Blog post by Tina Winterlik © 2012
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From WikipediaMonarchs 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. The monarch is the only butterfly that migrates both north and south as the birds do on a regular basis. But no single individual makes the entire round trip. Female monarchs deposit eggs for the next generation during these migrations.[22]
By the end of October, the population east of the Rocky Mountains migrates to the sanctuaries of the Mariposa Monarca Biosphere Reserve within the Trans-Mexican Volcanic Belt pine-oak forests 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, Santa Cruz, and Grover Beach. 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 last seven months or more.[8]
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 the position of the sun in the sky[23] and a time-compensated Sun compass that depends upon a circadian clock that is based in their antennae.[24][25]
New research has also shown that Monarch butterflies can use the earth's magnetic field for orientation. The antennae contain cryptochrome, a photoreceptor protein that is sensitive to the violet-blue part of the spectrum. In the presence of violet or blue light it can function as a chemical compass, which tells the animal if it is aligned with the earth's magnetic field, but it is unable to tell the difference between the magnetic North or South. The complete magnetical sense is present in a single antenna.[26][27]
http://en.wikipedia.org/wiki/Monarch_butterfly
