Pupa: What It Is, What Happens Inside, and What to Do If You Find One

What is a pupa?

A pupa is the stage in an insect’s life between the larva and the adult, when the insect stops eating and its body is rebuilt. Only insects with complete metamorphosis have one. That covers more than 80% of described insect species, including butterflies, moths, beetles, flies, bees, wasps, ants and fleas.

Most pupae barely move, but they are very much alive. The insect breathes, and inside the hard skin the wings, legs and eyes of the adult take shape. When the work is done, the skin splits and the adult climbs out.

The word comes from the Latin pupa, meaning girl or doll. Linnaeus picked it because many pupae look like a baby wrapped in swaddling clothes. Say it PYOO-puh. The plural is pupae, said PYOO-pee, although “pupas” is also correct.

What is another name for a pupa?

It depends on the insect. A butterfly pupa is called a chrysalis. A mosquito pupa is nicknamed a tumbler, because it tumbles through the water when you disturb it. The hard brown case of a house fly pupa is a puparium. A cocoon, on the other hand, is not another name for a pupa. It is the silk case some pupae lie inside.

Where the pupa fits in an insect’s life cycle

The pupa is the third of four stages: egg, larva, pupa and adult. Insects with this cycle, called complete metamorphosis, look completely different as young and as adults, so they need the pupa to rebuild. About 11 insect orders work this way. Insects such as cockroaches skip the pupa entirely.

The larva is the eating stage. It may be a caterpillar, a maggot or a grub, and it grows by shedding its skin several times. When it is fully grown, it stops eating, finds a safe place and sheds its skin one last time. What is underneath is the pupa.

Insects with incomplete metamorphosis grow up another way. Their young, called nymphs, already look like small wingless adults. They grow a little with each molt, and the last molt makes them adults. There is no resting stage, because there is nothing to rebuild. Young grasshoppers are a good example: each molt brings slightly bigger wing pads, until the last one gives them full wings.

Which insects have a pupa stage?

Eleven orders of insects have a pupa. The ones you are most likely to meet are beetles, flies, butterflies and moths, and bees, wasps and ants. The rest are fleas, lacewings and antlions, caddisflies, scorpionflies, snakeflies, dobsonflies and alderflies, and a small group of parasites called twisted-wing insects.

Do cockroaches and bed bugs have a pupa?

No. Cockroaches, bed bugs, stink bugs, crickets and termites hatch as nymphs and grow straight into adults. So a “roach pupa” does not exist. A small brown capsule near a cockroach is more likely an egg case, which holds many eggs in a row. A brown capsule on its own is often a fly pupa, covered below. Silverfish skip metamorphosis altogether.

Thrips and whiteflies are the odd ones out. They have resting stages that some guides call pupae, and thrips even have a “prepupa”. However, UC IPM notes that neither has a true pupal stage, and young whiteflies keep feeding the whole time.

Larva vs pupa: what is the difference?

A larva eats and moves, and a pupa does neither. The larva comes first. It is the caterpillar, maggot or grub that feeds and grows. The pupa comes next. It is the closed, still stage in which the larva’s body becomes an adult’s. So a maggot is not a pupa, but it turns into one.

What happens inside a pupa?

Inside the pupal skin, the insect digests much of its larval body and builds an adult from small groups of cells that were waiting all along. In a monarch this takes about 8 to 15 days in summer. Hormones set the timing: when juvenile hormone runs low, the next molt makes a pupa instead of a bigger larva.

Those waiting cells are called imaginal discs. A caterpillar carries a set for each adult part, such as the wings, legs, eyes and antennae. While the caterpillar feeds, they stay small. Once it pupates, enzymes break down most of its tissues, and the discs use that material to grow.

Does a caterpillar really turn into soup?

Only partly. You may have read that the caterpillar dissolves into a liquid soup. Ask Dr. Universe, a Washington State University science site, says it turns “into a liquid, soupy substance”. It is a good story, but not quite true.

Some parts are kept and remodeled. They include the gut, the breathing tubes and parts of the nervous system. In 2013, researchers used CT scans to look inside living painted lady pupae (Lowe and colleagues, Journal of the Royal Society Interface). They could see the breathing tubes about 12 hours after the pupa formed, and they found less rebuilding than expected.

