Why Do Fireflies Light Up

Fireflies have an organic compound luciferin in their abdomen. It reacts with oxygen, calcium, and ATP (adenosine triphosphate) in the presence of the enzyme luciferase to produce light. This chemical reaction generates cold light, so it doesn’t heat up the light producing organ of the fireflies. It is because the whole chemical energy of the reaction is converted to light without losing any of it as heat, unlike light bulbs (where about 90% of the energy is wasted as heat). The required oxygen enters their body through the trachea, a complex network of fine tubes, as they don’t have lungs. When oxygen is available the fireflies glow, and extinguish when no oxygen can be spared.

Since the muscles that transport oxygen work slowly, scientists have long wondered how they can blink so fast continuously. Recently it was found that it is nitric oxide that they use to control their glowing, and not oxygen. When there is no nitric oxide present in the body, the oxygen entering their light organ gets bound to the mitochondria (the energy-producing organelles of the cells) and is not available for producing light. But when nitric oxide is present, it itself binds to the mitochondria, freeing the oxygen to cause bioluminescence. However, nitric oxide decomposes fast, causing the mitochondria to re-trap the oxygen and put off the light. The process goes on.

Fireflies are mostly seen glowing in summer dusks and nights as it’s their breeding season and they prefer warm climates. You don’t usually find them in the day as at that time they find shelter in tall grass and stock up their energy for the night when their glows are more visible. Their lighting up serves the following purposes:

• Warding off possible predators that associate their lights with their bitter taste.
• Attracting mates; males of a particular species produce a certain pattern of light to attract females of the same species.
• Attracting prey (that could even be other fireflies).

So next time you watch the magical lighting show of the fireflies on a warm summer night, you know what it is for and how they do it. If you want to catch them in a jar, you now know that there should be a good oxygen supply to keep them glowing and alive.