What are Monarch Butterflies?

R. Kayne

A Monarch butterfly (Danaus plexippus) is one of the most beautiful and interesting butterflies in the world. Monarchs have distinctive bright orange wings with black veins and outer margins. The black outer margins are speckled with white spots as is the black body. The forewings have three dark orange patches near the tips, and the hind wings are rounded and lighter orange.

All types of milkweed plants produce a sticky, toxic sap within the stems and leaves.
All types of milkweed plants produce a sticky, toxic sap within the stems and leaves.

A Monarch butterfly looks like it only has four legs because the two front legs are dwarfed and stay curled up close to the body. They follow the 4-stage life cycle of all butterflies and moths, going from egg, to larva (caterpillar), to pupa or chrysalis (cocoon), to adult butterfly. This metamorphism is one of the great wonders of life.

Noxious milkweed invades farmland, making the Monarch butterfly a beneficial insect.
Noxious milkweed invades farmland, making the Monarch butterfly a beneficial insect.

An adult female lays a tiny ridged egg on the underside of a milkweed leaf. The egg hardens and after a few days a small caterpillar emerges. The caterpillar is ringed with brightly colored bands of cream, black and yellow. First it eats the egg case for nutrients, then the milkweed leaves.

Milkweed contains poison in the form of toxic cardenolide glycosides, which serves as the Monarch's natural defense against predators. Since the adult butterfly only sips food but cannot eat, it is up to the caterpillar to consume the leaves that will give the adult its inborn defense. Noxious milkweed invades farmland, making the Monarch a beneficial insect.

The Monarch larva eats day and night to increase its weight, and will molt 4 times in the process. When it reaches a length of about 2 inches (5 cm), it finds a suitable place to attach a hind leg and hang upside down. It uses its spinneret to create a silken glue to keep it secure, then begins shedding its skin or exoskeleton. The new skin hardens into a green waxy pupa. This cocoon becomes increasingly transparent as it nears the 10 — 14 day mark when the adult Monarch emerges, its wings wet and crumpled.

The adult Monarch butterfly will stay at the broken cocoon pumping blood into the veins in its wings to inflate them. Once inflated they are allowed to dry in the air, then the Monarch is ready to take flight. It will get its nourishment now from drinking flower nectar with a proboscis, or long tongue. When the proboscis is not in use it is coiled at the mouth.

The eyes of a Monarch butterfly are compound, made of many smaller eyes. They can see colors and images. The butterflies also posses the senses of hearing, touch, and taste/smell. The adult does not grow, and the life span of the adult Monarch varies. If it emerges from the cocoon in early summer it will live about 2 — 5 weeks, but if it emerges in late summer it will live over the winter months.

One of the most amazing things about the Monarch butterfly is that some species are migratory, flying over 2,200 miles (3,540 km) every fall from the cold north of the United States and Canada to warmer areas like California, Florida and Mexico. Monarch butterflies by the thousands settle in the same trees generation after generation. These habitats are shrinking, however, inspiring conservationists to work towards reserving some of these areas to ensure survival of the Monarch butterfly.

Monarch larvae made news in 1999 when a published report in Nature magazine showed that the pollen from genetically modified (GM) "B.t. corn" killed the caterpillars. (B.t. is a pesticide used by organic farmers because it evaporates away in the sun and can be easily washed off. GM corn, however, has been genetically modified to contain the B.t. pesticide in the DNA of the plant itself.) The tests were conducted on captive specimens, as Monarch larvae do not eat corn, but the deaths highlighted the concern that when wind and natural processes spread GM pollen into the surrounding environment it will have unintended effects, not just on Monarchs, but on many species of insects and even other plants and neighboring crops.

The bright colors of the Monarch butterfly are what warn predators of the poison it holds. A bird that takes a nip of a wing will vomit and remember not to try that distinctly colored insect again. For this reason the non-poisonous North American Viceroy butterfly (Limenitis archippus) has adapted itself to look almost exactly like the Monarch!

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Discussion Comments


When the monarch butterfly's caterpillar undergoes histolysis and histogenesis what organs or tissues (other than the histoblasts) survive to become part of the adult butterfly?


Butterflies use their feet as their taste buds.

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