Is a pupa alive?

Yes. It breathes through small holes along its sides called spiracles, and it burns stored fat while it changes. Many pupae also twitch or wiggle when touched, because they keep some of the larva’s belly muscles. It does not eat or drink, though. As a result, it lives on whatever the larva stored up.

Do insects remember being larvae?

At least one moth does. In 2008, Blackiston and colleagues trained tobacco hornworm caterpillars to avoid a smell by pairing it with a mild shock (PLoS ONE). The moths that grew from caterpillars trained late in life still avoided the smell. Caterpillars trained when younger did not keep the lesson. So some memory can pass through the pupal stage.

Pupa vs chrysalis vs cocoon

Four words get mixed up here. A pupa is the insect itself at this stage of life, and a chrysalis is simply a butterfly’s pupa. By contrast, a cocoon is a silk case that a larva spins around itself before it pupates. Last, a puparium is a fly’s old larval skin, hardened into a case.

What is a chrysalis?

A chrysalis is a butterfly pupa. The caterpillar spins a small pad of silk, hooks its tail end into it and sheds its skin one last time. Underneath, a new skin hardens into the chrysalis. Its tail hook is called the cremaster. Swallowtails also loop a thin silk belt around the middle to hold themselves upright.

A chrysalis is not a case the caterpillar builds. It is the pupa’s own skin, which is why you can sometimes see the shape of the wings and antennae on it. The word comes from the Greek chrysos, gold, after the gold dots on chrysalises like the monarch’s. For more than one, say chrysalises or chrysalides.

What is a cocoon?

A cocoon is a case of silk that a larva spins around itself before it pupates. The silk comes from glands near its mouth, and many larvae weave in leaves, soil, wood or their own hairs. The pupa forms inside and stays there. Most moths spin one. Fleas, caddisflies and many wasps make cocoons too.

A silkworm’s cocoon holds a single thread of 600 to 1,500 meters (about 2,000 to 4,900 feet), and that thread becomes silk cloth. Not every moth makes a cocoon, though. Hornworm moths, for example, dig into the soil and pupate bare.

Do any butterflies make cocoons?

A few do. Most sources say only moths make cocoons, but apollo butterflies (Parnassius) are the exception. They pupate inside a silk cocoon, as McCorkle and Hammond reported in 1985 (Journal of the Lepidopterists’ Society). Skippers also tie leaves together with silk and pupate in that shelter. So “a cocoon means a moth” is a good rule, but not a law.

What is a puparium?

A puparium is the hard brown case around the pupa of a house fly or blow fly. When the maggot is grown, its last skin shrinks into a barrel shape and hardens. The true pupa then forms inside. That is why fly pupae look like smooth brown capsules with no legs or wings showing at all.

The three types of pupae

Entomologists sort pupae into three types by how the legs and wings sit. Obtect pupae have them glued flat to the body. Exarate pupae have them free, like a pale, folded-up adult. Coarctate pupae are exarate pupae hidden inside a puparium. Most pupae are exarate.

There is a second, rarer split, by the jaws. Most pupae cannot move their jaws, but caddisflies and a few others can. They use them to cut their way out of the case, and some caddisfly pupae then swim to the surface before the adult comes out.

What does a pupa look like?

Most pupae are oval, smooth and brown, green or cream, from a few millimeters to over 2 inches (50 mm) long. Fresh pupae start pale and darken within hours. Butterfly pupae are often green or brown to match their surroundings, while moth and fly pupae are usually brown and shiny.

Sizes are typical, not exact.

Can you name an insect from its pupa alone? Sometimes. A gold-dotted green chrysalis is almost always a monarch, and a tomato hornworm pupa’s handle is hard to miss. Most brown moth pupae look alike, however, and even experts struggle with them. If you are not sure, keep it and see what comes out, or try our free photo bug identifier.

Butterfly pupa

A butterfly pupa, or chrysalis, is usually smooth and hangs in the open. A monarch chrysalis is jade green with gold dots and about 1 inch (25 mm) long. Black swallowtail pupae are green or brown depending on the surface. In one study, 97% were brown on a rough red surface, but only 2% on rough green.

