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Hexapods are oviparous i.e. reproduction results into egg laying by adult female. Although hatchlings are quite different different in morphology and feeding habits.
To achieve adult form these hatchlings undergo post embryonic development of several stages which may vary in size shape and feeding. The whole process is termed as Metamorphosis.

GENERAL MODEL OF METAMORPHOSIS

General Model of Metamorphosis depicting Hemimetabolous & Holometabolous

IMPORTANT TERMINOLOGY

NYMPH –  It is sexually immature young form whose exoskeleton looks like as that of adult but has only wing pads and under developed gonads
NAIAD – It is sexually immature young individuals whom have different structure and mode of life i.e. habitat.
LARVA –  It is the sexually immature young form whom are morphologically alike adult and determined for feeding only.
CRYSALIS – Prior to aquire adult morphology larva passes through a quiescent stage know as Pupa or CRYSALIS. This stage is marked by development of adult morphology.

TYPES OF METAMORPHOSIS

  • ‌Ametabolous
  • Hemimetaboly
  • ‌Holometaboly

AMETABOLY

Metamorphosis is absent.
Young hatchlings similar to adult in all respects of structure and feeding. But they are smaller in size and has under developed reproductive organ. They undergoes few moults to reach adult size. Insect passes through this type of post embryonic development are known as Ametabolous.

Ametabolous type of life cycle – Overview

HEMIMETABOLY

It is characterised by presence of Nymph or Naiad and absence of Pupa stage in the life history of the insect.

It can be define in 2 following forms:

  • Gradual Metamorphosis
    • Nymph undergoes gradual changes or moult to become an adult or Imago.
    • It has been defined as the series of subtle changes or moults through which sexually immature young i.e. Nymph passes to aquire adult morphological structure i.e. wings and become reproductively active.
    • EXAMPLE – Periplaneta americana (American cockroach), Dysdercus cingulatus ( Red cotton bug) Eggs are laid in bunches inside the soil. Hatchlings undergoes 3 moults or 4 Nymphal stages to become an adult.
  • Incomplete Metamorphosis
    • Naiad undergoes gradual moults to become an adult
    • It has been defined as the series of moults through which young individual whom is different in aspects of structure and mode of life, to acquire adult morphology and mode of life.
    • EXAMPLE – Insects belonging to order Odoneta: Dragon fly, Mayfly, Coenagrion puella (Damselfly) – Female lays eggs over aquatic weeds. Hatchlings are termed as Naiads and identify by presence of caudal gills as their habitat is aquatic. In order to aquire adult morphology and mode of life i.e. terrestrial habitat they undergoes few subtle moults.
Gradual Metamorphosis

HOLOMETABOLY

It is also termed as complement metamorphosis.

Insect passes through all four body forms in its life history i.e. EGG -> LARVAE or INSTAR -> PUPA or CRYSALIS -> ADULT or IMAGO.

EXAMPLE –  Insects belongs to the order Lepidoptera, Coleoptera, Hemiptera etc. Danaus plexippus (Monarch butterfly) – it lays eggs single egg over the leaf of host plant. Larva emerges out and passes through 5-6 instar stages and turn into pupa. Inside pupa adult morphological appendages develop such as wings covered with scales, siphoning moth part  and antenna etc.

Holometabolous

Nancy Gupta

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