Hypermetamorphosis is a term used in entomology that refers to a class of variants of holometabolism, that is to say, complete insect metamorphosis, but where some larval instars are distinct from each other.
Description
Hypermetamorphosis, as the term normally is used in entomology, refers to a class of variants of holometabolism. In hypermetamorphosis some larval instars are functionally and morphologically are distinct from each other.
In a typically holometabolic insect such as most flies, moths, or wasps, all the larval instars are similar except that earlier instars are smaller than later instars. In an insect that undergoes hypermetamorphosis however, at least one instar, usually the first, differs form and in function as well. Such hypermetamorphosis occurs in very similar forms in several orders of insects, and it is appropriate to avoid careless generalisations about the nature of that class of development strategy, because, biologically speaking, most of the examples are not homologous, but analogous; for instance examples of hypermetamorphosis in the Diptera are neither derived from examples among the Mantispidae, nor from any common ancestor with hypermetamorphosis. There are however important functional points of resemblance.
There also are examples of holometabolic species in which there are certain striking differences between the earliest instars and the later instars, though without their generally being regarded as hypermetamorphic. For example, early instars of many Papilionidae are of a colour, shape and texture that suggest bird droppings; later instars that are larger and would simply stand out in such camouflage, typically become leaf-green.
In hypermetamorphic species, the tendency is to produce the first instar in large numbers as tiny, very mobile larvae that must find their own way to the food they will need for their entire larval existence. The general term for such a mobile first instar is a planidium, from the Greek language πλάνος (planos) meaning "roaming". The planidial instar is specialised for active mobility and is equipped to seek out the prey or host on which subsequent instars are to feed. In typical examples the first-instar larval morphology is campodeiform (meaning: elongated, flattened, and active, more or less resembling the morphology of insects in the genus Campodea). There is however, considerable variety in the forms of planidia that occur in various families and orders; in the beetle family Meloidae, the three-clawed planidium originally was called a triungulin, and similar planidia for example, those of the Strepsiptera, may also be called triungula.
In their planidial form many species do not feed; they first change their skin and change their bodily form to a form suited to eating rather than seeking out food. Whether the larva feeds in its phase as a planidium or not, it must feed after it undergoes ecdysis. In its feeding phase it adopts a scarabaeiform (grublike) or vermiform (maggotlike) morphology.
As a rule the instars after the first ecdysis are of more or less constant form and not highly mobile, being specialised for feeding and growth until the final larval instar metamorphoses into the pupal form.
Various forms of Hypermetamorphosis
Hypermetamorphosis usually occurs as an adaptation of the ontogeny of certain parasitoid insects, notably:
- the beetle families Meloidae and Ripiphoridae,
- the order Strepsiptera
- flies in the families Acroceridae, Bombyliidae and Nemestrinidae,
- The Neuropteran family Mantispidae and
- the parasitic wasp family Eucharitidae.
Technically the subimago of the Ephemeroptera might be described as a stage in a form of hypermetamorphosis, but that is not common practice.
Examples of hypermetamorphosis in any given insect order are analogous and not homologous to those in any other order; for example hypermetamorphosis in the Acroceridae was not derived from the Strepsiptera, much less the Ephemeroptera.
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