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The Monarch butterflies of North America, east of the Rocky Mountains, are unique in the insect world. They undertake the longest migratory journey of any insect. There are other Monarchs in other places as well; the North American west coast population also migrates, wintering along the coast of southern California and venturing as far north as southern British Columbia in the summer. In the tropics of Central America, a sedentary Monarch population thrives, reproducing throughout the year. Monarchs have also been introduced, and have established breeding colonies, in Australia, New Zealand, Hawaii and other Pacific Islands, as well as Spain, the Azores, and Bermuda.
But it is the mysterious Monarchs of eastern North America that captures the imagination of nature lovers: why do these butterflies converge each winter at specific sites in the mountains of central Mexico? How do they find their way there? And how do they survive their 5 month semi dormancy in the mountainous forests?
Monarch Biology
The basic life cycle of a Monarch butterfly is no different from other butterflies. They undergo the four typical life stages: egg, larva, pupa, and adult.
Caterpillars are eating machines. A caterpillar’s first meal is the eggshell that once encased it. From there, it proceeds to devour the leaves of its milkweed home and begins to grow at a rapid rate. The milkweed leaves are not just a source of food. The leaves also contain toxins that are stored in the animal’s exoskeleton for the rest of its life, making both caterpillar and butterfly poisonous to most predators.
As the caterpillar grows, it sheds its skin to accommodate its growing body. During the summer, the caterpillar phase can take as long as 9 to 15 days. After 15 days of eating and eating, the caterpillar is more than 3,000 times heavier than the egg it emerged from!
The caterpillar stage ends in a transformation. After attaching itself under a leaf with silken threads, the caterpillar sheds its skin for the final time, and a pupa emerges. This stage, called chrysalis, is when the caterpillar makes its transformation to a butterfly, rearranging its internal organs to metamorphose from a voracious leaf-eater to a nectar-sipping winged beauty.
Ten to fifteen days later, an adult butterfly emerges, still moist, wings crumpled from being compactly folded in the pupa. After several hours of stretching and drying out, the butterfly takes to the air for the first time. A Monarch is as light as air, almost, weighing between one-tenth to one-third the weight of a dime. It will not grow anymore. All the sustenance it derives from nectar will serve as energy for body functions and flight. Only 3 to 8 days after emergence from its pupa, the butterfly is ready to reproduce. The life span of an actively reproducing Monarch butterfly is fleeting, only 2 to 5 weeks.
Summer Monarchs
Next > Monarchs – The Great Migration