Characteristics of Insects

General characteristics of all [most] insects.

1. Bilateral symmetry, segmented [metameric] bodies.
2. Advanced cephalization with an anterior brain and ventral nerve cord having segmental ganglia .
3. Jointed appendages.
4. Three body regions [tagmata]: head, thorax, and abdomen;
   1. Head:
      1. formed by tagmosis of primitive ancestral somites with segmental appendages modified to mouth-parts
      2. oral opening, or mouth
      3. one pair of sensory antenna
      4. compound eyes usually present, especially in adults
      5. simple eyes often present as well
   2. Thorax:
      1. composed of three body segments posterior to the head
         1. pro-, meso-, and meta-thoracic segments
      2. specialized for locomotion
      3. three pairs of legs, one or two pairs of wings [or wings absent]
      4. if present, wings are on the meso- and metathorax only
   3. Abdomen:
      1. simplest of insect tagmata
      2. 11 abdominal segments primitively, variously modified in modern insects
      3. genital openings usually on segments 8 and 9
      4. anal opening usually on terminal segment
      5. no appendages on abdominal metameres
5. Wings, if present, are not homologous with any vertebrate
6. Chitinous exoskeleton:
   1. covers the entire body surface
   2. provides semipermeable barrier, especially important in limiting water loss
   3. anchors muscles
   4. provides joint articulations
   5. composed of hardened, sclerotized plates [sclerites] and flexible, membranous sections
   6. requires periodic molts to permit growth and development
7. Stage-structured life-histories, usually delineated by molts.
8. Open circulatory system; hemocoelomic body; organs bathed in hemolymph
   1. dorsal "heart"
9. Ventral nerve cord; dorsal anterior ganglion; circumenteric connectives
10. Tracheal system for gas exchange:
    1. few insects possess chemical O₂ carriers
    2. tracheal tubes ramify throughout the body to allow direct atmospheric gas exchange for most body cells
11. Malphigian tubule system for nitrogenous waste disposal [contrast with metanephridia in annelids]
    1. system of tubules that filters the body fluid [hemolymph] and disposes of the filtrate via the alimentary canal
    2. fuctionally analogous to vertebrate kidneys

[From Arthropod evolution and general characteristics of insects M. Camann Humboldt State University