The Monarch Butterfly

Egg to Butterfly

The monarch is a medium-sized butterfly with bright orange wings, black veins, and a border of black dotted with white spots. Its wings stretch about nine or ten centimeters from tip to tip, which is roughly the width of a child's hand, and the whole insect weighs only about half a gram, which is lighter than a paperclip. Monarchs live across much of North America, and their scientific name is Danaus plexippus. They are famous for two things in particular: the dramatic way they change shape as they grow, and a migration that is longer than the journey of almost any other insect. Monarchs also live in places such as Hawaii, Australia, and Spain, but the North American ones are the famous long-distance travelers.

Like all butterflies, the monarch goes through complete metamorphosis, which means that it changes its body entirely during its life. There are four stages. First comes the egg, then the larva, which is the caterpillar, then the pupa, which is the chrysalis, and finally the adult butterfly. The caterpillar and the butterfly look so different that someone who had never seen the change would never guess they were the same animal. In summer the whole cycle, from a newly laid egg to a flying adult, takes about a month, so one monarch family can produce several generations in a single warm season. Butterflies and moths together belong to a group of insects called Lepidoptera, a Greek name that means scale wings, because their wings are covered with thousands of tiny overlapping scales.

A female monarch lays her eggs one at a time, almost always on the underside of a milkweed leaf. She can lay several hundred eggs during her life. Each egg is about the size of a pinhead, creamy white, and ridged like a tiny cone. Butterflies have taste sensors on their feet, so the mother checks that a leaf is milkweed by landing on it and tasting it with her legs. This matters a great deal, because milkweed is the only plant that monarch caterpillars can eat. After about four days, a tiny caterpillar chews its way out of the egg. Just before that moment, the egg darkens, because the black head of the caterpillar shows through the thin shell.

Caterpillars and Colors

The first meal for a newly hatched caterpillar is its own eggshell. After that it eats milkweed leaves almost without stopping. In about two weeks it grows from a body about two millimeters long to one about five centimeters long, and its weight increases roughly two thousand times. A caterpillar's skin cannot stretch very much, so it sheds its skin four times, each time wriggling out of a tight old covering into a new one underneath. Its body is striped with yellow, black, and white, and it has two pairs of long black filaments, one near the head and one at the tail, which look like thin antennae. During this time it does very little besides eat, rest, and grow.

Milkweed contains bitter chemicals that are poisonous to many animals. A monarch caterpillar is not harmed by them, and it stores the chemicals in its body, and they stay there even after it becomes a butterfly. Birds that try to eat a monarch quickly learn to avoid it, and the bright orange and black colors act as a warning signal. Birds that eat one often become sick, so they remember the pattern of colors and stay away from it next time. A different butterfly, called the viceroy, looks very much like a monarch, and birds avoid it too. So the colors of these butterflies are not meant to hide them, and in fact they are designed to be noticed.

When it is fully grown, the caterpillar hangs upside down from a twig in the shape of the letter J and sheds its skin one last time. Underneath is a smooth, jade-green chrysalis decorated with a row of tiny gold dots. Inside it, for about ten days, much of the caterpillar's body is broken down and rebuilt into the parts of a butterfly, including wings, long legs, and eyes. Just before the adult emerges, the chrysalis turns clear, and you can see the folded orange wings inside. The butterfly then pushes out, hangs from the empty case, and pumps fluid into its wings to expand them. It takes a few hours for the wings to dry and harden before the first flight.

Wings and the Long Journey

The adult monarch no longer chews anything. It has a long, thin tongue called a proboscis, which stays curled up like a party blower when it is not in use and uncurls to drink nectar from flowers. Monarchs also gather at wet mud to drink, and they smell with their antennae and taste with their feet. A butterfly's tongue is made of two halves that fit together like a zipper to form a single straw, which is how it manages to drink. In summer an adult usually lives only two to six weeks. During that short time it feeds, mates, and lays eggs, and this is why the summer generations of monarchs are so quickly replaced by new ones.

Monarchs cannot survive a freezing winter, so the monarchs that live east of the Rocky Mountains travel south each autumn. Some fly as far as four thousand kilometers from southern Canada and the northern United States to a few mountain forests in central Mexico. They fly in the daytime, often gliding on rising currents of warm air to save energy, and at night they rest together in trees. Along the way they stop to drink nectar from flowers and build up fat, which they burn as fuel on the long trip. A monarch can cover eighty to a hundred and sixty kilometers in a single day. The monarchs west of the Rockies make a shorter trip, spending the winter in groves of trees along the coast of California.

The most surprising fact about this migration is that no single monarch makes the whole round trip. The butterflies that hatch in summer live a few weeks, but the generation born in late summer is different. It lives for about eight months, and it does not mate until spring. Scientists call it the Methuselah generation, after a very old man in the Bible. These butterflies fly south, spend the winter in Mexico, and then fly north in spring, laying eggs in the southern United States before they die. Their children, grandchildren, and great-grandchildren continue north, and it takes about four generations to reach Canada. The trip north is a relay, with each generation flying one part of the way.

The Mountain Forest

That means the monarchs that reach Mexico each autumn have never been there before. Their great-grandparents left that forest the previous spring, and no butterfly alive can teach them the way. The same small group of mountain forests is used year after year, even though the butterflies arrive from places thousands of kilometers apart. Scientists have found that monarchs use the sun as a compass, and they have a kind of clock in their antennae that corrects for the way the sun moves across the sky during the day. There is also evidence that monarchs can sense the magnetic field of the Earth. Together these tools let a creature with a brain smaller than a grain of rice navigate across a whole continent.

The monarchs arrive in the mountain forests of Mexico, which stand about three thousand meters above sea level, at about the beginning of November. That timing is close to the Day of the Dead, a Mexican holiday on November first and second, and many local families have long said that the returning butterflies carry the spirits of loved ones. The monarchs cluster in huge numbers on the branches of fir trees, so thickly that the branches bend under their weight. In the cool air they stay still to save energy, but on a warm morning they take flight in a shower of orange. Because the forest keeps them cool, they burn very little fuel, and they live on the fat stored during the journey until spring arrives.

Monarch numbers have dropped in recent decades, because milkweed has disappeared from many fields and because some of the winter forests have been cut. People are helping in many ways. Gardeners plant native milkweed and nectar flowers, and scientists and volunteers attach tiny, light stickers to the wings of migrating monarchs. When someone finds a tagged butterfly, the sticker's number tells researchers where that monarch began its trip and where it ended. In this way, ordinary families have helped to show how far the butterflies fly and which routes they follow. A small patch of milkweed in a yard can feed several caterpillars, so even a little garden can make a difference, and the stickers do not slow the butterflies down.

Think about what this means. A butterfly lighter than a paperclip flies thousands of kilometers to a mountain forest it has never seen, in the exact region where its great-grandparents began their journey. You can look for monarchs yourself. In late summer and early autumn, watch for orange butterflies feeding on goldenrod and asters in a meadow or a garden, flying steadily southward. In summer, look under milkweed leaves for a pinhead-sized egg, or for a striped caterpillar that is eating as fast as it can. Seeing one is a small chance to witness one of the great journeys of the natural world, right where you live, if you look carefully.