Dear Charlie,

There are about 40 kinds of cats out there—like me. There’s only one kind of human on Earth now. But there are more than a million kinds of insects. That’s just the insect species we know about.

Every single one of those insects has six legs.

I talked about why that is with my friend Allan Felsot. He’s an insect scientist at Washington State University.

He told me there must be some evolutionary reason insects have six legs—like better stability when walking.

“In biology, every ‘why’ question has the same answer,” Felsot said. “Things are the way they are because of adaptations that have allowed organisms to live longer.”

An adaptation is a trait that helps organisms survive in their environment. It helps them live long enough to have babies and pass on their genes.

Insects and their closest relatives are arthropods. That includes crustaceans like lobsters and shrimp. It includes arachnids like spiders. It includes myriapods like millipedes and centipedes.

Arthropods have traits in common. They have limbs with joints. They have a hard shell on the outside called an exoskeleton. They’re symmetrical. If you drew a line down their bodies—from head to tail—they look the same on both sides.

A scarlet lily beetle with red elytra and black legs and antennae

But insects and only a few other arthropods have just six limbs. Crustaceans usually have 10 limbs. Arachnids have eight legs. Myriapods can have hundreds of legs. Insects have all the limb genes crustaceans have, but some of those genes are turned off.

That’s because having just six legs made life better for some insect ancestor millions of years ago. It survived and left six-legged babies.

One big advantage of having exactly six legs is balance. Felsot says it’s called static stability. That means an insect on six legs can speed up then stop fast. It can change the direction it’s moving—without falling over.

Being able to stop and change direction without wiping out helps insects dodge enemies—like hungry birds or people with fly swatters.

That balance has to do with the way insects use their legs while walking. Insects usually walk with three legs touching the ground at a time. They might start with one left leg and two right legs on the ground. Then they take a step by putting two left legs and one right leg on the ground and lifting the others up.

It looks like a tripod—the three-legged stand that supports a camera or telescope. Or a stool.

“You can’t have a two-legged stool,” Felsot said. “But a three-legged stool or tripod is probably the most stable configuration.”

It’s so stable that scientists who make robots use insects as models. That’s called biomimicry. They’re copying the balance insects can achieve, thanks to their biology.

Eventually insects got another adaptation, too—wings! They’re the only arthropods that did that. It’s one reason they’re so successful.

Now that’s fly.


Dr. Universe