Advantages of Metamorphosis for Insects

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we'll talk about how something as weird and dramatic as metamorphosis can possibly have evolved. Then, we'll talk about ways that metamorphosis benefits insects.

Resource Distribution

Imagine there's only one slice of chocolate cake left. You're home alone and hungry. What do you do? Very likely, you'll eat it. But now suppose you're sharing the house with a precious, well-behaved child. Now it feels a bit wrong to eat that cake by yourself, right? You might give it to the kid, or at least share it.

This is part of the problem that insects solve using metamorphosis, the biological process by which an animal abruptly and dramatically changes form during its development. No more sharing!

Ametabolous insects, like silverfish (Thysanura), have young that are essentially just tiny versions of adults. They do not undergo metamorphosis. Hemimetabolous insects, like cockroaches (Blattodea), have young that look similar to adults but are different in important ways. For instance, young hemimetabolous insects do not have wings. Sometimes their process of development is termed ''incomplete metamorphosis''. Holometabolous insects undergo complete metamorphosis. They go through an egg phase, a larval phase, and a pupal phase in which the larval tissue is completely digested and re-assembled before becoming an adult. Examples of holometabolous insects include caterpillars that turn into butterflies and maggots that turn into flies.

Silverfish are ametabolous.
Silverfish are ametabolous.

The Evolution of Metamorphosis

It seems somewhat confusing, at first, that a natural process could develop something as radical as the transformation from a caterpillar to a butterfly. The answer to that line of thinking began before the mechanisms of evolution were even understood. English physician William Harvey suggested that caterpillars were escaped embryos… back in 1651!

Though Harvey was wrong about some things (for instance, he thought that a pupa was another kind of egg), more and more genetic evidence has amassed to suggest that larvae, like caterpillars and maggots, really are just insect embryos that have escaped their eggs.

Hexapods - six-legged critters very much like insects - appear to have evolved around 479 million years ago. Based on fossil records, it appears that metamorphosis is about 280 million years old. In Illinois, paleontologists have found an intermediate form - a fossil of a young insect that looks sort of like a larva and sort of like a grasshopper.

Scientists Lynn Riddiford and James Truman have suggested one of the most comprehensive descriptions of how metamorphosis could have evolved. They point to the pro-nymph, a stage in early holometabolous insects. Pro-nymphs look and act differently from later stages. Some insects complete their pro-nymph phase while still in the egg, but some remain pro-nymphs for a brief period after they hatch. Riddiford and Truman suggest that maybe, at some point in the past, some pro-nymphs didn't fully absorb the yolk in their eggs. Rather than let it go to waste, they developed the ability to eat while still in the egg. It became useful for embryos like these to keep eating, and over time, these yolk-eating embryos became the caterpillars and maggots we know today.

Benefits of Hungry, Hungry Embryos

If you are an adult moth, the nice thing about a caterpillar is that it doesn't eat the same thing as you. Though Lepidoptera (the group of butterflies and moths), is a diverse group, many butterflies eat nectar and many caterpillars munch leaves, right? Therefore, unlike you and your cake-hungry child, butterflies don't have to share their resources with hungry babies. If you're a butterfly, you can have that cake-flower all to yourself.

Another advantage of holometabolous insects is that they can take advantage of resources that change with the seasons. Insects are very diverse, and some of them live for years and years, but most of them are pretty short-lived. So a change of seasons can be a huge deal for an insect. Holometabolous insects can synchronize their growth with seasonal changes. For instance, monarch caterpillars (Danaus plexippus) colonize the northern parts of their range during the summer, but only adult monarch butterflies, with wings, migrate south in the fall.

Adult monarch butterflies migrate to Mexico in the fall and cluster together on trees for the winter.
Adult monarch butterflies migrate to Mexico in the fall and cluster together on trees for the winter.

Pupae are a pretty good way to overwinter too. Many pupae have a hard shell to defend the insect against harsh climates, and some insects build a protective cocoon around their pupa. For instance, a swallowtail butterfly may be a hungry caterpillar all summer, overwinter as a pupa, and then emerge in spring to fly away as an adult. Since most pupae don't eat, being a pupa can be a good way to slow down during winter, when food is hard to find.

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