Butterflies belong to the insects, and a butterfly grows in four steps: egg, caterpillar, chrysalis, and adult butterfly, a pattern known as complete metamorphosis, which means the young look completely different from the adults. The big change from caterpillar to butterfly is called metamorphosis. A mother butterfly lays her eggs on the kind of plant her caterpillars will eat, and a female monarch lays her eggs one at a time, on milkweed leaves, while a female butterfly drums a leaf with her feet to taste whether it is the right plant for her caterpillars. A caterpillar spends its time eating and growing, and a monarch caterpillar eats and grows for about two weeks before it makes a chrysalis. It grows from a few millimeters long to about two inches (5 cm), going through five stages, called instars, and shedding its skin between them, a process called molting. Because adult insects do not grow, a butterfly's adult size is set by when its caterpillar stops growing and starts to change. Hormones, not the caterpillar's choice, steer growth and change, and juvenile hormone keeps a young insect from starting metamorphosis too soon.
The change itself is stranger than the popular picture of a caterpillar growing wings. Many caterpillars hang upside down from a button of silk just before they become a chrysalis, and a caterpillar fastens itself with silk, either a pad or a girdle, to a stem or twig before it does so. Inside the chrysalis, the caterpillar's body breaks down and is rebuilt as a butterfly: groups of cells called imaginal discs survive inside the chrysalis and use the broken-down caterpillar as fuel to build the wings, eyes, antennae and legs of the adult. Not everything in the caterpillar dissolves, however, since some muscles, parts of the nervous system, the breathing tubes and the gut carry on into the adult. A monarch stays inside its chrysalis for about two weeks; the monarch caterpillar, in particular, hangs from a twig and turns into a chrysalis, and this is the stage at which the whole body plan is rebuilt.
What comes out has a body plan that is easy to describe and hard to rival. A butterfly's body has three parts: the head, the middle (thorax) and the end (abdomen), and a butterfly has three pairs of legs, six legs in all, and four wings, two in front and two in back. Its wings are covered with tiny scales, and some butterfly colors come from tiny structures in the scales that bend and reflect light, and not from pigment, as in the blue of a Blue Morpho. Taken together, these features make the adult a very different animal from the caterpillar, which is just what the term complete metamorphosis describes, and the difference can be checked simply by counting parts: three body sections, six legs and four wings.
The adult senses the world in ways that differ from ours; a butterfly's large compound eyes let it see in almost every direction at once, and butterflies can sense polarized light, a pattern in skylight that people cannot see. A butterfly has two feelers called antennae, and most have a little club at the tip. It drinks with a long tongue called a proboscis, which curls up like a spiral when the butterfly is not drinking. A butterfly finds sugary food by touching it with its feet, which can sense dissolved sugar. The line between butterflies and moths is fuzzier than the usual rule suggests; butterflies fly in the day, and most moths fly at night, but some moths fly by day and some are brightly colored. A butterfly pupa is called a chrysalis, while a cocoon is the silk case a moth caterpillar spins. Scientists have named about 18,000 to 20,000 kinds of butterflies, as of November 2026, and there are many more kinds of moths.
Defense is a matter of chemistry and imitation. A monarch's bright colors warn birds that it tastes bad, and the reason is the plant it eats: milkweed has poisons called cardenolides, and monarch caterpillars store them and use them as a defense against predators. The poison stays in the monarch's body and makes it taste bad; the viceroy butterfly copies the orange-and-black look of bad-tasting butterflies like the monarch and queen. Scientists once thought the viceroy was only a harmless copy, but a study found that viceroys in northern Florida, where queens are rare, have become bad-tasting too. Seen together, these examples show two separate defenses, one chemical and one visual, and the viceroy case shows that scientists revise their explanations when a study finds something new, which is how an older idea about harmless copying came to be replaced.
Monarchs are also known for a long seasonal journey, and the facts about it start with where the monarchs live. North American monarchs form two groups, an eastern group and a western group, divided by the Rocky Mountains, and the eastern group is much bigger. Monarchs east of the Rocky Mountains spend winter in Mexico, and some fly as far as 3,000 miles to reach their winter home, while monarchs west of the Rockies spend winter in California. Monarchs begin the trip south when shorter days, cooler weather and aging milkweed tell them that winter is coming. The longest one-day flight of a tagged eastern monarch was 265 miles, as of November 2026. Because each of these facts concerns a different stage of the trip, the journey is best read as a sequence: first the cues that start it, then the route, and then the distances that individual butterflies manage to cover in a single day.
How does an insect that has never been to Mexico find it? The monarchs that fly to Mexico have never been there before, yet they find the way, because a monarch's sense of direction for the trip south is inherited, not learned, because each migrant is on its first trip. Monarchs use the sun to stay on course; the sun moves across the sky but the monarch's goal stays put, so monarchs use an inner clock to correct their sun compass for the time of day, and the timing clocks are in its antennae, not only in its brain. Monarchs may use polarized light with the sun to steer, and flight-simulator tests show that migrating monarchs have a magnetic compass that senses the tilt of Earth's magnetic field, probably a backup for days when the sun is hidden. Together, these cues work as a layered navigation system rather than a single instrument.
Winter, and the spring return, work through a pause in growth. Diapause is a long rest in growth or breeding that lets insects get through a hard season, and monarchs that migrate stop breeding in the fall. Monarchs in diapause can live for many months, up to about nine, while summer adults breed for only two to five weeks. In Mexico, monarchs huddle together on oyamel fir trees, and the oyamel fir forests stay cool but above freezing, about 0 to 15 degrees Celsius, so monarchs use little of their fat while they wait out winter. In early spring, monarchs wake from diapause at the winter sites, mate, and then spread north, and they fly north in spring over three to five generations, because each generation lays eggs and then dies. The painted lady butterfly makes a similar trip between Africa and Europe, over several generations, so no single butterfly makes the whole trip.
Counting monarchs is itself a puzzle that shows how scientists measure what they cannot count. Scientists do not count each eastern monarch; they estimate the population by measuring the area of Mexican forest covered by roosting monarchs, in hectares. A hectare of roosting monarchs holds an estimated 21 million butterflies, though the number varies from year to year, as of November 2026. In winter 2023-24, eastern monarchs covered only 0.9 hectares of Mexican forest, less than half the winter before, and in winter 2024-25 they covered 1.79 hectares, nearly double the winter before. In winter 2025-26, eastern monarchs covered 2.93 hectares (about 7.24 acres) of forest, up about 64 percent from the winter before, as of November 2026. Keep in mind that these figures describe forest area first and butterflies only by calculation, so a rise or fall in one winter says less than a trend that lasts for years.
Over the longer run the numbers point downward. Western monarchs have fallen by more than 95 percent since the 1980s, and eastern monarchs have fallen by 80 percent or more since the 1990s, as of November 2026. Monarchs have fewer places to lay eggs because milkweed has been lost from many fields and roadsides, and weed killers kill the milkweed caterpillars need and the wildflowers adults feed on, while insect sprays can kill monarchs. Forests where monarchs spend the winter are shrinking because of cutting of trees, storms and building, and storms and big swings in temperature can kill huge numbers of monarchs, and climate change is making odd weather more common. In December 2024 the U.S. Fish and Wildlife Service proposed to list the monarch as threatened under the Endangered Species Act; as of November 2026 it is a proposal. People can help monarchs by planting milkweed in yards and gardens, and the best milkweed to plant is the kind that grows naturally where you live.
Words to know
- metamorphosis: body-changing growth
- diapause: long growth pause
- proboscis: butterfly's tongue
