The march of the Monarch

Many of the monarchs we see this time of year in Missouri are part of the massive southward migration of monarchs to their winter sanctuaries in Mexico (track their migration at www.learner.org/jnorth/monarch). Though monarchs produce several broods during the summer and fall months, it is the final fall generation that makes the arduous cross-country trip. This generation will live longer than the others — 7 to 8 months rather than the typical 4 to 5 weeks. These same butterflies will return to the southern U.S. in the spring. Their shorter-lived offspring will continue migrating northward over subsequent generations.

An equally fascinating aspect of the monarch is the transformation that takes place when the caterpillar becomes an adult butterfly. This process, known as metamorphosis, is not unique to monarchs, but is nonetheless a stunning display.

Monarch caterpillars are very tiny when they first hatch from the egg. The adult female monarch butterfly lays its eggs on the underside of milkweed leaves. When the caterpillar hatches, it feeds almost continuously on the milkweed for about two weeks while it grows to a full size of about 2 inches long. When the caterpillar is full grown, it usually leaves the milkweed plant in search of a safe place to pupate.

When the caterpillar is ready to pupate, it attaches itself to a branch or other similar anchor by making a silk-like pad. The caterpillar hangs upside down from this pad by its last pair of legs in a J-shape.

After hanging in a J-shape for about a day, the caterpillar begins to shed its skin for the fifth and last time. Previous sheddings allowed the caterpillar to grow bigger. This time, the caterpillar sheds its skin to reveal a green casing called a chrysalis. As the skin is pushed to the bottom of the body, the cremaster appears. This spiny appendage attaches the chrysalis to the silk pad. This whole process happens in as little as 60 seconds.

The chrysalis is very soft at first, but within an hour it hardens into a protective shell. Inside this casing a dramatic transformation takes place. Wings are formed and chewing mouthparts are replaced with a straw-like tongue, or proboscis, that the adult butterfly uses to sip nectar. Tissue, limbs and organs are all changed during this process. The beginnings of different body parts of the adult butterfly can be recognized in the chrysalis itself. The large, leaf-like flaps at the bottom become the wings, and the upper, ridged, portion defines the thorax.

In about two weeks the chrysalis will start becoming transparent, revealing the orange and black wings of the newly formed butterfly.

Once the monarch has completed its transformation, the chrysalis will suddenly crack open, and the fully formed monarch butterfly will crawl out in as little as 30 seconds. The butterfly’s wings are wet and crumpled when it emerges. The monarch is very vulnerable to predators at this stage, as it cannot fly. It clings to the empty chrysalis and pumps a blood-like substance through its wings to enlarge and strengthen them.

After about an hour, the monarch’s wings are dry and ready for flying. In about a week the Monarch will be mature enough to reproduce and begin the life cycle of the next generation.