As night falls on Laguna Azul, an ecotourism lodge on the outskirts of the Mexican hamlet of Nanacamilpa, it looks like rain, and a group of eight tourists nervously watch the sky. Their guides look unconcerned. Cold rain makes for a miserable walk, but the fireflies love it.

A younger guide introduces himself in a strong voice as an older one looks on, leaning on a thick walking stick. The guide lists the basic rules of the tour: walk only on the path, no wandering, no music, no alcohol, no insect repellent, no lamps, and absolutely no cell phones. Any light, even that of a cell phone display, can distract the fireflies.

The walk follows sprawling farmland up a hill to the