What if a caterpillar never became a butterfly?

A caterpillar munches leaves all day long. It gets bigger and bigger.

Then it hangs up and makes a chrysalis. Out comes a butterfly! What if that never happened?

A caterpillar spends its days munching leaves and growing. Then one day it stops eating, hangs upside down and becomes a chrysalis. Some time later, a butterfly climbs out.

But what if the caterpillar never changed? What if it just stayed a caterpillar forever? In this thought experiment you can look inside the big change, see why caterpillars and butterflies have such different jobs, and meet other insects that change too.

Butterflies and moths go through complete metamorphosis: egg, larva, pupa and adult. The larva and adult look, eat and behave so differently that they could be different animals.

This lab asks what would be lost if the larva never became an adult. To answer it, we look at what really happens inside a pupa, at how the life stages divide the work of feeding, traveling and breeding, and at how other insects grow up without a pupa at all.

A butterfly lays a tiny egg on a leaf. A little caterpillar hatches out.

It eats and grows. Then it becomes a chrysalis. Inside, it changes into a butterfly.

Every butterfly starts as a tiny egg laid on a leaf. Out hatches a caterpillar, which eats and eats. The American Museum of Natural History says caterpillars can grow up to 1,000 times heavier than when they hatched.

A monarch caterpillar, for example, starts out less than 1 centimeter long and grows to about 5 centimeters. Then it forms a chrysalis. Inside, its body is completely rebuilt, and an adult butterfly comes out with wings.

The monarch shows the four stages clearly. The U.S. Forest Service notes that a female lays 100 to 300 eggs in her life; they hatch about four days later. The larva grows from under 1 cm to about 5 cm through a series of molts, each stage between molts called an instar.

The chrysalis, the American Museum of Natural History explains, is the last form of the caterpillar's outer skeleton. Inside it the larva breaks down at the level of its cells and reorganizes into an adult. AMNH gives several weeks for this stage in butterflies generally; for the monarch, the Forest Service says it can take as few as five days. The adult's emergence is called eclosion.

Now imagine the caterpillar never makes a chrysalis. It just keeps being a caterpillar.

It has no wings, so it cannot fly. And caterpillars do not lay eggs. So there might be no new baby caterpillars.

In our imaginary world, the caterpillar keeps eating but never forms a chrysalis. It would have no wings, so it could never fly to find flowers or a mate.

That matters, because in butterflies it is the adult that finds a mate and lays eggs. A caterpillar that never grew up might never have young of its own. If every caterpillar stayed a caterpillar, there might soon be no butterflies, no eggs and, in the end, no more caterpillars either.

Stopping metamorphosis would leave the animal in its feeding form for life. In insects with complete metamorphosis, it is the winged adult that is able to reproduce, as a University of Kentucky entomology guide puts it, and adult butterflies spend most of their time looking for a mate.

So the most direct consequence is an idea that follows from the life cycle: a larva that never became an adult could not breed, and the species could not continue. Other effects, such as flowers losing butterfly visitors, are reasonable ideas but would depend on the species.

A caterpillar has strong jaws for chewing leaves.

A butterfly has a long tongue like a straw for sipping nectar. Each body is good at its own job.

A caterpillar and a butterfly do different jobs, so they need different bodies. The caterpillar's job is to eat and grow, so it has chewing mouthparts for leaves. The butterfly's jobs are to travel, find a mate and lay eggs, so it has wings and a long tube for sipping nectar.

Inside the chrysalis, the change is huge. Scientific American explains that the caterpillar's body mostly breaks down, while small groups of cells survive and build the wings, legs, eyes and antennae of the butterfly.

Entomologists describe the larva as a specialized feeding stage, very different from the adult, which may not feed at all; caterpillars chew, while adult moths and butterflies have siphoning mouthparts for nectar (University of Kentucky).

Scientific American describes the rebuild: enzymes dissolve most larval tissue, but imaginal discs, groups of cells formed while the caterpillar was still developing inside its egg, one for each adult structure, survive and divide rapidly using that protein-rich mix. A fruit fly's wing disc can grow from about 50 cells to more than 50,000. Some muscles and parts of the nervous system are largely kept, and one study suggests moths remember things they learned as caterpillars.

Monarch butterflies fly very far. Some fly all the way to Mexico!

They also sip nectar from flowers and help the flowers make seeds.

Wings let butterflies go places caterpillars never could. The U.S. Forest Service says migrating monarchs, each weighing less than a gram, fly 2,000 to 3,000 miles to spend the winter in Mexico. Some of these butterflies live as long as 9 months, while summer monarchs live 2 to 5 weeks.

As they feed on nectar, monarchs also pollinate many kinds of wildflowers, the National Park Service explains. A caterpillar that never changed would never make that journey or visit a single flower.

Metamorphosis gives one species two very different lives. Monarch caterpillars feed on milkweed, and the National Park Service notes that the plant's toxins stay in the monarch even after metamorphosis, so animals that eat one become sick and learn to avoid them. As adults, monarchs drink nectar and pollinate many wildflowers.

The U.S. Forest Service describes up to four summer generations, each traveling a little farther north, and a final generation that does not breed but flies 2,000 to 3,000 miles to overwinter in Mexico, some living as long as 9 months. None of that would be possible for a larva without wings.