Metamorphosis (biology)

Metamorphosis is a biological process by which an animal physically develops after

birth

or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell

growth

and differentiation. Some

insect

s,

fish

, amphibians, mollusks, crustaceans,

cnidaria

ns, echinoderms, and tunicates undergo metamorphosis, which is often accompanied by a change of nutrition source or behavior. Animals can be divided into species that undergo complete metamorphosis (" holometaboly"), incomplete metamorphosis (" hemimetaboly"), or no metamorphosis (" ametaboly").

Scientific usage of the term is technically precise, and it is not applied to general aspects of cell growth, including rapid

growth spurt

s. References to "metamorphosis" in mammals are imprecise and only colloquial, but historically idealist ideas of transformation and morphology, as in

Goethe

's ''Metamorphosis of Plants'', have influenced the development of ideas of

evolution

.

Etymology

The word ''metamorphosis'' derives from Greek , "transformation, transforming", from ('), "after" and ('), "form".

Hormonal control

Metamorphosis is iodothyronine-induced and an ancestral feature of all chordates.

In insects, growth and metamorphosis are controlled by

hormone

s synthesized by endocrine glands near the front of the body (anterior). Neurosecretory cells in an insect's brain secrete a hormone, the prothoracicotropic hormone (PTTH) that activates prothoracic glands, which secrete a second hormone, usually

ecdysone

(an ecdysteroid), that induces ecdysis.
PTTH also stimulates the corpora allata, a retrocerebral organ, to produce juvenile hormone, which prevents the development of adult characteristics during ecdysis. In holometabolous insects, molts between larval instars have a high level of juvenile hormone, the moult to the pupal stage has a low level of juvenile hormone, and the final, or

imaginal

, molt has no juvenile hormone present at all. Experiments on

firebugs

have shown how juvenile hormone can affect the number of nymph instar stages in hemimetabolous insects.

Insects

All three categories of metamorphosis can be found in the diversity of insects, including no metamorphosis ("ametaboly"), incomplete or partial metamorphosis ("hemimetaboly"), and complete metamorphosis ("holometaboly"). While ametabolous insects show very little difference between larval and adult forms (also known as "direct development"), both hemimetabolous and holometabolous insects have significant morphological and behavioral differences between larval and adult forms, the most significant being the inclusion, in holometabolus organisms, of a

pupa

l or resting stage between the larval and adult forms.

Development and terminology

In hemimetabolous

insects

, immature stages are called nymphs. Development proceeds in repeated stages of growth and ecdysis (moulting); these stages are called instars. The juvenile forms closely resemble adults, but are smaller and lack adult features such as wings and genitalia. The size and morphological differences between nymphs in different instars are small, often just differences in body proportions and the number of segments; in later instars, external wing buds form.

In holometabolous insects, immature stages are called larvae and differ markedly from adults. Insects which undergo holometabolism pass through a larval stage, then enter an inactive state called

pupa

(called a

"chrysalis

" in butterfly species), and finally emerge as adults.

Evolution

The earliest insect forms showed direct development (ametabolism), and the evolution of metamorphosis in insects is thought to have fuelled their dramatic radiation (1,2). Some early ametabolous "true insects" are still present today, such as

bristletails

and

silverfish

. Hemimetabolous insects include

cockroach

es,

grasshopper

s,

dragonflies

, and

true bugs

. Phylogenetically, all insects in the

Pterygota

undergo a marked change in form, texture and physical appearance from immature stage to adult. These insects either have hemimetabolous development, and undergo an incomplete or partial metamorphosis, or holometabolous development, which undergo a complete metamorphosis, including a

pupa

l or resting stage between the larval and adult forms.Gullan, P.J. & Cranston, P.S. 6.2 ''Life History Patterns and Phases in The Insects: An Outline of Entomology''. pp. 143–153. 2005 by Blackwell Publishing

A number of hypotheses have been proposed to explain the evolution of holometaboly from hemimetaboly, mostly centering on whether or not the intermediate stages of hemimetabolous forms are homologous in origin to the pupal stage of holometabolous forms.

More recently, scientific attention has turned to characterizing the mechanistic basis of metamorphosis in terms of its hormonal control, by characterizing spatial and temporal patterns of hormone expression relative to metamorphosis in a wide range of insects.

Temperature-Dependent Metamorphosis

According to a 2009 study, temperature plays an important role in insect development as each individual species are found to have specific thermal windows that allow them to progress through their developmental stages. These windows are not significantly affected by ecological traits, rather, the windows are phylogenetically adapted to the ecological circumstances insects are living in.

Recent research

According to research from 2008, adult ''

Manduca sexta

'' is able to retain behavior learned as a

caterpillar

. Another caterpillar,

the ornate moth caterpillar

, is able to carry toxins that it acquires from its diet through metamorphosis and into adulthood, where the toxins still serve for protection against predators.

Many observations published in 2002, and supported in 2013 indicate that programmed cell death plays a considerable role during physiological processes of multicellular organisms, particularly during

embryogenesis

, and metamorphosis.

Below is the sequence of steps in the metamorphosis of the butterfly (illustrated):

1 – The larva of a butterfly

2 – The pupa is now spewing the thread to form chrysalis

3 – The chrysalis is fully formed

4 – Adult butterfly coming out of the chrysalis

Pieris rapae