A natural science passage at pace

1On a summer evening in much of North America, the first fireflies rise from the grass just as the daylight begins to fade. To a person sitting on a porch, the blinking looks almost random. To a firefly, it is a language, and the chemistry that makes it possible is one of the most efficient ways of producing light that any living thing has found.

2A firefly is a beetle, not a fly, and its light comes from a special organ near the tip of its abdomen called the lantern. Inside the lantern, cells hold a substance called luciferin. When an enzyme called luciferase helps luciferin combine with oxygen, the reaction releases energy as light. (An enzyme is a protein that speeds up a chemical reaction without being used up by it.) The reaction also requires a molecule called ATP, which cells use to carry energy, so the firefly pays for every flash with energy from its food.

3What makes the firefly’s light remarkable is how little of that energy is wasted. An old-fashioned incandescent bulb turns most of the electricity it uses into heat rather than light, which is why it grows hot to the touch. The firefly’s reaction turns a much larger share of its energy into light, and the lantern stays cool. For this reason, biologists sometimes call it “cold light.” Producing light through a chemical reaction inside a living thing is called bioluminescence, and it also occurs in some fungi, in certain bacteria, and in many animals of the deep sea.

4A steady glow would be easy to explain. A flash is harder, because the firefly must switch the reaction on and off within a fraction of a second. Because the reaction cannot run without oxygen, researchers have long suspected that the firefly flashes by controlling how much oxygen reaches its light-producing cells. Air enters the lantern through a branching network of tiny tubes. One proposed explanation is that a nerve signal releases a gas called nitric oxide, which briefly stops other parts of the cell from using up the incoming oxygen, so that the oxygen reaches the light-producing reaction and a flash begins. Experiments in which fireflies exposed to nitric oxide glowed more brightly fit this explanation, but the details of the switch are still being worked out.

5Whatever the switch is, fireflies use it to communicate. In many species, a male flies through the dark flashing a pattern that belongs to his species alone: one long flash, say, or a quick double flash repeated every few seconds. A female of the same species, usually waiting on a leaf or a blade of grass, answers after a set delay. The male turns toward her flash, flashes again, and the two continue the exchange until he finds her. Because the patterns differ from species to species, a firefly can pick out a partner of its own kind even in a field where several species are flashing at once.

6The code can also be broken. Females of certain firefly species imitate the answering flashes of other species. When a male of the other species flies down expecting a partner, the female catches and eats him. Biologists who have studied these hunters report that some can switch among the flash patterns of more than one kind of prey.

7Fireflies glow long before they are old enough to court. Their larvae, which live on the ground and in leaf litter, glow faintly, and even the eggs of some species give off a dim light. Larvae do not flash to find mates, so their glow must serve another purpose. Many fireflies contain chemicals that make them taste bad to predators, and the leading explanation is that the larvae’s glow works as a warning: animals that have tried a bad-tasting firefly learn to leave glowing insects alone.

1On a summer evening in much of North America, the first fireflies rise from the grass just as the daylight begins to fade. To a person sitting on a porch, the blinking looks almost random. To a firefly, it is a language, and the chemistry that makes it possible is one of the most efficient ways of producing light that any living thing has found.

2A firefly is a beetle, not a fly, and its light comes from a special organ near the tip of its abdomen called the lantern. Inside the lantern, cells hold a substance called luciferin. When an enzyme called luciferase helps luciferin combine with oxygen, the reaction releases energy as light. (An enzyme is a protein that speeds up a chemical reaction without being used up by it.) The reaction also requires a molecule called ATP, which cells use to carry energy, so the firefly pays for every flash with energy from its food.

3What makes the firefly’s light remarkable is how little of that energy is wasted. An old-fashioned incandescent bulb turns most of the electricity it uses into heat rather than light, which is why it grows hot to the touch. The firefly’s reaction turns a much larger share of its energy into light, and the lantern stays cool. For this reason, biologists sometimes call it “cold light.” Producing light through a chemical reaction inside a living thing is called bioluminescence, and it also occurs in some fungi, in certain bacteria, and in many animals of the deep sea.

4A steady glow would be easy to explain. A flash is harder, because the firefly must switch the reaction on and off within a fraction of a second. Because the reaction cannot run without oxygen, researchers have long suspected that the firefly flashes by controlling how much oxygen reaches its light-producing cells. Air enters the lantern through a branching network of tiny tubes. One proposed explanation is that a nerve signal releases a gas called nitric oxide, which briefly stops other parts of the cell from using up the incoming oxygen, so that the oxygen reaches the light-producing reaction and a flash begins. Experiments in which fireflies exposed to nitric oxide glowed more brightly fit this explanation, but the details of the switch are still being worked out.

5Whatever the switch is, fireflies use it to communicate. In many species, a male flies through the dark flashing a pattern that belongs to his species alone: one long flash, say, or a quick double flash repeated every few seconds. A female of the same species, usually waiting on a leaf or a blade of grass, answers after a set delay. The male turns toward her flash, flashes again, and the two continue the exchange until he finds her. Because the patterns differ from species to species, a firefly can pick out a partner of its own kind even in a field where several species are flashing at once.

6The code can also be broken. Females of certain firefly species imitate the answering flashes of other species. When a male of the other species flies down expecting a partner, the female catches and eats him. Biologists who have studied these hunters report that some can switch among the flash patterns of more than one kind of prey.

7Fireflies glow long before they are old enough to court. Their larvae, which live on the ground and in leaf litter, glow faintly, and even the eggs of some species give off a dim light. Larvae do not flash to find mates, so their glow must serve another purpose. Many fireflies contain chemicals that make them taste bad to predators, and the leading explanation is that the larvae’s glow works as a warning: animals that have tried a bad-tasting firefly learn to leave glowing insects alone.