Holometabolism, also called complete metamorphosis, is a form of insect development which includes four life stages: egg, larva, pupa, and imago (or adult). Holometabolism is a synapomorphic trait of all insects in the superorder Endopterygota. Immature stages of holometabolous insects are very different from the mature stage. In some species the holometabolous life cycle prevents larvae from competing with adults because they inhabit different ecological niches. The morphology and behavior of each stage are adapted for different activities. For example, larval traits maximize feeding, growth, and development, while adult traits enable dispersal, mating, and egg laying. Some species of holometabolous insects protect and feed their offspring. Other insect developmental strategies include ametabolism and hemimetabolism.
Developmental stages
There are four general developmental stages, each with its own morphology and function.
Egg
The first stage of the insect life cycle is the egg, or embryo, for all developmental strategies. The egg begins as a single cell which divides and develops into the larval form before hatching. Some insects reproduce by parthenogenesis or may be haplodiploid, and produce viable eggs without fertilization. The egg stage in most insects is very short, only a few days. However, insects may hibernate, or undergo diapause in the egg stage to avoid extreme conditions, in which case this stage can last several months. The eggs of some types of insects, such as tsetse flies, or aphids (which are hemimetabolous), hatch before they are laid.
Larva
The second stage of the holometabolous life cycle is the larva (plural: larvae). Many adult insects lay their eggs directly onto a food source so the larvae may begin eating as soon as they hatch. Larvae never possess wings or wing buds, and have simple rather than compound eyes. In most species, the larval stage is mobile and worm-like in form. Larvae can be classified by their body type:
Elateriform: wireworm-like, as in the beetle family Elateridae. Eruciform: caterpillar-like, as in the Lepidoptera and Symphyta. Some that lack legs, such as the larvae of Nematoceran flies such as mosquitoes, are called apodous eruciform. Scarabaeiform: grub-like, with a head-capsule, as in the beetle family Scarabaeidae. Vermiform: maggot-like, as in most species of Brachyceran flies. Campodeiform: similar to members of the genus Campodea, elongated, more or less straight, flattened, and active, with functional legs.
The larval stage is variously adapted to gaining and accumulating the materials and energy necessary for growth and metamorphosis. Most holometabolous insects pass through several larval stages, or instars, as they grow and develop. The larva must moult to pass from each larval stage. These stages may look very similar and differ mostly in size, or may differ in many characteristics including, behavior, color, hairs, and spines, and even number of legs. Differences between larval stages are especially pronounced in insects with hypermetamorphosis. The final larval stage in some insects is called a prepupa. Prepupae do not feed, and become inactive. It is not uncommon that larval tissue that is broken down during metamorphosis increase in size by cell enlargement, while cells and tissues that will turn into imago grows by an increase in numbers.
Pupa
To enter the third stage of homometabolous development, the larva undergoes metamorphosis into a pupa. The pupa is a quiescent, non-feeding developmental stage. Most pupae move very little, although the pupae of some species, such as mosquitoes, are mobile. In preparation for pupation, the larvae of many species seek protected sites or construct a protective cocoon of silk or other material, such as its own accumulated feces. Some insects undergo diapause as pupa. In this stage, the insect's physiology and functional structure, both internal and external, change drastically.
Pupae can be classified into three types: obtect, exarate, and coarctate. Obtect pupae are compact, with the legs and other appendages enclosed, such as a butterfly chrysalis. Exarate pupae have their legs and other appendages free and extended. Coarctate pupae develop inside the larval skin.