Butterflies are perhaps most famous for the process by which a plump little caterpillar transforms into a winged work of art. But they’re not unique in going through this drastic life change, called complete metamorphosis, or holometabolism.
A whopping 75 percent of known insects—among them bees, beetles, flies, and moths—develop in four stages: egg, larva, pupa, and adult. Most striking about complete metamorphosis is how different the larva looks and behaves from the adult. (Watch a time-lapse video of a caterpillar becoming a butterfly.)
Other species, such as grasshoppers and dragonflies, experience incomplete, or simple, metamorphosis, which involves three life stages—egg, larva or nymph, and adult or imago. The nymphs look like tiny adults, eating and shedding their skins until they reach adulthood.
Eggs and larvae
Nearly all insects start out as eggs and then hatch into larvae. Caterpillars are a type of larvae that many people are familiar with, but others resemble worms or tiny insects, as occurs in ladybugs (aka ladybirds).
A larva’s main job is to grow and molt, a process triggered by hormones. Each stage of molting is called an instar, and some insects molt up to five times before moving onto the next stage. (Read about a venomous caterpillar with a toxic “toupee.”)
Larvae eat as if there’s no tomorrow because, in a way, there isn’t. Metamorphosis changes almost everything.
In insects that undergo incomplete metamorphosis, the larvae are called nymphs. Many, such as grasshoppers, look and behave much like tiny versions of the adult insects. Others, such as leafhoppers, look a bit different from adults, with small wing buds. But these insects eat the same things as adults and move the same way, going through multiple molts until they mature. Cicadas can take 17 years to metamorphose into adulthood, spending most of that time underground.
After shedding their final instar, insects that experience complete metamorphosis become pupae. In some cases, pupae enclose themselves inside a hard cocoon, or chrysalis, which butterflies and moths make from their own silk. Once complete, they’ll hang upside down from a perch on a silken thread.
Others deploy different techniques. After a worm-like larval stage of nearly two years, Hercules beetles of the American tropics store up enough feces to form sturdy cocoons.
It's odd that these giant beetles make a cocoon at all, explains Richard Jones, an author and entomologist in the U.K not affiliated with a university or organization.
“Most beetles do not make a pupal chamber or cocoon,” Jones says. “They simply shimmy out of their last larval skin and pupate. Ladybirds do this attached to a leaf”—no sleeping bag required.
Other beetles, such as the eastern firefly of the United States, nestle in soil.
Some caddisflies build cases out of rocks and shells from their native rivers and streams and pupate inside after sealing them up. Honeybee larvae resemble white grubs, pupating inside sealed cells within the honeycomb.
After emerging from its chrysalis, a newly minted butterfly may look wilted—its wings are wet and need a couple of hours to expand before taking flight. Hercules beetles emerge with their spectacular horns, and caddisflies cut their way out of their found creations and swim to the surface for one final molt before flying off.
Generally, adult insects don’t live long—dragonflies, for example, live only about a month, but before that, they’re in their larval state for around three years.
Many insects—such as fireflies and crane flies—don’t eat at all during their short adulthood, spending their precious time looking for mates. The luna moth doesn’t even have a mouth or digestive system.
That doesn’t mean adult insects are mere shadows of their former selves. In 2008, researchers from Georgetown University, in Washington, D.C., taught caterpillars of the tobacco hornworm moth to dislike—and avoid—a particular scent. As butterflies, they avoided the odor as well—suggesting that the adults can remember experiences from their time as caterpillars.
Why such an elaborate life cycle?
Metamorphosis is ultimately a successful strategy because juveniles and adults eat different things. Caterpillars munch nutrient-rich leaves to enable all that developmental change, and butterflies just need to sip nectar (essentially sugary water).
For species with such different developmental forms, “you’ve suddenly created a competitive-free space,” says Katy Prudic, an entomologist at the University of Arizona in Tucson. Parents and offspring aren’t competing for resources, allowing both life stages to develop independently. (Read about a butterfly that can fly 2,500 miles.)
“It’s this wonderful process of really reinventing yourself,” Prudic adds. “When insects metamorphose, they're able to explore and go places that they couldn’t as maggots, caterpillars, and grubs.”