Monarch butterfly (Danaus plexippus)

Monarch butterflies are unique and amazing butterflies. Every fall, monarchs in North America migrate to warm, winter roosting sites. Monarchs in the east migrate to Mexico in the winter. Monarchs in the west migrate to the coast of California in the winter. In the spring, they migrate back. Their migratory round-trips may be as far as 5,000 km (about 3,000 miles). Monarchs are the only butterflies that migrate round-trip.

Year after year, migrating monarchs choose the same few sites for winter roosting. This is amazing, because different butterflies migrate every year! When the forests where they roost are cut down, they have no place to go. Monarchs breed throughout the spring and summer. Most monarchs live about a month. Once they lay eggs, they die. Only the last generation of monarchs, born in the fall, migrates. This generation of monarchs lives about 6 to 9 months. After returning from migration in the spring, they lay their eggs and die. Therefore, the monarchs that migrate in fall are the great-grandchildren of the last generation of monarchs to migrate. We do not understand how monarch butterflies know how or where to migrate year after year, given that they have no migration experience.

Butterflies undergo "metamorphosis". That is, they change body forms throughout their life. They go through four stages: Eggs > Larvae > Pupae > Adults. Butterflies eggs grow into larvae. The larvae of butterflies and moths are called "caterpillars". Monarch caterpillars have a special diet. They feed on milkweed plants. You may have seen the bright orange blossoms of the Butterflyweed on the roadside in August and September. Other members of the Milkweed family are not so pretty, but they are just as important to the butterflies. When the larvae have grown to about the size of an adult, they are ready to become pupae. At this time, they spin a small mat of silk on a plant. They attach themselves to the silk, and hang upside down. During this pupal stage, they develop wings and legs. Unlike moths, butterflies do not spin a cocoon of silk around themselves during the pupal stage. From the pupal stage, the butterflies become adults. This whole process takes about one month.

Once the monarchs are adults, it is easy to tell males apart from females. Males have a black spot on each hind wing, whereas females do not. Also, females have much thicker black veins on their wings (see diagram).

Monarch butterflies have an interesting form of defense against predators: chemistry. The milkweed plant that the butterfly larvae eat has poisonous compounds. The milkweed chemicals are not toxic to monarchs, but they are toxic and taste bad to its potential predators, such as birds, reptiles, and mammals. The monarch butterfly advertises its poison with its bright colors. In general, insects and other animals with bright yellow, orange, and red colors are poisonous.