Butterfly Life Cycle — Metamorphosis
What causes the radical transformation from caterpillar to butterfly?
How does a caterpillar rearrange itself into a butterfly?
What happens inside a chrysalis or cocoon?
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.
In insects growth and metamorphosis are controlled by hormones synthesized by endocrine glands
near the front of the body. Neurosecretory cells in an insect's brain secrete a hormone that activates glands,
which secrete a second hormone to produce juvenile hormone, which prevents the development of adult
characteristics during the shedding of the outer cuticle (skin).
Metamorphosis is a Greek work meaning transformation or change in shape. Butterflies undergo a
complete metamorphosis which means that the butterfly looks different in each stage:
egg, larva, pupae and adult.
The butterfly egg is spherical or ovate shaped, varying in color and size. It is covered by chorion, a hard shell that
protects the egg, and is lined with a waxy layer that prevents dehydration. It is full of nutrients and is commonly the
first meal for the newly hatched larva. There is a small opening called a micropyle which allows sperm to enter the
egg for fertilization while inside the female.
To protect the egg and embryo that is forming, the eggs are attached to a leaf by the butterfly using an
adhesive-like substance that glues the egg to the plant leaf. The nature of the “glue” is still fairly unknown in the
scientific community. For protection, the eggs are often laid on the underneath side of the leaf so as not to be
visible to predators. It is essential for the eggs to be laid on the host plant for that particular butterfly species.
Eggs are sometimes laid in groups, although many butterflies such as the Monarch lay their eggs singly. The
average time for the embryo to develop is 7-14 days but in instances where the egg is the overwintering stage, the
embryo may take months to develop.
To protect the egg and embryo that is forming, the eggs are attached to a leaf by the butterfly using an
adhesive-like substance that glues the egg to the plant leaf. The nature of the “glue” is still fairly unknown in the
scientific community. For protection, the eggs are often laid on the underneath side of the leaf so as not to be
visible to predators. It is essential for the eggs to be laid on the host plant for that particular butterfly species. Eggs
are sometimes laid in groups, although many butterflies such as the Monarch lay their eggs singly. The average
time for the embryo to develop is 7-14 days but in instances where the egg is the overwintering stage, the embryo
may take months to develop.
A caterpillar grows really fast. This is because they eat a lot. A caterpillar is really small when it is born. It starts to
grow fast. This is because it eats all the time. When a butterfly moth larva (also known as a caterpillar) first
hatches from its egg, it is very small! This young caterpillar is referred to as a first Instar caterpillar
Caterpillars face a challenge as they grow! Unfortunately, their skin cannot grow with them! In
order for a caterpillar to grow larger than the skin it had when it hatched, it must make a new, larger skin! The
caterpillar does this by first growing a new skin underneath the outer skin. Then, when it is ready, it sheds the
old skin and the newer, larger skin underneath is exposed. This process is properly called molting. After the
caterpillar has molted for the first time, it is referred to as a second Instar and it has some room to grow.
The third instar caterpillar also eats and grows until it is too big for its skin. It molts again, and the caterpillar with
its new skin is referred to as a Fourth Instar caterpillar. Most butterflies have 5-6 instars.
The pupa, though sedentary, is a stage of great internal turmoil and transformation. Nearly all
of the larval tissues and organs (eyes, mouth parts, legs, glands, muscles) will be digested and
reorganized into the body of the adult butterfly. Prior to pupation, the larva loses much of its
patterning and its length decreases. In addition, the gut is evacuated.
The pupa stage of the butterfly’s life cycle is a time for complete change. The pupa is also known as a chrysalis
and is a vessel in which the slow moving caterpillar changes into a winged butterfly. The process begins with the
prothoracicotropic hormone (PTTH) triggering the caterpillar to quit eating and start wandering to a place to
pupate, which is usually on twigs or other safe areas around their host plants but not necessarily on the host
plants. Some butterfly species that over winter as a chrysalis may pupate in the leaf litter close to the ground.
The chrysalis is immobile so it is important that it be camouflaged into the
environment. For example, the Spicebush Swallowtail’s chrysalis takes on a brown or green color depending on
the color of the environment. Many chrysalides are leaf shaped and vary in color from yellow, brown, green to
silver and gold. Some chrysalides have the ability to wiggle and make a hissing sound in order to frighten
predators.
This is not a "resting" stage as many people think. Quite to the contrary, a lot is happening to the pupa! The body
of the caterpillar is transforming into an adult butterfly! Wings are fully formed (the beginnings of the wings were
actually forming underneath the caterpillar's skin before its last molt) in the chrysalis. Antennae are formed and the
chewing mouth parts of the caterpillar are transformed into the sucking mouth parts of the butterfly.
Adults emerge from the pupa with short wings that must be expanded with hemolymph.
Newly emerged adults, especially those with large, heavy wings, must be able to hang from a
twig or other object to successfully enlarge their wings.
The transformation from a hardly recognizable creature to a flight-ready adult is rapid, occurring
over a span of only 20-60 minutes. It is one of nature's most captivating and awe-inspiring phenomena.
Some species cycle through an entire generation in as few as 4-5 weeks with the Monarch butterfly
being a familiar example.
Near the end of each stage, the larva undergoes a process called apolysis, mediated by the release
of a series of hormones. During this phase, the cuticle, a tough outer layer made of a
mixture of chitin and specialized proteins, is released from the softer epidermis beneath,
and the epidermis begins to form a new cuticle. At the end of each instar, the larva molts,
the old cuticle splits and the new cuticle expands, rapidly hardening and developing pigment.
Development of butterfly wing patterns begins by the last larval instar. Internally, most of
the body cavity is taken up by the gut, but there may also be large silk glands, and special
glands which secrete distasteful or toxic substances. The developing wings are present in
later stage instars and the gonads start development in the egg stage
Butterflies spin the button of silk to
which the chrysalis will attach. Most butterflies have 5-6 instars.
| Vocabulary |
| Apolysis |
The process where the cuticle separates from the epidermis
when a larva is molting. |
| Chitin |
A fibrous substance forming the major constituent in the exoskeleton
of butterflies. |
| Endocrine Glands |
A gland that secretes a hormone into the bloodstream. |
| Hemolymph |
The transformation stage between the larva and the adult. |
| Instar |
A phase between 2 periods of molting in the development of
the butterfly larval. |
| Prothoracicotropic Hormone |
A hormone acts on the prothoracic glands, which respond by
releasing a molting hormone. |
| Pupation |
To go through metamorphosis and become a pupa. |
Butterflies and Moths of North America
WisconsinButterflies.org
Southern Wisconsin Butterfly Assn (NABA)
The Butterfly Site