Why Do Fireflies Die So Soon?

In the 1988 anime film Grave of the Fireflies, a little girl named Setsuko buries a bunch of the little dead insects. With tears in her eyes, she asks her older brother, “Why do fireflies have to die so soon?” The question is haunting in the context of the movie, which details the tragic plight of a pair of Japanese orphans in the wake of World War II. But the same question can be asked in a biological sense: why do fireflies die so soon? What is their life cycle, and how much of their lives do they spend in the flashy, flying adult stage? Read on to find out.

What is a Firefly’s Life Cycle?

What is a Firefly’s Life Cycle

Fireflies begin their lives as eggs, which are laid in mid to late summer. Some firefly species lay these eggs on plant leaves, though most deposit them directly in the soil.

Some firefly eggs are bioluminescent, meaning they produce a faint glow. That said, they are usually buried under the soil, so few people ever see these glowing eggs.

Firefly eggs hatch after about three to four weeks. The larvae, often called glow worms, typically live in the soil.

Glow worms live throughout the late summer, fall, and winter months. Some will continue to live as larvae through the following year, surviving two winters as larvae before maturing the following spring.

In spring, the glow worms enter the pupal stage. Most pupate in the ground, though some climb onto trees and pupate upside down.

While in their pupas, the glow worms gradually morph into adult fireflies. The process can take anywhere from ten days up to several weeks, depending on the species.

Eventually, in late spring or early summer, the adult fireflies make their appearance. 

Check out this video to learn more:

How Long Do Adult Fireflies Live?

Adult fireflies can be seen filling summer evenings with twinkles of light. Like stars at ground level, these bioluminescent beetles add a touch of magic and wonder to the warmest months of the year.

But you may have noticed that the nightly light shows they produce never last more than a couple of months. Fireflies first begin making their appearance in late May through late June, and they have all but disappeared again by early August.

Why is that?

The truth is, adult fireflies have very short lifespans. Some live up to two months in the adult stage, while others may only live two or three weeks.

Again, this depends on species, as some species are much longer-lived than others. That said, even the longest-lived fireflies don’t spend more than a couple of months as adults.

Why Do Fireflies Die So Soon?

How Long Do Adult Fireflies Live

Like Setsuko in Grave of the Fireflies, you may be wondering: why do fireflies have to die so soon? What is the purpose of their short lives when their lights are extinguished so quickly?

Similar to many other insects, adult fireflies have one important task: to continue the species. Their entire life during the adult stage is geared toward this ultimate goal.

Fireflies use their glowing abdomens primarily to attract mates. Each species produces a different flash pattern, and each male within a species may develop his own individual pattern in an effort to attract the attention of a female.

Once the males and females have paired up, they mate. The act of copulation may last several hours in some species, and once they have finished, both the male and female may continue to mate with other partners throughout the breeding season.

This ensures the greatest chance of producing viable offspring.

Mating and laying eggs seems to be the sole task of adult fireflies; some have such a short lifespan that they don’t even eat while in the adult stage. They simply mate, lay eggs, and then die.

In this case, it’s possible that a lack of food is at least partially responsible for the firefly’s early death. But other fireflies do eat as adults, and they still die within a month or two.

Ultimately, it appears that fireflies simply aren’t designed to live long after mating and laying eggs. Like many insects, their bodies begin to break down, and the act of producing offspring is one of the last things they will do in life. 

So, it’s unclear exactly what causes fireflies to have such a short lifespan, but it is clear that they live long enough to do what they were born to do: make sure their species doesn’t go extinct.  


Fireflies spend most of their lives in the larval stage. Once they finally reach adulthood, they typically die within a few weeks to a couple of months. It would appear that their lives as adults last just long enough to allow them to mate and lay eggs; like most other insects, they die shortly after producing offspring.

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