The first firefly is thought to have lived over 100 million years ago, illuminating the night with a glowing green light. And today, there are around 2,000 firefly species, found all over the world, from forests and grasslands to marshes and deserts. So, why do these insects glow? Emily A. Geest digs into the bioluminescent signals of fireflies. Lesson by Emily A. Geest, directed by Lou Morton.