These fruit flies are throwing down.

They're like fruit fly gladiators.

They lunge.

Flip each other over.

Roll around.

And even toss each other, sumo-wrestler style.

Normally, fruit flies don't get this worked up over a drop of apple juice.

But these guys have had their genes manipulated, something scientists have been doing to fruit flies for more than a century.

Yep, these little insects that we only notice when they get into our kitchen are unsung heroes of science.

In 1910, biologist Thomas Hunt Morgan noticed a rare white-eyed fly among his normal red-eyed ones.

The discovery led to our fundamental understanding of how genes get passed on from generation to generation.

Since then, fruit flies have been key to figuring out how human diseases work.

That's because, when it comes down to it, fruit flies are more like us than you might think.

They have about the same number of genes: 20,000 or so.

In fact, 75 percent of the genes that make humans sick are found, in a very similar form, in fruit flies.

They're a simpler version of us, a kind of genetic mirror that scientists have used to learn about cancer, Parkinson's, Alzheimer's.

And the fighter fruit flies?

They're helping researchers learn about conditions that tap into our emotional states, like depression or addiction.

Neuroscientist Eric Hoopfer - who studies fruit flies in David Anderson's lab at Caltech - flashes them with a red light.

He has altered these research flies to be sensitive to this light.

It activates a cluster of neurons in the flies' brains.

And they fight.

But change the intensity of the light, and they do something very different.

See how they're vibrating just one wing?

That's fruit fly flirting.

The researchers were surprised to see that the same neurons controlled such different behaviors.

What do fighting and "flirting" have in common?

In both, flies are pretty hot and bothered.

These neurons control something like emotional arousal.

Pinpointing these circuits in their brain could help us figure out where trouble starts in the human brain... and maybe one day, lead to better treatments for mental illness.

So the next time you see one of these guys on your leftover piece of banana, you might want to think twice about swatting it.

A little thanks might be in order instead.

Hey guys.

It's Lauren.

These flies seem to be having fun.

If you're curious about the amorous habits of other animals, check out our video on newts.

Every year they travel back to the pond where they were born to get some underwater lovin'.

And watch how garden snail babies are made.

It involves a tiny spear called a "love dart."

Thanks for watching.