Phylum: Arthropoda

This is the largest phylum in Kingdom Animalia. Majority of the members are insects. Over two-thirds of all named species on earth are arthropods. The name is derived from the presence of jointed appendages (arthros-joint, poda-appendages) on the body.

Habitat: They are found in a wide variety of habitats.


  • Economically important insects: Apis (Honey bee), Bombyx (Silkworm), Laccifer (Lac insect)
  • Vectors: Anopheles, Culex and Aedes (Mosquitoes)
  • Gregarious pest: Locusta (Locust)
  • Living fossil: Limulus (King crab).

Apis Bombyx Mosquito Limulus

Body plan: They have organ-system level of organization and a tube-within-tube body plan.

Symmetry: They are bilaterally symmetrical and metamerically segmented.

Germ layer: Arthropods are triploblastic, segmented and coelomate animals. But the coelom is highly reduced. The blastocoels remains as a fluid (haemolymph) filled cavity called haemocoel.

Body characteristics:

  • The body is divided into head, thorax and abdomen.
  • A thick cuticle made of chitin forms an exoskeleton around the body.
  • The body bears lateral, paired and jointed appendages.
  • Mouth parts are adapted for various kinds of nutrition: sucking, piercing, cutting etc.


  • Respiratory organs are gills, book gills, book lungs or tracheal system.
  • Circulatory system is of open type.
  • Sensory organs like antennae, eyes (compound and simple), statocysts or balancing organs are present.
  • Excretion takes place through malpighian tubules or green glands.

Reproduction: They are mostly dioecious.

Embryo Development: Fertilisation is usually internal. They are mostly oviparous. Development may be direct or indirect.


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