Metamorphosis
in
biology
means the process of transformation from an immature form to an adult form in two or more distinct stages. Good examples are
insects
and
amphibians
. Life for most insects begins as a
larva
or
nymph
then progresses to the
pupa
stage and ends as an
adult
.
[1]
A
cicada
in the process of shedding.
Animals that metamorphose include
tadpoles
into
frogs
,
caterpillars
into
butterflys
, and generally
insect
larva
into adult insects.
Six stages of development, from newly hatched nymph to fully winged adult. (
Melanoplus sanguinipes
)
There are two main types of metamorphosis in
insects
:
simple metamorphosis or
hemimetabolism
, and complete metamorphosis or
holometabolism
.
[2]
Simple metamorphosis or
hemimetabolism
[
change
|
change source
]
Insects who go through simple (or incomplete) metamorphosis have three life stages. These insects start as
eggs
, which are usually very small. When the egg hatches, a larva or
nymph
comes out. Nymphs are just baby insects. Most of the time, the nymph looks similar to the adult, but it is smaller, may have different colouration, and does not have
wings
. The nymph grows through stages called
instars
, shedding its 'skin' (
epicuticle
) at each stage (
ecdysis
). Finally, it changes into a mature adult with wings.
Some insect nymphs are
aquatic
, which means they live in water. These nymphs usually have
gills
and look very different from the adults they will turn into. Nymphs that live in water are called
naiads
.
Some insects that have a
life cycle
of egg-nymph-adult are:
Complete metamorphosis or
holometabolism
[
change
|
change source
]
Insects that have complete metamorphosis have four life stages. These insects start as
eggs
, which are very small. The egg hatches and a
larva
comes out. The larva looks like a
worm
and eats and eats so that it can grow much bigger. When the larva has grown it changes into a
pupa
. The pupa usually can not move or eat. The pupa is a special time when the insect is changing into an adult that will look very different from the larva or the pupa.
Moth
pupae (
plural
of pupa) are inside
cocoons
. When the pupa opens, the adult insect comes out.
Many insects have a
life cycle
of egg-larva-pupa-adult. Some of these insects are:
The monarch butterfly life cycle
Metamorphosis: comparative lengths of life stages
Species
|
Egg
|
Larva/Nymph
|
Pupa
|
Adult
|
Housefly
|
1 day
|
2 weeks
|
1 week
|
2 weeks
|
Ladybird
|
4 days
|
2 weeks
|
2 weeks
|
3?9 months
|
Monarch Butterfly
|
4 days
|
2 weeks
|
10 days
|
2?6 weeks
|
Periodical
Cicada
|
1 month
|
13/17 years
|
no such stage
|
2 months
|
Mayfly
|
1 month
|
3 years
|
1 day
|
Cockroach
|
1 month
|
3 months
|
9 months
|
The
monarch butterfly
goes through four stages of development. Life for a monarch butterfly begins as an egg hatched from an adult. This egg then develops into a worm-like larva
caterpillar
. In the pupa or chrysalis stage, the caterpillar spins a silk pad on a twig or leaves and hangs from this pad by its last pair of prolegs. It hangs upside down in the shape of a "J" and then molts leaving it encased in a green exoskeleton. The mature butterfly emerges after about two weeks and feeds on a variety of flowers including
milkweed
flowers, red clover and goldenrod.
Just before metamorphosis, only 24 hours are needed to reach the stage in the next picture
Almost functional
common frog
with some remains of the gill sac and a not fully developed jaw
In typical
amphibian
development, eggs are laid in water and larvae are adapted to an aquatic lifestyle.
Frogs
,
toads
, and
newts
all hatch from the egg as larvae with external gills. Afterwards, newt larvae start a predatory lifestyle, while
tadpoles
mostly scrape food off surfaces with their horny tooth ridges.
Many
marine
species have evolved a system where they release their
eggs
and
sperm
into the water at the same time, and fertilisation takes place in the sea over one or two days. This increases their chance of reproductive success.
Then, larvae join the other
plankton
, eating and growing before metamorphosing into adults. Most marine invertebrates and many fish have a life cycle with
pelagic
larvae or even pelagic eggs. These have the capacity to be transported long distances, and so spread the species around to other places.
The need for metamorphosis is clear when the relative size of eggs and adults is understood. A mature
Atlantic cod
can reach two metres long, and weigh over 200 pounds (96 kg). The female will produce over 100,000 eggs in a single spawning, and the resulting larvae are quite tiny. There is no way they could live like the adults, which are more or less top
predators
. The larvae cannot even swim against the current. Therefore, they must live a different kind of life before they can live as the adults do.
Their development is packaged into stages, and each stage has
adaptations
to life at its stage. The sharp distinction between stages (best seen in insects) is presumably because a longer transition would leave the juvenile less
fit
, either at its younger or older ways of life.
- Davies, R.G.
Outlines of Entomology
. Chapman and Hall: chapter 3