A firefly flashes when oxygen, breathed in through the abdominal tracheae, is allowed to combine with a substance called luciferin under the catalytic effect of the enzyme luciferase. This reaction produces a very efficient light, with almost no energy lost as heat. The timing of the firefly\'s flashes is controlled by the abundant nerves in the insect\'s light-making organ. The duration of the flashes depends on how long the luciferin takes to oxidize. In the pyralis, a common North American firefly, for example, the male flies around and flashes about every five seconds. The female stays on the ground and flashes in response about two seconds later, thus providing the crucial cue to their union.