Insects that are endopterygotes have holometabolous development and undergo complete metamorphosis, meaning they exist as 2 separate active life stages (a larva and an adult), with a transitional stage in between (the pupa) where they change from larva to adult form.
A redesign of the entire body plan is necessary for an insect larvae to progress to become an adult, usually involving a dramatic transformation in form and function of the insect. When a larvae has accumulated enough energy, it moults (pupates) into a transitional stage known as the pupa, where they may be enclosed in a protective case (the puparium). Within this, the massive internal and external reorganisation using previously dormant DNA allows an adult (imago) to emerge and fill it’s new ecological role.
Larval types. Lepidopteran caterpillars are characteristic of polypod larvae (eruciform), where cylindrical bodies are supported by thoracic legs and abdominal pro legs (pseudopods). Symphytan hymenoptera (sawflies) have polypod larvae, but these can be distinguished by presence of 7 prolegs. Oligopod (campodeiform) larvae lack abdominal prolegs but have functional thoracic legs, as in members of diptera, lepidoptera and coleoptera. Apodous larvae lack true legs, are worm or maggot like and usually live in the soil or somewhere hidden.
Pupal types. Several different types of pupae exist, all adapted to the particular environment they are found in (like the larvae). Most pupae are exarate, meaning their appendages are free from the body, whereas obtect pupae have appendages cemented to the body. Exarate pupae can have articulated mandibles (decticous) to cut through the cocoon upon emergence, or non-articulated (adecticous) where the outer layer of the cocoon is shed instead of eaten through.
Many orders of insect undergo incomplete metamorphosis, where they grow progressively in size through nymphal stages by moulting. For more about incomplete metamorphosis, click here.