I want a bumper sticker to describe my current fascination: I brake for milkweeds.

Every time I drive past a patch of common milkweeds I have to fight the urge to stop and peek under all the leaves. My quarry is the striped caterpillars of the monarch butterfly.

These days, Vermont's last few monarch caterpillars are munching leaves and preparing to transform themselves into adult monarchs -- those photogenic black, orange and white butterflies. Mature eastern monarch butterflies, like our Vermont brood, will feed on flower nectar for a few days and then fly 2,000 miles or more to just a dozen overwintering sites west of Mexico City.

These overwintering sites are a precise fit for a monarch butterfly to survive the winter. At about 10,000 feet, the sites are cold enough to temporarily halt the butterfly's reproduction, but not so cold that they will freeze. Butterflies cling by the millions to oyamel fir trees, protected by the tree canopy and warmed by heat rising from the ground at night.

About 250 million butterflies from eastern Canada and the United States will converge on the few high-altitude oyamel forests where their great-great grandmothers spent the winter. Populations west of the Rockies migrate to about 25 spots along the California coast. Another population lives year-round in Florida.

But I'm getting ahead of myself.

Monarch butterfly caterpillars feed exclusively on milkweeds. They've got a lot to choose from -- about 27 species of milkweeds are found, and eaten by monarchs, in North America. Female adults lay one egg per plant, usually on the underside of a leaf near the top of the plant. The eggs take about four days to hatch.

The tiny caterpillar that emerges is barely visible. In a day, it will grow to about one-quarter inch long. They're striped in thin yellow, white and black bands with long, black, droopy antennas on their heads that make them look ... well, goofy.

In the two weeks or so that it will take them to mature to become chrysalises, the caterpillars grow to two inches long and are as thick as a pencil. They amble slowly around their milkweed leaves, nibbling here, gnawing there, until they've grown large enough to form a chrysalis.

After about two weeks of munching, the caterpillar is ready. To make its transformation into a chrysalis, the caterpillar spins a silk mat and hangs upside down. It stabs a black stem, called a cremaster, from its rear end into the mat. It hangs from its bottom with its head curved up toward the sky, in the shape of a big squishy letter ''J.'' Then it sheds its skin for the last time.

This is fascinating to watch. The striped skin splits, revealing a green, maggot-like creature underneath. The now-green caterpillar wriggles mightily to work the skin up to the cremaster, where the skin can drop off to the ground. This effort is like watching someone remove a very tight piece of spandex. After the old skin is shed, the outside skin of the caterpillar hardens into its green chrysalis shape, marked by a row of chrome dots near its top.

The butterfly takes about two weeks to form inside the chrysalis. You can tell the monarch is ready to emerge when the chrysalis appears transparent. What's actually happening is that the chrysalis was always transparent -- the green is the caterpillar, and the change in color at the end is the pigment is finally forming on the butterfly's scales.

After the butterfly emerges, it pumps fluid from its abdomen into its wings. The wings must harden before it can fly. Then the butterfly is ready to travel as far as 90 miles a day to the high-altitude forests in Mexico, a place it has never been.

In the spring, these Vermont butterflies will migrate north from Mexico to the southern Gulf Coast states, where they will lay eggs and die. Their offspring will migrate to the Great Lakes states, lay eggs and die.

These offspring will fly east, and some will find their way to Vermont to lay eggs again, completing a great circle of migration.