The Science of monarchs

Monarch life cycle

A single, greenish egg attached to the underside of a milkweed leaf is the beginning of a monarch's life. The worm-like larva grows inside the egg. When it is ready, the larva chews a small hole in the egg shell and wriggles its way into the world. After a few minutes, the newly hatched larva has its first meal -- the remains of its egg. Female monarchs lay one to three eggs on the underside of milkweed leaves. This process is repeated until the female has laid hundreds of eggs. 

The larva begins to eat milkweed leaves after eating its shell. Milkweed is the only plant that monarch larvae will eat. The larva eats and grows, grows and eats. The larva grows so much that it outgrows its skin, much like outgrowing old clothes.

Molting must occur in order for the larva to keep growing, . The old skin splits, revealing the new skin underneath. The larva wriggles free of the too-tight skin. After freeing itself, the molted larva often eats its old skin before moving on to more milkweed leaves. 

After shedding their skins, monarch larvae continue to grow and will have to molt four more times. The last molt is much different than the others. The larva crawls away from its milkweed plant, searching for a suitable place. Some larvae will travel longer distances than others. When the larva has found a suitable place, it weaves a silk mat with a "button" in the center. Once the mat and button are ready, the larva grabs the silk with its legs and hangs upside down. The front part of its body will curve to make a "J-shape."

Once in the "J", the larva molts for the last time. The skin splits behind the head, and the larva wiggles while it hangs upside down to remove the old skin. This final molt is the trickiest, because the larva must shed its old skin and still hang onto the silk button. Once the larva embeds a hook-like structure at its rear end into the button, the rest of the skin can slip off. 

The larva becomes a pupa, when the skin fall off. The monarch has no eyes and no antennae. It has no legs, and it cannot move. All of the major changes in body shape, size, and arrangement happen. In monarchs, this stage can last as long as a week. At the end of this stage, an adult butterfly will emerge from the chrysalis.

Monarch Anatomy

Monarch Anatomy

A newly emerged butterfly will wait two or more hours before it can fly. New wings are small and shriveled, so the butterfly pumps body fluid through its wing veins in order to make them get bigger. Then, the monarch has to wait for air to replace some of the fluid. Until this happens, the monarch cannot fly, and its wings are easily damaged. 

After the wings have hardened, the butterfly flies away to find its first meal. From this point on, the monarch drinks all of its food. The butterfly will visit several different kinds of flowers to get obtain nectar.

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Sexual Dimorphism

Male and female adult monarch butterflies look different from one another.  Specifically, males have a black spot on each hindwing and females have slightly thicker black veins on their wings.  Some other differences exist too, such as the somewhat longer abdomen of the female.  The phenomenon whereby males and females can be distinguished by physical characteristics is called sexual dimorphism.

Warning Coloration and Mimicry

Warning coloration, or aposematism, is a term used to describe colors and/or patterns that warn predators that a potential prey species is unpalatable, toxic, or dangerous.  As a result of monarch caterpillars feeding upon milkweed which contains toxins called cardenolides, both the caterpillar and winged adult form of the monarch butterfly are toxic to most predators.

Various studies have shown that all vertebrates, including insectivorous birds associate red, orange, yellow, and white coloration as signs of danger. Toxic or unpalatable butterflies, such as the monarch, have evolved color schemes that exploit this phenomenon, and thereby dissuade birds and other predators from attacking them. Aposematic signals are beneficial for both the predator and prey, since both avoid potential harm.

Relatedly, some palatable species, like the viceroy butterfly, have evolved coloration which mimics that of a toxic species (in this case, the monarch) and resultantly, are infrequently attacked.

Annual Breeding Cycle and Migration

Monarchs spend the winter in roosting spots. Unlike most other insects in temperate climates, monarch butterflies cannot survive a long cold winter.Monarchs west of the Rocky Mountains travel to small groves of trees along the California coast. Those east of the Rocky Mountains fly farther south to the forests high in the mountains of Mexico. The monarch's migration is driven by seasonal changes. Daylength and temperature changes influence the movement of the monarch.

No other butterflies world wide migrate like the monarchs of North America. They travel much farther than all other tropical butterflies, up to three thousand miles. They are the only butterflies to make such a long, two-way migration every year. Amazingly, they fly in masses to the same winter roosts, often to the exact same trees. Their migration is more the type we expect from birds or whales. However, unlike birds and whales, individuals make the round-trip only once. It is their children's grandchildren that return south the following fall.

Some other species of Lepidoptera (butterflies and moths) travel long distances, but they generally go in one direction only, often following food. This one-way movement is properly called emigration. In tropical lands, butterflies do migrate back and forth as the seasons change. At the beginning of the dry season, the food plants shrivel and the butterflies leave to find a moister climate. When the rains arrive, the food plants grow back and the butterflies return.

When the late summer and early fall monarchs emerge from their pupae, or chrysalides, they are biologically and behaviorally different from those emerging in the summer. The shorter days and cooler air of late summer trigger changes. In Minnesota, this occurs around the end of August. Even though these butterflies look like summer adults, they won't mate or lay eggs until the following spring. Instead, their small bodies prepare for a strenuous flight. Otherwise solitary animals, they often cluster at night while moving ever southward. If they linger too long, they won't be able to make the journey; because they are cold-blooded, they are unable to fly in cold weather.

Fat, stored in the abdomen, is a critical element of their survival for the winter. This fat not only fuels their flight of one to three thousand miles, but must last until the next spring when they begin the flight back north. As they migrate southward, monarchs stop to nectar, and they actually gain weight during the trip! Some researchers think that monarchs conserve their "fuel" in flight by gliding on air currents as they travel south. This is an area of great interest for researchers.  There are many unanswered questions about how these small organisms can travel so far.

Another unsolved mystery is how monarchs find the overwintering sites each year. Somehow, they know their way, even though the butterflies returning to Mexico or California each fall are the great-great-grandchildren of the butterflies that left the previous spring. No one knows exactly how their homing system works; it is another of the many unanswered questions in the butterfly world.