The Monarch (Danaus plexippus) is a milkweed butterfly (subfamily Danainae), in the family Nymphalidae. It is perhaps the best known of all North American butterflies.

Monarchs are especially noted for their lengthy annual migration. In North America they make massive southward migrations starting in August until the first frost. A northward migration takes place in the spring. Female monarchs deposit eggs for the next generation during these migrations. By the end of October, the population east of the Rocky Mountains migrates to the sanctuaries of the Mariposa Monarca Biosphere Reserve in the Mexican states of Michoacán and México. The western population overwinters in various sites in central coastal and southern California, United States, notably in Pacific Grove and Santa Cruz.

Dr. Taylor, who runs the Monarch Watch project at the University of Kansas, used the tags to follow one of the great wonders of the natural world: the annual migration of monarch butterflies between Mexico and the United States and Canada.

The 4,000-mile round trip made by millions of monarchs holds a central mystery that Dr. Taylor and a network of entomologists are trying to solve.

“You want to lay it right on this cell here, the one shaped like a mitten,” the scientist, Orley R. Taylor, told the group, a dozen small-game hunters, average age about 7 and each armed with a net. “If you pinch it for about three seconds, the tag will stay on for the life of the butterfly, which could be as long as nine months.”

The length of these journeys exceeds the normal lifespan of most monarchs, which is less than two months for butterflies born in early summer. The last generation of the summer enters into a non-reproductive phase known as diapause and may live seven months or more. During diapause, butterflies fly to one of many overwintering sites. The generation that overwinters generally does not reproduce until it leaves the overwintering site sometime in February and March. It is thought that the overwinter population of those east of the Rockies may reach as far north as Texas and Oklahoma during the spring migration. It is the second, third and fourth generations that return to their northern locations in the United States and Canada in the spring. How the species manages to return to the same overwintering spots over a gap of several generations is still a subject of research; the flight patterns appear to be inherited, based on a combination of circadian rhythm and the position of the sun in the sky.

Reference

Fly Away Home, By DONALD G. McNEIL Jr. // NY Times, October 3, 2006 http://www.nytimes.com/2006/10/03/science/03butter.html

http://en.wik ipedia.org/wiki/Monarch_(butterfly)

The Long Journey Home on YouTube (Reported by Donald G. McNeil Jr., Produced by Jigar Mehta):
http://www.youtube.com/watch?v=RFoSpaNqqeQ&feature=fv w