Monarch butterfly (Danaus plexippus) is a well-known North American
butterfly. Since the 19th century, it is also found in New Zealand, and
in Australia where it is also known as the Wanderer Butterfly. In Europe
it is resident in the Canary Islands and Madeira, and is found as a migrant
in Mexico, Azores, Portugal and Spain. Its wings feature an easily recognizable
orange and black pattern, with a wingspan of 8.5–12.5 cm (3.34 in–4.92in).
(The Viceroy Butterfly has a similar size, color, and pattern, but can
be distinguished by an extra black stripe across the hindwing.) Female
monarchs have darker veins on their wings, and the males have a spot in
the center of each hindwing from which pheromones are released.
Monarchs are especially noted for their lengthy annual migration. 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 of the Rocky Mountains migrates to the sanctuaries in the
area of Angangueo, Ocampo, Zitácuaro and El Rosario in Michoacán,
Mexico. The western population overwinters in various sites in central
coastal California, United States, notably in Pacific Grove and Santa
Cruz. 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 up to 7 months. 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 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.
Monarch butterflies are one of the few insects capable of making transatlantic
crossings. They are becoming more common in Bermuda due to increased usage
of milkweed as an ornamental plant in flower gardens. Monarch butterflies
born in Bermuda remain year round due to the island's mild climate.