JOHN YANG: Finally tonight, for many, fireflies are part of the story of summer, evoking memories of lazy, hot evenings spent trying to catch them in glass jars.

Now we meet a man who's still trying to catch them, not with a jar, but with a camera.

JOHN YANG (voice-over): In Amazonian folklore, the light of fireflies is a beacon of hope and guidance from the gods.

Japanese legend has it that they're symbols of love, war, and departed souls.

And for National Geographic explorer and firefly specialist Sriram Murali -- SRIRAM MURALI, National Geographic Explorer: Coming from India, fireflies are symbolic of the night and the jungles.

JOHN YANG (voice-over): Murali has been fascinated by fireflies since the night.

Years ago, he was searching for stars near his hometown in southern India.

SRIRAM MURALI: I stumbled upon a forest with millions and millions of fireflies synchronizing their flashes across vast expanses of the forest.

And there's been no looking back.

The first time I saw it, I almost felt like I was transported to another planet.

JOHN YANG (voice-over): Fireflies once shared earth with dinosaurs, but now their numbers are declining.

This summer, Murali came to the United States to document how fireflies are coping in big cities.

SRIRAM MURALI: The fireflies in D.C. and New York are found in some of the most urban conditions, like in Battery Park in New York, which is just a couple of blocks from Wall Street, and in the National Mall in D.C. right in front of Lincoln Memorial.

To me, how these fireflies have adapted to live in such bright, harsh conditions really amazes me.

And I wanted to tell a story about these fireflies, about how resilient they are.

JOHN YANG (voice-over): There are more than 2,000 species.

Eastern fireflies have proven to be adaptable to urban environments, but many other species haven't fared as well.

Their enemies include climate change, pesticides, habitat loss, and light pollution.

SRIRAM MURALI: Fireflies flash to communicate, especially to find a mate.

And in lit areas, artificial lighting drowns their flashes, so the males and females are not able to see each other.

They don't reproduce and there are no offsprings.

And in a couple of seasons, the population vanishes.

JOHN YANG (voice-over): Murali has some suggestions to prevent that.

SRIRAM MURALI: Support native vegetation and do not use pesticides.

And keep your outdoor lighting to a minimum.

Use a motion sensor if possible.

It helps the migrating birds as well.

JOHN YANG (voice-over): A little less light could provide a healthier ecosystem so that fireflies will be able to continue to shine for generations to come.