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Holometabolism, also called complete metamorphism, is a term applied to insect groups to describe the specific kind of insect development which includes four life stages – as an embryo or egg, a larva, a pupa and an imago or adult. Holometabolism is a monophyletic trait common to all insects in the superorder Endopterygota. In some species the holometabolous life cycle prevents larvae from competing with adults because they inhabit different ecological niches. Accordingly, their morphology can be adapted to just one phase of activity, such as larvae feeding for growth and development, as opposed to adults flying for dispersal and seeking new supplies of food for their offspring. Conversely, in some insects, the adults can protect and feed the younger stages.
There are four developmental stages, each with its own morphology.
The first stage is from the fertilization of the egg inside the mother until the embryo hatches. The insect starts as a single cell and then develops into the larval form before it hatches.
The second stage lasts from hatching or birth until the larva pupates. In most species this mobile stage is worm-like in form. Such larvae can be one of several general varieties:
- 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), or
- vermiform (maggot-like, as in most species of Brachyceran flies).
Other species however may be campodeiform (a form reminiscent of members of the genus Campodea, elongated, more or less straight, flattened, and active, with functional legs). This stage is variously adapted to gaining and accumulating the materials and energy necessary for growth and metamorphosis.
The third stage is from pupation until eclosion. The pupae of most species hardly move at all, although the pupae of some species, such as mosquitoes, are mobile. In preparation for pupation, the larvae of many species construct a protective cocoon of silk or other material, such as its own accumulated faeces. There are three types of pupae: obtect, exarate, and coarctate. Obtect pupae are compact, with the legs and other appendages enclosed. Exarate pupae have their legs and other appendages free and extended. Coarctate pupae develop inside the larval skin. In this stage, the insect's physiology and functional structure, both internal and external, change drastically.
Adult holometabolous insects usually have wings (excepting where secondarily lost) and functioning reproductive organs. In this stage, reproduction is the top priority for queens and males.
The Orders that contain holometabolous insects are :
- Coleoptera – Beetles
- Diptera – Flies
- Hymenoptera – Ants, bees, sawflies and wasps
- Lepidoptera – Butterflies and moths
- Mecoptera – Scorpionflies
- Megaloptera – Alderflies, dobsonflies and fishflies
- Miomoptera (extinct)
- Neuroptera – Lacewings, antlions, etc.
- Protodiptera (extinct)
- Raphidioptera – Snakeflies
- Siphonaptera – Fleas
- Strepsiptera – Twisted-winged parasites
- Trichoptera – Caddisflies