Arthropodas

General Characteristics and Classification of Arthropoda 4.09/5 (714)

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General Characteristics and Classification of Arthropoda

Arthropoda (Arthon- Jointed; podos- legs)

  • Arthropoda, animals having jointed appendages or legs.
  • It is a phylum of triploblastic, haemocoelomic, segmented invertebrates having head, thorax and abdomen, a chitinous exoskeleton and jointed legs and appendages.
  • Most successful phylum on the Earth that has ever existed.
  • Arthropods were first studied by Aristotle.
  • Von Siebold coined the term Arthropoda.

 

General characters

  1. Cosmopolitan in distribution found in aquatic, terrestrial and aerial forms. 
    Some are ectoparasitic and vectors of disease.
  2. Body have jointed appendages or legs (which are modified to different structures to perform different functions like jaws, gills, walking legs, paddle). There may be 3 pairs, 4 pairs, 5 pairs, many pairs.
  3. Body is triploblastic.
  4. Bilaterally symmetrical.
  5. Organ system level of organization.
  6. Body is divisible into head, thorax and abdomen.
    NOTE: In some (crustacean and arachnida) body is divisible into cephalothorax (head and thorax is fused) and abdomen.
  7. This is the first group to develop a true head, which contains sense organs and feeding organs specialized for their particular habitats.
  8. Body is covered with chitinous exoskeleton.
  9. They are haemocoelomate. Coelom i.e. body cavity is filled with blood or fluid.
  10. Head bears a pair of compound eyes and antenna.
  11. Locomotion takes place by jointed appendages.
  12. Digestive system is complete, straight and well developed.
    The mouth bears mouth parts for ingestion of foods. Mouths are modified for chewing, biting, sponging, piercing, siphoning.
  13. Respiration takes place by general body surface or gills (in Crustaceans) or trachea ( in insects, diplopoda and chilopoda) or booklungs (Arachnida) and book gills (in king cobra).
  14. Circulatory system is of open type i.e. do not have blood vessels and enters directly into the body chambers. The blood is colorless.
  15. Excretion takes place through Malphigian tubules (in terrestrial form) or green glands or coxal glands (in aquatic forms).
    NOTE: Aquatic forms are ammonotelic, terrestrial forms are uricotelic.
  16. Nervous system is of annelidian type, which consists of brain and ventral nerve cord.
  17. Unisexual i.e. sexes are separate.
  18. Fertilization is internal or external.
  19. They are either oviparous or ovoviviparous.
  20. Development may be direct or indirect.
  21. Sensory organ include antennae, sensory hairs for touch and chemoreceptor, simple and compound eyes, auditory organs (in insects) and statocysts (in crustacean).

Examples

Zoological name Common name
Limulus Horse shoe crab or King crab
Daphnia Water flea
Cancer Crab
palaemon Prawn
Astacus Cray fish
Scolopendra Centipede
Julus Millipede
Aranea Spider
Periplaneta americana Cockroach
Bombyx Moth
Pieris Butterfly
Musca Housefly
Apis Honeybee
Melanopus Grasshopper
Palamnaeus Scorpion

Insects as disease carriers

Insects Disease
Housefly Typhoid, Cholera, Dysentry, Diarrhoea
Mosquitoes Malaria, Filariasis, Encephalitis, Dengue fever
Kissing bug Chaga’s disease
Rat flea Bubonic plaque.
Sand fly Kala-azar
Bed bug Typhus fever
Tsetse fly African sleeping sickness
Body louse Relapsing fever

Classification

On the basis of jointed appendages, arthropods are classified into following classes:

Character Crustacean Myriapoda Arachnida Insect
Habitat Mainly aquatic, few live in moist place.Few are parasitic form Terrestrial, found under logs of wood stones etc. Terrestrial. Some are parasitic. Terrestrial and rarely aquatic. Some are parasitic.
Appendages/ walking legs Five pairs Many pairs Four pairs Three pairs.
Division of body Cephalothorax and abdomen. Head, thorax and abdomen. Cephalothorax (prosome) and abdomen (opisthosoma) Head, thorax and abdomen.
Antenna Two pairs One pair No antenna One pair
Respiration takes place by Gills Trachea Trachea and book lungs Trachea
Eye Compound eye Compound eye Distinct eye Compound eye
Excretion Coxal gland or Green gland or Antennary Malpighian tubules Green gland Malpighian tubules
Examples Prawn Millipede Spider, Scorpion Butterfly, Moth
     

Arthropodas

Things to Remember

  1. Largest phylum in the animal kingdom.
  2. Body is covered by chitinous exoskeleton which undergoes periodical ecdysis. The process of casting off of skin or integument is called ecdysis or moulting. 
  3. Cuticle is secreted by epidermis of skin. It protects the internal organs and provides space for the attachment of muscles.
  4. Spider produces web by means of spinnerets.
  5. Scorpion produces toxin by means of telson (tail).
  6. In prawn, only oxygenated blood flows.
  7. Prawn has 19 pairs of appendages.
  8. Larva of cockroach is called Nymph.
  9. In cockroach heart is thirteen chambered.
  10. Female cockroach lack anal style. Whereas male cockroach are stylish.
  11. Cockroach receives ultrasonic sound by means of anal circuit.
  12. Mouth parts adapted for various modes of feeding in Arthropods are:
  • Biting and chewing type: Cockroach, grasshoppers
  • Chewing and lapping type: bees and wasps
  • Piercing and sucking type: Bugs, Aphids, Mosquitoes
  • Sponging type: Housefly
  • Siphoning type: Butterflies and moth
  1. Haemoglobin absent in blood so called haemolymph.
  2. Periplatus is a connecting link between Annelida and Arthropoda.

General Characteristics and Classification of Arthropoda

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