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Egg

Mature adult Aquatic naiad

Aerial

imago

• Gradual metamorphosis or paurometabolous
  development : In this type of metamorphosis, the
  life history includes egg, nymph (young) and imago
  (adult). The nymph resembles an adult in general body
  form, but lacks wings and external genital appendages.
  E.g., cockroaches, grasshoppers, locusts, termites, stick
  insect, praying mantis, bed bug and lice etc.


• Complete metamorphosis or holometabolous
  development : In this type of metamorphosis, the
  life history includes four developmental stages – egg,
  larva, pupa and imago (adult). Larva after hatching,
  moults several times to become a fully grown adult.
  It later becomes a pupa within a secreted case, called
  puparium. Pupa differentiates into the young adult
  that breaks the puparium open and emerges out. It
  grows to a mature form, e.g., butterflies, moths, beetles,
  houseflies, mosquitoes, fleas, honey bees, ants, wasps,
  etc.

• Metamorphosis or post embryonic growth of insects, is
under hormonal control. The various hormones secreted
are as follows :

• Brain hormone (BH) : It is secreted by the
  neurosecretory cells of the brain. It serves to activate
  corpora cardiaca.


• Prothoracicotropic hormone (PTTH) : This hormone
  is secreted by the corpora cardiaca, which in turn
  stimulates the prothoracic glands.


• Prothoracic gland hormone (PGH) : This hormone is
  secreted by the paired, bilateral sheet of cells in thorax,
  constituting the prothoracic glands. This hormone is
  known to trigger moulting as it acts on the tissues to
  promote all of the changes characterising a moult.
  
  
  • Juvenile hormone (JH) : This hormone is secreted by
    corpora allata. This hormone regulates morphogenesis
    and so promotes metamorphosis, that is development
    of the larvae into adult through pupal stage.
    • The Class Insecta is divided into 30 orders. The most
    important orders with common examples are listed in the
    table given as follows :
    Table : Important Orders of Class Insecta
    with examples
    Orders Examples
    Thysanura Lepisma (Silver fish)
    Orthoptera Schistocerca (Locust), Poecilocerus
    (grasshopper), Gryllus (House cricket)
    Periplaneta (Cockroach), Mantis (Praying
    mantis).
    Isoptera Microtermes (Termites or white ants).
    Odonata Sympetrum (Dragon fly).
    Anoplura Pediculus (Human louse).
    Ephemeroptera Ephemera (Mayfly).
    Hemiptera Cimex (Bed bug), Dysdercus (Red cotton
    bug).
    Homoptera Aphis (Aphid = Plant louse), Laccifer
    lacca or Tachardia lacca (lac insect).
    Coleoptera Coccinella (Lady bird beetle), Sitophilus
    (Grain Weevil).
    Lepidoptera Bombyx (Silk moth), Pieris (Cabbage
    butterfly).
    Diptera Musca (Housefly), Anopheles, Culex,
    Aedes (All the three mosquitoes).
    Siphonaptera Xenopsylla cheopis (Rat flea).
    Hymenoptera Apis (Honey bee), Vespa (Wasp).
  
  
  
  

•    Subphylum Onycophora that consists of unsegmented

terrestrial, primitive worm-like arthropods which

resemble both annelids and arthropods is considered as

connecting link e.g., Peripatus.

•    They resemble annelids in worm like body, indistinct head,

thin flexible cuticle, unjointed appendages, segmentally

arranged nephridia and ciliated genital ducts.

•    They resemble arthropods in having clawed legs,

antennae, haemocoel, salivary glands, tracheae and

dorsal tubular heart with lateral ostia.

onycophora : the Connecting link