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Monarch Migration

The monarch butterfly (Danaus plexippus), long appreciated for its beauty, has gained additional recognition in recent years for its startling migratory abilities. Previously, the southward movement each fall of these bright orange and black butterflies had been observed, but their winter destination remained a mystery. The mystery was finally solved with the discovery of overwintering grounds 9,000 feet up in the Sierra Madre mountains of Mexico. Like many insects, monarch butterflies undergo a four-stage metamorphosis, evolving from egg to larval, pupal, and finally adult stages. The larvae have voracious appetites and literally increase their weight 3,000-fold in only 14 days' time.

During this larval stage, the monarch caterpillar feeds exclusively on various species of the milkweed plant. This plant contains an organic compound that is poisonous to birds and causes a violent reaction if ingested. One such distasteful experience is enough to discourage a bird from ever again pursuing a monarch as a meal.

A monarch's wings are covered with tiny, colorful, overlapping scales that are the "dust" that comes off on the hands when holding a butterfly too firmly. The wing patterns formed by these scales define a monarch's appearance and give the insect its beauty. But of even greater significance, the scales help predatory birds quickly recognize the insect and serve as a warning that this is one butterfly to avoid.

Anywhere from three to five generations of monarchs may be produced during a single summer season. The last generation of a given season will not reach sexual maturity, and this generation will follow a powerful instinct to migrate southward, sometimes flying more than 2,500 miles to reach the Mexico wintering site. Apparently, a dormant period in the relatively mild winter of this mountainous region is required for the butterflies to become sexually mature. When spring arrives, the insects then begin the northward return journey, mating and producing offspring en route. It is one of these newer generations that actually reaches the southern Appalachians each summer. It remains both a mystery and a miracle as to how the season's final generation makes the southern migration without ever having been to Mexico.

Much of the recent information on monarch migration has been gained through the pioneering efforts of Fred and Nora Urquhart of the University of Toronto. They developed miniature lightweight tags, attached them to the butterflies' wings, and have been compiling migration data for many years. In Western North Carolina, zoologist Dr. Hal Mahan and his wife, Laura, a botanist, have cooperated in this research effort, tagging hundreds of monarchs over the years, many of which have been recovered in Mexico. The mountains of North Carolina lie directly along the migration route, making the area perfect for monarch tagging. Such efforts have helped unlock some of the secrets of the monarch and provided insight into other species as well.

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