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Walking through the woods the other day, I noticed many small cocoons on the trees. You may know that a caterpillar turns into a butterfly after a long sleep in its cocoon. Butterflies are flying insects. There are nearly 160,000 species in the world. In this article, I look at how caterpillars turn into butterflies.

Their life cycle has four stages:

  • Egg
  • Caterpillar (larva)
  • Chrysalis (pupa)
  • Butterfly (imago)

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At each of these stages of their evolution, their size and appearance vary enormously from one species to another. Let us take the example of the swallowtail (or large hairtail).

The female lays her eggs, one by one, on the leaves of various food plants such as carrot or common fennel. They are first green-yellow, then brown, and finally dark blue.

The tiny caterpillars are born after about a week. They are black and measure just 2 mm, but have powerful jaws (mandibles). They spend most of their time eating.

In a few weeks, they reach a size of about 5 cm. During this period, they change their skin several times: this is the phenomenon of molting.

During the last molt, the pale green chrysalis appears. Using a silk thread, it attaches to a plant stalk, such as a carrot. At this stage, the animal no longer feeds.

Inside the chrysalis, it’s the real upheaval. The organs adapt to the future life of the butterfly: the mandibles change into proboscis, and wings grow on the body. The transformation is called metamorphosis.

After about two weeks, the chrysalis is brownish, and the butterfly emerges. They are still all crumpled up and curled upon themselves.

It inflates its wings with air mixed with a liquid within half an hour. They unfold and harden.

The butterfly takes flight. It forages the nectar of wildflowers to feed and looks for a partner to reproduce.

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Coming out of a tiny egg, the caterpillar spends its time eating. Once satisfied, she prepares for her incredible metamorphosis to become an imago, a butterfly. The caterpillars of the moths weave a silk cocoon, in which they will become a chrysalis, the intermediate stage between the caterpillar and the butterfly.

Butterfly caterpillars, for their part, attach their chrysalis directly to a branch or a leaf with a few silk threads that they make. They often take on the color of their support, thus becoming invisible.


In the chrysalis, it is a real upheaval. In one to two weeks, the caterpillar’s organs evolve, giving birth to the nymph, the future butterfly, the brain and the eyes enlarge, the antennae lengthen, the mandibles shrink, and the proboscis appears.

The butterfly will finally see the light of day when the external conditions (sun, humidity) are favorable. This can take up to several months for some species of moth. During this time, the nymph lives on the food reserves made by the caterpillar.

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The emergence is when the imago finally breaks its chrysalis to prepare to take off. It swells its abdomen more and more and, little by little cracks the chrysalis from which it is released in a few minutes. It begins by sticking out the head, the legs, and, lastly, the wings.

All crumpled up, he then needs a few hours of respite, wings down, to finish getting ready. When the butterfly’s wings are very rigid and dry, it can finally take off to discover the vast world, but that’s another story.

The caterpillar will feed on plants and grow by gradually molting. After 4 or 5 molts comes the moment of metamorphosis. It then clings to a leaf or a branch producing silk.

She sheds her skin one last time, revealing the chrysalis, an epidermis that will harden to protect the caterpillar during its transformation.

Some species weave a silk cocoon to add extra protection.

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The Caterpillar Digests Itself 

Inside the chrysalis, the caterpillar will digest itself: its whole body is dissolved by enzymes and reduced to a semi-liquid state.

Left intact are the imaginal discs: made up of stem cells, which will absorb proteins from the remains of the caterpillar and create a whole new body.

The length of the process varies depending on the species, but after about two weeks, the butterfly emerges from its chrysalis.

For the caterpillar to transform into a butterfly, it undergoes an anatomical change (transformation in its shape and body structure) called metamorphosis. The process begins when the insect becomes a pupa, a period in which it does not move and remains within a kind of protection.

The caterpillar-like phase of the animal lasts about 45 days. Pupa time takes up to 15 days. In winter, the tendency is to take longer. This happens because the insect’s metabolism slows down and delays the transformation cycle, affecting the entire process of change.

When the caterpillar is inside the shell, it forms wings, a food device, and the new organs undergo an evolution to take care of the unique needs of the being to come.

All caterpillars must go through this process. However, in some cases, errors can occur, preventing them from turning into butterflies. This is a common situation and happens due to an inadequate genetic reorganization (a common phenomenon when dealing with animals in captivity) when it is about to become a pupa. Finally, the results are insects with physical problems.

To identify whether the butterfly is female or male, observe the variations in its body, called sexual dimorphism (details in the body that differ between the sexes). However, the modifications vary depending on the species, as each has particular characteristics different from others.


All insects of the Holometabola class (those that make a complete metamorphosis) undergo the entire transformation process. Some examples include butterflies, moths, flies, bees, and beetles. 

The butterfly, for example, lays eggs. It is born in the shape of a caterpillar, which turns into a pupa and gives rise to the complete butterfly with wings. The multi-stage process is known as complete metamorphosis, as the insect has gone through all the stages of modification.

However, we also have insects in the Hemimetabolo class in nature (which do incomplete metamorphosis). These pass only through the transformation part, cases of locusts, crickets, cicadas, termites, and cockroaches.

In incomplete metamorphosis, the insect hatches from the egg and is already similar to the adult. After maturity, the appearance gets closer and closer to that of the older and more complete ones.