Glowing spots seen on the ground Saturday night (click photos to enlarge)

On Saturday night a couple of friends and I went on an impromptu night hike down the road to see what we might see (plus to help work off a large meal). It was fairly warm for early November but there was not a lot of activity other than a distant Barred Owl calling and the reflected light of what seemed like hundreds of spider eyes in the grass along the road. On the way back, I was ahead of the others and was inspecting some chewed leaves on a low branch when I noticed something on the ground. It was a slight glow in the grass.

Firefly larva

I turned my light on it and found a firefly larva, also called a glow worm. Since there are several other glowing creatures around the world commonly called glow worms, I prefer to call these lightning bug or firefly larvae.

Firefly larva top view

At first glance, they remind me of a short millipede without all the legs, or maybe a large sow bug. The dorsal segments look like overlapping plates that extend to the back and sides. Larvae emerge from eggs in late summer, and live through the winter before pupating in the spring. In some species, the larval stage lasts two years.

Light organs on underside of eighth abdominal segment of firefly larva

Firefly larvae in the family Lampyridae all have a light organ on their eighth abdominal segment which appears as two white spots. When they are crawling about, the light is diffuse and causes a soft glow on the ground. When you flip them over, the light looks like two tiny glowing eyes (the first photo shows two larvae on their backs). And in some species the eggs are also bioluminescent.

Light from the photic organ on the underside of a firefly larva

Adult fireflies use their flashing lights to attract mates. But the slow changing glow of the larvae must have another function. The light produced by the larvae is also a different color than that from adults – it tends to be shifted more to the green spectrum. Like the adults, larval fireflies contain some toxic compounds and are distasteful. Since the larvae are active primarily at night, it is believed that bioluminescence originated as a warning display, suggesting to predators that they are unpalatable. Research has also shown that the eyes of certain predators of larval fireflies are more sensitive to the green region of the light spectrum.

Firefly larva consuming snail

Firefly larvae live in the soil and leaf litter and crawl about at night hunting slugs, snails, earthworms, and other invertebrates. On a museum workshop in the Smokies several years ago, I came across a large firefly larva with its head inside a snail shell, presumably consuming the occupant. They apparently inject their prey with toxins that also serve as digestive enzymes, essentially liquefying the tissues.

Turns out that all of us on the hike had spotted the tiny night lights at about the same time. Walking back, we kept finding more of the faint glimmers in the grass. It was a great moment of shared wonder.  As always, when you take the time to look around your surroundings, nature has a way of putting on an amazing show, including an eye-catching glow.

Firefly larva with head outstretched

We are all worms, but I do believe I am a glowworm.

Winston Churchill