Moth pupa

A moth pupa is usually a hard, shiny, brown shell, either bare in the soil or inside a cocoon. Hornworm pupae have a loop that holds the moth’s long tongue. Giant silk moths, such as the cecropia and polyphemus, spin large cocoons on twigs or among dead leaves and stay in them all winter.

Beetle pupa

A beetle pupa looks like a pale, soft adult with its legs, antennae and wing cases folded against the body. Most form in soil, wood or stored food. Some, including mealworm pupae, can snap the gaps between their belly segments shut. These “gin traps” pinch small attackers such as ants.

Fly pupa

A fly pupa looks like a small brown capsule with rounded ends. A house fly’s is about 1/3 inch (8 mm) long, cream when new and dark reddish brown later. Maggots crawl away from their food to pupate, so you often find them a few feet from the source. Gnat and drain fly pupae are far smaller and hide in soil or slime.

Mosquito pupa

A mosquito pupa is a dark, comma-shaped swimmer that hangs at the water’s surface and breathes through two short tubes on its back. When you disturb it, it dives in a tumbling jerk, which is why people call it a tumbler. It does not eat, and it turns into a mosquito in about 2 to 3 days in warm weather.

Ant, bee and wasp pupae

These pupae look like pale adults, folded up. Bees and wasps keep theirs in wax or paper cells. Ants carry theirs around the nest, and the white “ant eggs” you see when you lift a stone are mostly pupae. Some ants wrap them in tan silk cocoons, while fire ant pupae are bare.

How long does the pupa stage last?

For most insects in warm weather, the pupa stage lasts about 3 days to 3 weeks. House flies take 2 to 6 days, monarchs 8 to 15 days, and many moths 2 to 4 weeks. Cold slows every pupa down, and insects that spend the winter as pupae may wait 8 or 9 months.

Times are typical at room or summer temperatures.

Pupae that wait out the winter

Many insects get through winter as pupae, in a pause called diapause. In that state, development nearly stops until warmth or longer days restart it. Late-summer swallowtail chrysalises, cecropia moth cocoons and hornworm pupae in the soil all wait until spring this way. So a chrysalis from August that has not hatched by October may simply be waiting, not dead.

The 30-year “pupa” that is not one

You may read that some pupae can wait 30 years. The record is real, but the stage is not a pupa. In 2001, Jerry Powell reported yucca moths that came out after 30 years in a resting state (Annals of the Entomological Society of America). They had spent those years as fully grown larvae, sealed in hard cysts. They pupated only at the end.

Found a pupa? What it is and what to do

Where you found it usually tells you what it is. Brown capsules about 1/3 inch (8 mm) long on a floor are almost always fly pupae. Silk cocoons on a ceiling are often pantry moths, and a shiny brown pupa in garden soil is usually a moth. Most need nothing more than a broom and a clean-up.

Each of the indoor ones has its own guide: clothes moths, carpet beetles, phorid flies and fungus gnats.

Brown capsules on the floor: fly pupae

Small, reddish-brown capsules about 1/3 inch (8 mm) long, scattered on a floor, are fly pupae. Maggots leave their food and crawl to a drier spot to pupate, so the source is usually a few feet away: a trash can, pet food or a dead mouse in a wall. Each full case will release one fly, so sweep them all up and find the source. Our guide to fly pupae covers how to tell them from mouse droppings and roach egg cases, which fly made them, how soon they hatch, and the step-by-step clean-up.

Cocoons on the ceiling or walls

Small silk cocoons where the ceiling meets the wall, or along shelf edges, are usually Indianmeal moths. Their larvae live in flour, cereal, nuts, birdseed and pet food. When they are grown, they leave the food and climb, so the cocoons can be far from the source. Find the infested package and throw it out. Then clean the shelves and store dry food in sealed containers.

A pupa in your garden soil

A shiny brown pupa you dig up in spring or fall is usually a moth, often a cutworm or a hornworm. It cannot hurt you or your plants while it rests. If you grow tomatoes and want fewer hornworms, crush it, since tilling destroys up to 90% of them. Otherwise, rebury it a few inches deep.

Some gardeners keep a pupa in a jar of slightly damp soil to see what hatches. Give the adult a stick to climb, so it can hang and spread its wings. Expect a wait: a pupa dug up in fall may not hatch until late spring.

