Danaus plexippus
monarch butterfly

Danaus plexippus ranges from North and South America and the Caribbean to Australia, New Zealand, the oceanic islands of the Pacific, Mauritius, the Canary Islands of the Atlantic, and, most recently, Western Europe.

D. plexippus is a predominantly open country, frost intolerant species whose range of breeding habitats is greatly dependent upon the presence of asclepiad flora (milkweeds). The monarch requires dense tree cover for overwintering, and the majority of the present sites in California are associated with Eucalyptus trees, specifically the blue gum, Eucalyptus globulus. These trees were introduced from Australia and have filled the role of native species that have been been reduced by logging.

Both male and female monarchs are bright orange with black borders and black veins. The veins on the female are thicker than those of the male. Male monarchs also have a swollen pouch on both of their hind wings.

Monarchs are poisonous to vertebrates. Their poison comes from the milkweed they feed on.

Monarchs also use their appearance to ward off predators. Orange is considered a warning color, which will warn predators that monarchs are poisonous, and not to attack them. From a distance, monarchs can blend into their surroundings. Sometimes, their spots will appear to be the eyes of a larger animal, and will ward off predators.

Small caterpillars hatch from eggs laid by female Monarchs. They grow, shedding their skin to get bigger. Eventually each caterpillar stops growing and forms a case around itself called a chrysalis. Inside the chrysalis it changes its body its body in a process called metamorphosis. When it is done it emerges as an adult butterfly.

Like birds, D. plexippus follows a pattern of seasonal migration. There are two distinct populations in the North America, those that breed in the East and those that breed in the West. Each autumn millions of these butterflies leave their breeding grounds and fly to overwintering sites. The Eastern population overwinters in the volcanic mountains of eastern Michoacan in central Mexico. The Western breeders spend their winters along the California coast. Similar migratory behavior has been observed in Costa Rican and Australian populations.

The larva feed on a wide range of milkweeds of the genus Asclepias. From these plants they acquire and store cardiac glycosides, secondary plant compounds that protect them from predation. The adults of the species forage for flower nectar.

Many species of milkweed occurring in parts of the United States and Mexico are known to be poisonous to cattle ,and D. plexippus is considered beneficial because it helps reduce the abundance of these plants. Overwintering sites are of interest to tourists.

The annual monarch migration is considered a "threatened phenomena" by the International Union for Conservation of Nature and Natural Resources. Steps have been taken by both the United States and Mexican governments along with numerous private individuals and organizations to protect the overwintering sites of these butterflies.