Nestled inside this leaf, something is making its squiggly mark.

It’s a larva.

And it’s walking a very thin line – well, wriggling one – between life and death.

Larvae like these are called leaf miners.

They tunnel their way through leaves, just below the papery layer called the epidermis.

Different types of insects etch  a mine pattern all their own.

This one will grow up to be a fly.

Scaptomyza, a cousin of the fruit fly.

But to get there, it needs food -- plants like arugula, kale, wild mustard.

It craves the pungent chemicals – isothiocyanates  – that give these leafy greens the bitter taste   that some of us find delicious.

Those chemicals also happen to be  toxic to small animals like insects.

The plant makes them precisely to keep pests away.

That’s good for Scaptomyza flies and their  larvae.

They have the plants all to themselves.

So how does a Scaptomyza larva make a home here?

It has to do with an evolutionary arms race.

Larva and plant battle it out, each side bringing its strongest defenses.

It starts with a fly and her ovipositor, that zipper-looking part.

She uses these sharp bristles  to dig a hole in the leaf.

This is what it looks like from  the other side of the leaf.

The ovipositor opens and closes to scrape a hole that goes almost all the way through.

The hole, called a stipple, fills up  with sap.

The fly sucks it all up.

But she isn’t drinking just to feed herself.

She’s looking for just the right place to lay  her egg.

When the plant senses the wound made   by the fly’s ovipositor, it fights back  by producing even more toxic chemicals.

The fly’s eggs and larvae  can handle the chemicals,   up to a point.

Mom boosts their chances  by tasting for a spot that is less toxic.

Her little taste test paid off.

This wriggling  larva is getting ready to hatch out of its egg.

Next, the leaf miner sets off tunneling  in the spongy layer of the leaf.

Sometimes it moves to another leaf and starts over, having chowed through the first.

After a couple of weeks, the larva hardens into  a pupa.

See it here in the dirt below the plant,   or in this leaf?

A few days  later it emerges as a fly.

The survivors of this chemical race come out  stronger.

The plants with the most powerful   toxins get to multiply.

So do the flies that can withstand them.

But both the plants and the insects need to constantly   ratchet things up a notch ...  or they just might go extinct.