Metamorphosis: Change of Plans

Resource for Grades K-5

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Running Time: 4m 45s
Size: 14.1 MB

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Metamorphosis: Change of Plans (Audio Description) (Video)

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Background Essay

For many creatures, the transformation from infant to adult is a gradual process. In many mammals and birds, for example, development unfolds over the course of years. This is not to say that the transformation is constant. The color of a young bird's plumage often changes when the bird molts, and human children periodically experience growth spurts. However, none of the changes that occur in these animals is as dramatic as the metamorphosis that takes place between the larval stage and adulthood in amphibians and most insects.

What's so remarkable about the process of metamorphosis is that the same animal at each stage looks and behaves nothing alike. Bullfrog larvae, better known as tadpoles, are strict herbivores, while adult bullfrogs are carnivorous. Tadpoles, at least before they grow legs, look more like fish than frogs. In fact, they even breathe with gills. Adult frogs, by contrast, have powerful back legs and webbed feet and breathe with lungs.

Like bullfrog tadpoles, dragonfly larvae, called nymphs, are fully aquatic. The nymphs lack the long slender wings of their adult counterparts and, like tadpoles, acquire oxygen from the water through gills. Similarly, monarch butterflies undergo dramatic changes in their transition from larva to adult. Both phases are highly specialized and very different from each other. While the larva is specialized for eating and growing, gaining as much as 2,700 times its original weight, the adult only sips nectar as it searches for mates and good locations to lay eggs.

The level of specialization of the larval and adult phases of these animals suggests a reason for metamorphosis. Many scientists hypothesize that specialization minimizes competition between larvae and adults and thus allows these species to exploit their environment more effectively than other animals.

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Discussion Questions

What are some ways in which metamorphosis helps the animals that undergo it?

What is the difference between how you grow up and how a caterpillar grows up?

Why do you think the caterpillar and butterfly are so brightly colored?

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Transcript

NARRATOR: When some animals grow up, they not only get bigger, their whole body shape changes. Tadpoles are baby frogs that live in the water. Like fish, they breathe oxygen from the water through their gills. After about two years, this bullfrog tadpole is finally ready to change into a frog, in a process called metamorphosis.

First, tiny legs appear near the tail. A front leg grows, elbow first. The small mouth and eyes change. As the eyes move upward and stand out, the frog can see above water (frog croaking). The mouth becomes wider, and the frog can eat insects.

The froglet uses up nutrients stored in its tail. Slowly the tail is absorbed into the body. A strong pair of legs and feet grow, good for jumping on land. The gills disappear and lungs develop, and the frog can breathe air, just like us. Within only ten days, the tadpole has become a frog and is ready to live mainly on land.

Dragonflies are found in ponds, rivers and streams in most parts of the world. Dragonfly babies are called nymphs. Like tadpoles, they live in the water. When they're old enough to change into grownup dragonflies, they climb up out of the water.

This nymph is holding on to a stalk of grass with its feet. The outer skin on its back splits open and the new dragonfly begins to climb out. At first, its wings are soft and crumpled. As blood pumps through the veins, the wings and body get bigger and harden. It takes about five hours before the adult dragonfly is ready to fly and feed. If you look carefully, sometimes you can find the old skins of nymphs still stuck to the grass.

This caterpillar will grow up to be a beautiful monarch butterfly. As a caterpillar, it lives on the milkweed plant, where it eats a leaf a day...which is like a person eating 40 pounds of salad a day.

The caterpillar goes through metamorphosis, too. When it reaches about two inches, or five centimeters long, it attaches to a twig head down. It wriggles out of its skin and forms a hard chrysalis—a kind of caterpillar sleeping bag. Inside, an extraordinary transformation takes place.

After about two weeks, the monarch butterfly breaks out of the see-through chrysalis. Like the dragonfly, it pumps blood through the veins of its wings. In a few hours, with its wings dried and hardened, the monarch takes to the air.