In the middle of the night, you walk into the bathroom.

What *is* that?

It’s shaped like a fish and even has shimmering scales.

But it also has legs – six of them.

This insect is a firebrat.

Firebrats and their close relative the silverfish don’t come out of your drain, as it might seem.

They just happened to wander into your sink and can’t climb out.

Sorry, pal.

They live in dark places, hiding in your walls … or a stack of newspapers.

If they’re near a source of warmth, all the better.

They get the name firebrat from their attraction to heat.

And they like to live where they eat.

This page could make a tasty meal.

Firebrats and silverfish are the original bookworms: They love the books you keep meaning to open.

They also love the cereal in your kitchen.

Look closely.

You’ll see traits some of the earliest insects had around 400 million years ago.

Take these three long filaments.

The two outer ones are called cerci.

They work like antennae, detecting chemicals and predators … like this house centipede.

Other insects, like cockroaches, have a short pair of cerci.

But very few insects have this third one, called the median caudal filament.

Its tiny hairs detect the faintest air currents.

A firebrat is born as a mini version of itself.

That’s rare for insects.

This ancient way of developing is called ametaboly.

It’s totally different than the metamorphosis an insect like a butterfly goes through, from caterpillar to pupa to adult.

That’s called holometaboly.

This complete transformation is an advantage: The caterpillar feeds on leaves; the adult, nectar.

So, a butterfly doesn’t compete with its younger self for food.

But a firebrat does, sharing its food at every life stage.

Luckily it can go months between meals.

Like the original insects, firebrats don’t grow wings.

Most insects do.

I mean, wings are useful: You can reach different kinds of food and steer clear of predators.

The diversity of insects exploded when wings appeared.

That’s why there’s hundreds of thousands of species of flies – yes, mosquitoes are flies – and only a few hundred species of firebrats and silverfish.

Maybe you’re thinking, “Why didn’t firebrats just change to be more like other insects?” The thing is, evolving doesn’t mean an animal or plant keeps moving toward some ideal version of itself.

All it has to do to survive is keep up a big-enough population.

And firebrats have – for millions of years.

Also, as far as pests go, they’re not so bad.

They don’t bite or sting.

And they have enzymes in their gut that digest tough cellulose.

One day we might even be able to harness those enzymes to make biofuels from plants.

Not bad at all for an insect firmly set in its ways.