Across the North American continent, the massive southward migration of the Monarch Butterfly is an amazing spectacle to witness. In the waning days of summer, within days of emerging from their chrysalises, the fluttering insects embark on their long journey - a staggering 4,000 kilometers (2,500 miles). Unlike the other arthropods in their summertime home ranges, Monarch Butterflies cannot withstand the frigid winter temperatures that loom on the horizon. With no experiential memories to act on, they are guided by an incredible instinct, following the same flight paths to the very same trees that their ancestors have used for countless generations.
From the rolling Appalachians to the Great Plains to the majestic Rockies to the California coastline, roaming clouds of orange and black drift toward their annual hibernation locations. Their flight appears effortless, as if they are floating on air, their wings extended into a V-formation during long periods of gliding. By the end of October, the migration is over at last. Populations to the east arrive at the Mariposa Monarca Biosphere Reserve, high in the mountainous regions of central Mexico, to settle on the local oyamel fir trees. Those west of the Rocky Mountains colonize conifer groves and eucalyptus trees along the southern California coastline in select spots like Pacific Grove and Santa Cruz.
For the next six months, the Monarch Butterflies thrive in these sanctuaries, gathering in such abundant clusters that they can completely obstruct any sight of a tree branch or trunk. When this happens, the scene resembles an artistic masterpiece as much as it does a natural phenomenon. Thick black veins and margins spread throughout otherwise orange or tawny wings, producing a pattern that evokes images of intricate stained glass designs. Small white spots speckle the bordering black margins, as do a few flecks of orange on the forewing tips.
While females tout darker veins, males display an androconium spot in the upper rear of each wing, which functions both as a source for pheromone release as well as a device for defense against predatory mammals and birds, who often mistake the sport for a large, intimidating eye. Yet, the androconium spot is not the only warning sign that keeps hungry threats at bay. The Monarch Butterflies' brightly colored wings serve as an aposematism, sending a clear, direct message to all potential predators: stay away, or risk ingesting a vile dose of poison.
The Monarch Butterfly feeds on a diverse array of nectar plants, but none more so than the milkweeds that fall into the Asclepiadaceae family - rich in numerous nutrients and sugars, notably cardiac glycosides. These sugary compounds, contained in particularly high concentrations within swamp milkweed, are harmless to the insect, but terrible tasting or toxic to most species of mammals and birds. Through trial and error, predators learned to avoid eating the Monarch Butterfly.
This dietary defense mechanism has ensured the species an extended existence, but it is not entirely foolproof. In Mexico, black-headed grosbeaks have discovered their immunity to the cardiac glycoside toxins and devour the butterflies in massive quantities; as much as 14% of the hibernating population can be consumed during a single winter. To the north, several species of small birds - including orioles, jays, and robins - have evolutionarily outsmarted the insects, learning to only eat the less poisonous thoracic muscles and abdominal contents. Combined with hungry mice and parasitic insects, the Monarch Butterfly has its fair share of adversity to overcome, and that does not even include the gravest threat of all: humans.
In recent years, as humans have inflicted widespread habitat destruction upon the butterfly's annual hibernation sites, Danaus plexippus has become a threatened species. While deforestation devastates wide expanses of vital habitat, the Monarch Butterfly's ultimate undoing may be its own inflexible instincts, so deeply ingrained that is impossible to consider the species will suddenly adapt and learn to seek out new overwinter grounds each year. Thus, it is man's responsibility to conserve this creature, by protecting the sacred habitats it so desperately depends on for survival.
With warmer temperatures comes the end of hibernation and the beginning of the journey back north. Before departing, however, the Monarch Butterflies must attend to one last order of business: mating. Courtship commences with an aerial pursuit, as the male flies after the female, prodding and pushing until he has pinned her to the forest floor below. On the ground, the two insects remain attached for upwards to an hour during copulation. In addition to fulfilling its primary purpose - fertilizing eggs - the male's spermatophore also supplies the female with essential nutrients that will sustain her energy throughout the long remigration ahead.
Immediately upon their return north, females seek out milkweed plants on which to lay their eggs, and die soon after. They have lived long and fruitful lives, some lasting as long as eight months - much longer than their offspring will ever live. When their eggs hatch days later, the hungry caterpillars that emerge do not need parenting, anyways. These yellow, white, and black-banded critters - measuring a mere 5 centimeters (2 inches) long - devote their existence almost entirely to eating milkweed. Consuming as many fats and nutrients as possible is crucial for the next stage of life.
After two weeks, the caterpillars hang upside down, attaching their last pair of prolegs to a silk pad they spin on the underside of twig or leaf, and begin to envelop themselves in a chrysalis. Once complete, the caterpillar has disappeared into a smooth turquoise encasement. Over the next ten days, the only visible changes are external, as the chrysalis darkens in color. Inside, however, a tremendous transformation is transpiring. When the chrysalis finally opens up, it reveals a fully developed Monarch Butterfly.
At first, the metamorphosed insect remains sedentary, waiting for its wings to fully stiffen and dry. After a few hours, the Monarch Butterfly takes off for its first flight. Unlike its parent, this butterfly will only live for about 2 to 8 weeks. It will never endure a grueling migration, and never travel far from the meadow, prairie, or park where it was born. It will fly, float, feed on milkweed, and reproduce. It is all part of the an annual cycle. Every year, Monarch Butterflies cycle through four generation: the first three enjoy short lives during the spring and summer - only the fourth embarks on the autumn migration.
A male and female Monarch Butterfly fly near the branches of a conifer Abies religiosa in Oyamel Forest of the Sierra Madre Occidentale of Mexico. (The wing venation is darker and wider in the female.) Monarchs migrate long distances over the United States to reach these refugia in Mexico. Those that reach these endangered forests mate and then die, never to return to their northern homes.
5 centimeters (2 inches)
9-10 centimeters (3.5-4 inches)
2-8 weeks after chrysalis during summer 6-8 months after chrysalis during winter
Fields, prairies, meadows, and urban and suburban parks during summer Conifer groves and forests during winter
North America, Australia, New Zealand, western Europe, and select Caribbean and Pacific islands
Milkweed, nectar plants
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Monarchs are the only butterflies to migrate both north and south in the same fashion that birds do on an annual basis.
Less than 1% of the global Monarch Butterfly population displays a grayish-white hue on its wings in lieu of orange. Yet, likely as a result of selective predation on the Hawaiian island of Oahu, this physical variation can appear in as many as 10% of individuals.
Through a process known as puddling, male Monarch Butterflies will absorb nutrient-rich minerals and moisture by sitting in damp soil or wet gravel.
The Monarch is the state insect in Alabama, Idaho, Illinois, Minnesota, and Texas. It is also the state butterfly in Vermont and West Virginia.
In 1990, it was proposed that either the Monarch Butterfly or honeybee be designated as the national insect of the United States of America, but legislation did not pass.
The Monarch earned its nickname from Samuel H. Scudder, who first dubbed the species in 1874 by writing: "It is one of the largest of our butterflies, and rules a vast domain."
Indigenous to North America, the Monarch Butterfly's impressive stamina, combined with the right wind conditions, has allowed the species to cross the vast bodies of water - including the Atlantic Ocean - and populate several areas in western Europe and the Caribbean.