when it is hard since muscles cannot exert force on cuticle (muscles are attached to cuticle)
• Instars: juvenile stages between moults

  Intermoult: period of time between moults
• Incomplete metamorphosis

  • Young looks like adult
  • No drastic change to body form and body shape
  • Possibility that young can grow bigger
  • Nymph and adult use same resources thus increasing competition
  • Difference in nymph and adult: sexual maturity, presence of wings
• Complete metamorphosis

  • Larva specialised for feeding and growing
  • Adult specialised for reproduction
  • Pupa stage may be disadvantageous – vulnerable since it cannot move to defend itself
  • Specialisation increases efficiency and larva and adult do not compete for resources
• Moulting


  Under endocrine control, ecdysone (steroid hormone) stimulates epidermis to secrete new cuticle beneath old one
• Moulting process

  1. Initiate moulting by causing old cuticle to separate from epidermis

  2. Signal epidermis to stimulate epidermal cells to release moulting fluid

  3. Moulting fluid in space between old cuticle and epidermis contains chitinases and proteases to break down chitin and protein in cuticle

  4. Old cuticle is broken down to recycle monomers of proteins and to help shedding by breaking and splitting old cuticle

  5. Individuals of the same species exit old cuticle from same spot (line of weakness)

  6. Organism takes in air or water to cause body to swell up and exert more force to make it easier to split up

  7. Stimulate epidermis to produce new cuticle
• Lipoprotein layer

  Between new cuticle and moulting fluid to protect cuticle from being broken down by enzymes
• New cuticle is still soft, wrinkled, incomplete and weakly pigmented and needs additional time to stretch out, deposit pigments and harden cuticles

  • Stretching out smooths out wrinkles to make the new cuticle larger and increase the length or size of the animal
  • Hardening done by proteins via sclerotization (forming cross-links in protein chain)
  • Deposition of mineral salts in protein matrix via mineralisation to harden
• Arthropods have paired, jointed segmental appendages

  • Appendages consists of linear series of articles
  • Each article consists of cylinder of hardened exoskeleton
  • Articular membrane allows article to articulate with body or another article
  • Articular membrane contains muscles and has a thinner layer of chitin to allow for movement of appendages
  • Muscles inside cylinders extend across flexible joints, and contract to produce movement
• Flexor and extensor muscles are antagonistic

  • Flexor muscle contracts, extensor muscle relaxed – muscle is flexed
  • Flexor muscle relaxed, extensor muscle contracts – muscle is extended
• Functional specialisation of appendages

  • Head appendages specialised for sensory (both mechanical and chemical) reception and feeding (because it is close to the mouth)
  • Thoracic and abdominal appendages are adapted for locomotion, food manipulation, gas exchange, reproduction and other functions
  • Appendages may be lost from some tagmata
• Chelicerae


  • 1 pair at head
  • Sharp to pierce prey and inject venom to kill/paralyse prey or for injecting enzymes to partially digest food
• Pedipalps


  • 1 pair
  • Hairy for sensory reception and catching prey
• Walking legs

  • 4 pairs
• Pedipalps on male crabs

  • Hook-like to hook onto the abdominal spines of females during mating to fertilise eggs as soon as they are laid
• Gnathobase of walking legs

  • Around mouth has spines to grind up food
  • Can only eat while it is walking
• Telson

  • Triangular on coastal horseshoe crabs
  • Rounded on mangrove horseshoe crabs
• Scorpion with bigger sting


  Depends on sting to catch prey, venom is more potent
• Scorpion with bigger pedipalps


  Depends on pedipalps to catch prey, venom not as potent
• Antennae and antennules for sensory reception

  • Antennae is longer and unbranched, antennule is shorter and branched