How Does a Caterpillar Turn Into a Butterfly? Metamorphosis

What is complete metamorphosis?

Butterflies and moths develop in four distinct stages: egg, larva (the caterpillar), pupa (the chrysalis) and adult. This is called complete metamorphosis, and it is shared by beetles, flies, bees, wasps and ants. The young and the adults look nothing alike and usually eat different food: caterpillars chew leaves, while adult butterflies sip liquids.

Insects such as grasshoppers, dragonflies and true bugs follow incomplete metamorphosis instead. Their young, called nymphs, look like small wingless adults and change gradually with each molt, with no pupa. Frogs go through their own dramatic metamorphosis, described in how do tadpoles turn into frogs?

The monarch butterfly is the best-studied example, so it is used for the timings below.

Stage 1: the egg

A female monarch lays each egg singly, usually on the underside of a young milkweed leaf, gluing it in place. The egg is cream colored, ridged and only about 1.2 mm (0.05 in) long. Inside, the embryo develops for about 3–8 days, depending on temperature.

Just before hatching, the dark head of the tiny caterpillar can be seen through the shell. The newly hatched caterpillar usually eats its own eggshell as its first meal.

Stage 2: the caterpillar eats and molts

A caterpillar is an eating machine. A monarch caterpillar hatches at about 2–6 mm (0.1–0.2 in) long and grows to about 4.5–5 cm (1.8–2 in) in roughly two weeks, ending up thousands of times heavier than when it hatched.

Its outer skin, the exoskeleton, cannot stretch indefinitely, so the caterpillar must molt. It grows a new, larger skin underneath and wriggles out of the old one. The stages between molts are called instars, and a monarch has five.

Two hormones control the process. Ecdysone triggers each molt, while juvenile hormone decides what kind of molt it will be. As long as juvenile hormone levels are high, each molt produces a bigger caterpillar. In the final instar, juvenile hormone levels drop, and the next pulse of ecdysone switches the caterpillar toward becoming a pupa.

Meanwhile, the future adult is quietly taking shape. The imaginal discs, small folded pockets of cells set aside inside the caterpillar, begin to grow quickly during the last instar. The wing discs in particular expand, although they stay hidden under the skin.

Stage 3: inside the chrysalis

When it is fully grown, a monarch caterpillar stops eating and often wanders away from its milkweed plant. It spins a pad of silk on a sheltered surface, grips it with its rear legs and hangs head-down in a “J” shape for about 12–16 hours. Then its skin splits behind the head and is wriggled off, revealing a soft green chrysalis. A hook at the tail end, called the cremaster, anchors the chrysalis in the silk pad. Within hours it hardens into the familiar jade case with gold dots.

Inside, one of the most dramatic transformations in the animal world begins. Enzymes break down many caterpillar tissues, including most of the old muscles and the leaf-chewing mouthparts. The material released helps feed the rapidly growing imaginal discs, which build:

• four wings, with their scales and color patterns
• six long, jointed legs
• large compound eyes and long antennae
• the coiled proboscis used to drink nectar
• flight muscles and reproductive organs

Not everything starts from scratch. The nervous system, the breathing tubes (tracheae), the heart and parts of the gut are reorganized rather than destroyed. In a monarch, the chrysalis stage usually lasts about 8–15 days in summer. A day or so before emergence, the case turns clear and the orange and black wings show through.

Stage 4: the adult emerges

The butterfly splits the chrysalis open and climbs out with small, crumpled wings and a swollen abdomen. It hangs from the empty case and pumps body fluid into the veins of its wings, which expand to full size. The wings then dry and stiffen over the next couple of hours or so.

The newly emerged butterfly also has to assemble its tongue: the proboscis forms as two halves that the butterfly zips together by curling and uncurling it. It also expels a reddish fluid, called meconium, the waste left over from life in the chrysalis. Within a few hours it is ready for its first flight. Adult butterflies never grow; their size is fixed when they emerge.

How long does each stage take?

Timings depend strongly on temperature. These are typical figures for monarchs in summer:

Other butterflies vary. The painted lady completes egg to adult in about four to six weeks, which is why it is a favorite in classrooms, while species that spend the winter as a chrysalis may stay in that stage for many months. For lifespans across species, see how long do butterflies live?

Myths and surprising facts

Myth: The caterpillar completely dissolves into soup. Fact: A lot of tissue is broken down, but key organs are remodeled, and the adult’s parts grow from imaginal discs that were there all along.

Myth: Butterflies come out of cocoons. Fact: A butterfly pupa is a chrysalis, a hard case formed from the insect’s own skin. Cocoons are silk cases spun mainly by moth caterpillars. More differences are covered in butterfly vs moth.

Fact: Some memories may survive. In a 2008 study, tobacco hornworm caterpillars, the young of a large moth, were trained to avoid a particular smell. Those trained in the final caterpillar stages still avoided it as adult moths, suggesting that parts of the nervous system carry information through metamorphosis.