2006-08-18 / Columnists

Drawing On Science

by Stephen Yaeger

Monarch Butterfly Migration

You may have noticed many Monarch Butterflies passing through our area. These insects are migrating ... winging their way toward Mexico where they will settle in for the winter. They are among the most recognized of all butterflies being having orange color with a black body and black markings on their wings with white spots.

Monarch butterflies have four large, scale-covered wings and belong to a large group of insects called Lepidoptera (leh-peh-DOP-tuh-ra; lepido= scale, tera= wings). As insects they have six jointed legs; three body parts: a head, thorax and abdomen; two antennae, compound eyes and an exoskeleton . The legs and wings are attached to the thorax muscles which control their movement. The Monarch, as with all butterflies, has a long sucking tube called a proboscis (pro-BO-sis), with which it sucks up the nectar of flowers. As such, they are very good at pollinating flowers. Male Monarchs can be distinguished from females by the scent glands present on each hind wing. They appear as black spots and are actually modified scales.

The life cycle and feeding habits play a dominant role in the insect's migratory habits. It was only a few years ago that the migration patterns of Monarchs was actually revealed. It begins in the northernmost regions of the United States as well as in Canada. Millions of Monarchs begin to group in late August to start their journey south. This journey probably takes place because the insect cannot survive the northern winters. The migratory instinct is most likely prompted by the shortening daylight hours as the months pass. The Monarchs making this journey are reproductively dormant. Their sole purpose is to survive a hazardous flight to their overwintering grounds in the south. Once they reach their winter sites in California and the Transvolcanic Plateau of Mexico, the insects go into a resting state... roosting in the trees for some eight to nine months waiting for winter to end.

At the first sign of spring the reproductive organs of the females become active and mating begins. Then the Monarchs make their way north. It's probable that they do this because the milkweed plant, the dominant food of the insect's caterpillar, does not grow in the regions where they spend the winter. White, spherical, ridged eggs are laid on the milkweed plants that they find along the way; then this generation soon dies out.

When the eggs hatch the larvae (caterpillars) emerge and begin to feed on the milkweed. At a given period in time a caterpillar is ready to metamorphose . It secretes a special liquid inside its body and then passes it out of its mouth. The liquid hardens into silk threads and they are used by the caterpillar to stick to surfaces to form a pupa ( chrysalis ). Once the pupal stage passes, the adult butterfly emerges.

The new adult pumps blood into its wrinkled, folded wings. The wings expand and spread out. When they dry the insect is ready to continue the flight north periodically landing on flowers to feed on the nectar. Mating, egg laying and metamorphosis are repeated at least two more times until the last generation reaches its summer homes. It is this last generation that will eventually make its way back south in the fall to repeat the life cycle/migration of the Monarch. Milkweed is a toxic plant synthesizing a chemical, cardenolide glycoside , which is a cardiac poison absorbed by the Monarch caterpillar and stored in the adult butterfly's system. If a bird, such as a Blue Jay, captures a Monarch it will rip it apart and swallow the pieces. Soon it becomes sick and vomits because of the poison. Any animal that experiences the result of eating a Monarch butterfly will remember the black stripes, orange color and white spots of the insect, keeping it off its menu from then on.

There is another butterfly called the Viceroy which has no natural protection of its own. It closely resembles the Monarch butterfly having similar colors and patterns. This type of adaptive mimicry is called Batesian mimicry . The Viceroy butterfly is smaller than the Monarch and doesn't feed on the milkweed plant, but a prospective predator, which has tried to feast on a Monarch, will not know the difference and will gladly avoid the Viceroy also.


E-mail Steve: Drawingonscience@aol.


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