Pupae worth leaving alone

Several pupae turn into insects that protect your garden, so they are worth leaving alone. Lady beetle pupae are orange and black and fixed to leaves, like the pupa of the orange ladybug. Hover fly pupae are tan teardrops about 1/2 inch (13 mm) long. Lacewing cocoons are small white silk balls on leaves. All three become predators of aphids. On a hornworm, white cocoons like grains of rice belong to a tiny wasp, so leave that caterpillar on the plant.

Can a pupa hurt you?

No. A pupa has no working mouth or stinger, and it does not eat. The worst it can do is wiggle in your hand. A few cocoons are a different story, though, because the caterpillar wove its hairs into the silk. Browntail moth, tussock moth and puss caterpillar cocoons can all leave an itchy rash.

Is my pupa or chrysalis dead?

A pupa that wiggles when you touch it is alive. Some never move, though, so stillness alone proves nothing. Keepers also look for one that stays firm and keeps its color. A monarch chrysalis that turns dark all over is usually about to hatch, and you will see the orange wings through it the day before.

The Monarch Larva Monitoring Project lists the warning signs. White, silk-like threads hanging from a chrysalis mean a parasitic fly has left it. Small black dots can mean a tiny wasp is inside. A chrysalis that turns black without clearing, or leaks, is usually diseased. Remove it. Monarch Health, a University of Georgia project, advises cleaning the container with a 20% bleach solution before you reuse it.

Butterfly keepers sometimes re-hang a chrysalis that falls, tying dental floss to the silk just above the cremaster, never to the cremaster itself. Wait until the chrysalis has hardened before you handle it, because a fresh one is soft. If you raise monarchs, the project advises raising a few at a time, one per container, and releasing them close to where you found them.

Who to call

Still not sure what you found? Our six checks for naming any bug work on adults, and the insect identifier takes a photo of almost anything.

How the adult gets out

The adult splits the pupal skin along a seam, pulls itself free and hangs to dry, which takes several hours for a butterfly. This is called eclosion. A butterfly pumps fluid into its crumpled wings to stretch them, then lets them harden before it can fly.

Insects in cases need a trick to get out. Silk moths use an enzyme to loosen one end of the cocoon. A fly inflates a balloon-like sac on its forehead to push the cap off its puparium.

Soon after it comes out, a butterfly or moth drips a red or orange fluid. This is meconium, the waste left over from the pupa. In tortoiseshell butterflies and their relatives it is bright red. Arthur Shapiro wrote in 1981 that it once caused “superstitious terror” among European peasants, who feared a rain of blood. It is waste, not blood.

Pupa myths

Seven myths about pupae turn up again and again, in picture books, cartoons and even some science sites. Each one is listed below with what is actually true. The biggest is that a pupa is an empty shell, when it is really the living insect.

• Myth 1

  Myth: “A pupa is just a shell.”

  Truth: It is the living insect. It breathes, often moves, and is building an adult inside.

• Myth 2

  Myth: “The caterpillar turns completely to soup.”

  Truth: Most tissue breaks down, but the imaginal discs, the gut, the breathing tubes and parts of the nervous system carry through.

• Myth 3

  Myth: “Chrysalis and cocoon mean the same thing.”

  Truth: A chrysalis is a butterfly's pupa. A cocoon is a silk case around a pupa, usually a moth's.

• Myth 4

  Myth: “Only moths make cocoons.”

  Truth: Apollo butterflies pupate in silk cocoons, and fleas, caddisflies and many wasps make them too.

• Myth 5

  Myth: “Cockroaches have a pupa.”

  Truth: Cockroaches, bed bugs and crickets grow from nymphs straight into adults. There is no pupa.

• Myth 6

  Myth: “Mosquito dunks kill every stage in the water.”

  Truth: Bti, the bacteria in mosquito dunks, only works on larvae that eat it. Washington State's Department of Health says it “does not affect mosquito pupae or adults”.

• Myth 7

  Myth: “A pupa can live 30 years.”

  Truth: The record belongs to yucca moth larvae resting in cysts. The true pupal stage is short.

Pest control disclaimer. Photo-based identification can help, but it is not always perfect. For dangerous spiders, large infestations, or anything you are unsure about, contact a licensed pest control professional.