The monarch is also known as the milkweed butterfly because its larvae feed on the milkweed plant. In fact, this plant is the only source of food for the larvae.
The life cycle of this butterfly can be categorized into four developmental stages. The first stage is the egg, the second the caterpillar (larva), the third the pupa (chrysalis), and the last the adult, which heralds the emergence of the most beautiful creature on Earth.
The female butterfly begins the next cycle by laying one small, pinhead-sized white egg underneath the leaf of a milkweed plant. The caterpillar begins to develop inside, drawing upon the yolk material inside the egg for nutrition. It takes the caterpillar about 3 to 5 days to eat a hole in the egg case and emerge onto the surface of the leaf.
This caterpillar is about 0.1 inches long and weighs about 0.55 milligrams. Its body is covered by nine chocolate-colored rings apart from a black head, three pairs of front true legs with claws, and five pairs of prolegs extending backwards. This feature adds a unique and functional form to its body. It also has a stomach, mouth, and a silk gland among other characteristics of a full-grown caterpillar. Its rapid growth leads to four or five molts (shedding of its external skin).
At this stage, its weight increases to 1.5 grams. It is about 2 inches long with yellow, white, and black stripes on its body. It now stops eating and spins a white silk pad on a stiff object, attaching itself by its two rear prolegs to the silk pad. Once this is done, it hangs head-down in a J-shape in a seemingly inactive state for about 12 hours.
Once this stage ends, the caterpillar begins to convulse in rhythmic jerks, breaking off its outer skin. At this developmental stage, its head capsule and 16 legs are shed in about 60 seconds, with the emergence of a pupa or chrysalis that is 1 inch long and 0.4 inches in diameter. Jade-green in color, the chrysalis at this point has 24 metallic appearing gold spots surrounding half its upper abdomen. The remaining part is surrounded by 12 metallic-like gold spots.
As the caterpillar's head capsule is discarded, it enters the chrysalis stage without any vision and with only the ability to distinguish light from darkness. The inside portion of the chrysalis that is located below the gold crown turns to a jade-green liquid within the first 16 hours, because of the disintegration of the various parts of the caterpillar's body. The outline of the butterfly's wings can now be viewed through the somewhat transparent outer shell. One can also see a pattern of lines that mark the position of the future butterfly proboscis (tubular sucking organ, used to suck sweet nectar from the flowers), first and second pairs of legs, and also the two antennae.
The caterpillar emerges from the chrysalis after 8 to 14 days. It pumps fluid from its abdomen into the wing veins and is ready to fly in about 15 minutes. It takes approximately 2 hours for the wings to dry, and the butterfly is full-grown. It has six legs and four beautiful wings that are surrounded by a narrow black border studded with white spots. At this stage, the adult has two compound eyes and is about 1.3 inches long and 0.15 inches wide and weighs about 0.41 grams.
In autumn, the Eastern populations of monarch butterflies migrate south for two months to the Neovolcanic Mountains of Mexico. It is here that they cluster in trees between 9,000 to 11,000 feet elevation for about 4 months of November to March. This period also allows them to hibernate. It is during this interval that they eat and drink little, which in turn enables them to live as long as one year. It is during the spring that these butterflies migrate from Mexico to the northeast towards the U.S. At this time, some butterflies return the long distances to where they were born while others fly only part of the distance to Texas or the lower southern states. Those located to the west of the Rocky Mountains travel to the west coast from Baja California to the north of San Francisco. There are others that come from British Colombia in the autumn to winter in California. These butterflies have life spans of about six weeks.
It is unfortunate that the population of the monarch has been greatly reduced because of the recent illegal deforestation. Steps to classify it as a protected species and restore its habitat are underway. Another threat is that these creatures are susceptible to Ophryocystis elektroscirrha. This parasite results in decreased weight, shortened life span, rapid weight loss, and weakened wings. These effects vary between butterfly populations and parasite strains.