Why are they called railroad worms?

Their lights look like lit train windows

How does a larva stop a millipede releasing its defence chemicals?

It paralyses it fast with digestive juice from its hollow jaws

What is the New Zealand glowworm?

The larva of a fungus gnat, a kind of fly

Why do medical researchers like the railroad worm's red enzyme?

Red light passes through blood-rich tissue better

It is the brightest light in nature

Glowworm beetle

Phengodidae

Glowworm beetles (family Phengodidae), also called railroad worms, are soft-bodied beetles found almost only in the Americas. Their larvae and wingless, larva-shaped adult females glow from pairs of lights along the body, while the small winged males have feathery antennae. Railroad worms of the genus Phrixothrix also carry a red light on the head: the only true red light known from a land animal.

Also known as Railroad worm, Phengodid beetle

Glowworm beetle abilities

11 pairs steady side lights
Railroad worms glow from a row of paired lights along the body. They shine steadily, like the windows of a night train, instead of flashing like many fireflies.
- Phrixothrix has 11 pairs of side lights and 2 head lights.
- The side lights glow green to yellow, peaking at about 535–565 nm.
- Females often glow nonstop while curled around their eggs. In some groups the males glow, in one scientist's words, “continuously and uncontrollably” all their adult life.
Flash code 12.6 s, repeats
flash 12 s · dark 600 ms
Sources [1, 3, 4]
628 nm reddest beetle light
Phrixothrix railroad worms carry a red light on the head. It is the only true red light known from any land animal.
- The head lights of Phrixothrix hirtus peak at about 628 nm in the live animal. Its relative P. vivianii has green side lights at 542 nm.
- Both colours use the same fuel, luciferin. The difference is the enzyme: head and side lights use different forms of luciferase.
- Exactly what the red light is for is still an open question.
Peak colour of the light
Railroad worm head light628 nm
Orange railroad worm (Euryopa)600 nm
Railroad worm side light542 nm
Australian glowworm (a fly)488 nm
Higher numbers mean redder light.
Sources [1, 2, 3, 8, 9]
2 hollow, sickle-shaped jaws
Glowworm beetle larvae hunt millipedes. They stop their prey so quickly that it cannot release its bad-tasting defence chemicals, then drink a liquid meal.
- In one study, a Phengodes larva pierced a millipede's skin with its hollow jaws and passed in digestive juice.
- The millipede stopped moving almost at once, so it could not release its benzoquinone defences.
- The larva then drank the softened insides. Only the hard outer skeleton and the sealed chemical sacs were left.
Sources [4, 5]
24 lights on a PhrixothrixStill debated
When disturbed, a glowworm beetle larva or female may curl up and glow. Scientists think the light warns night-time predators that it tastes bad, but this is not fully proven.
- Photos in a University of Florida guide show a Phengodes curled up after being disturbed, and curled up while glowing.
- Current evidence suggests the glow is a warning signal to nocturnal predators.
- Another idea is that switching the lights on suddenly may startle an attacker.
- A 99-million-year-old fossil relative had a light organ that probably served for defence too.
Sources [1, 4, 10, 11]
12 segments in each male antenna
Male glowworm beetles have big, feathery antennae. They use them to track the scent of a female through the night air.
- The antennae have comb-like branches on 12 segments.
- Males find females by following their pheromone, not their glow.
- Scientists found an expanded set of odour-binding genes in the genome of Phrixothrix hirtus.
Sources [2, 4]
3 vs 12 antenna segments, female vs male
Females never grow wings. They become adults but keep the soft, segmented shape of a larva, while males turn into small winged beetles.
- A female keeps larva-like antennae with 3 segments, larva-like legs with a single claw, and simple eyes instead of compound eyes.
- She can still be told from a larva: she has an opening for laying eggs near the tail.
- Males are brown to black beetles about 6–35 mm long, with short wing covers.
- Females are much larger than males.
Sources [4, 7]
15 in 100 reactions give off red light
Inside each light, cells mix a fuel called luciferin with oxygen and energy, helped by an enzyme called luciferase. The fuel is the same for every colour; the enzyme sets the colour.
- Railroad worms use the same luciferin as fireflies and click beetles, even though beetles seem to have evolved light more than once.
- Different forms of luciferase make the green side lights and the red head lights. The two enzymes are 71% identical.
- The red enzyme is less efficient: about 15 in every 100 reactions make light, compared with more than 40 for green beetle enzymes.
- Scientists use the red enzyme to make cells glow in medical research, because red light passes through blood-rich tissue better than green light.
Light made per 100 reactions
Red railroad worm enzyme15 %
Green beetle enzymes>40
Green beetle enzymes reach more than 40.
Sources [1, 3, 12, 13, 14, 15, 16]
53 eggs laid by one female
A mother glowworm beetle lays her eggs in a group on the ground and curls her body around them, often glowing as she guards them.
- One Phengodes female laid 53 eggs, in a record from 1906.
- Western banded glowworm eggs are about 2.75 mm across; most species have smaller eggs.
- The eggs glow too, though it can take up to a month after laying before they light up.
- She may stay curled around them until they hatch, but nobody is sure.
Sources [4]

Tap a pin on the model or a part below.

628 nm deep red light
- Railroad worms of the genus Phrixothrix have two small lights on the head that glow red.
- In Phrixothrix hirtus the red glow peaks at about 628 nm, the reddest light measured from any glowing beetle.
- It is the only true red light known from a land animal.
Sources [1, 2, 3]
11 pairs lights along the body
- Phrixothrix has 11 pairs of lights along its body: one light on each side of a segment.
- They glow green to yellow, at about 535–565 nm.
- In the dark they look like the lit windows of a train at night, which is how railroad worms got their name.
Sources [1, 3, 4]
Hollow sickle-shaped jaws
- Each jaw is curved like a sickle and has a channel running inside it from base to tip.
- The larva uses its jaws to pass digestive juice into its prey, which softens the prey's insides.
- Then it drinks the liquid meal.
Sources [4, 5]
- Larvae and adult females have tiny simple eyes with a single lens, not the big compound eyes most adult insects have.
- This is one of the clues that a female keeps a larva's body all her life.
Sources [4]
12 segments in a male antenna
- A female's antennae are short and larva-like, with just 3 segments.
- Most males have large, feather-like antennae with 12 segments and comb-like branches.
- Males use them to follow the scent (pheromone) a female gives off, not her light.
- The genome of Phrixothrix hirtus has an expanded family of odour-binding genes.
Sources [2, 4]
< ½ wing covers vs wings
- Only males have wings. Their hard front wings (elytra) are less than half as long as the hind wings.
- Males fly at night and are often caught at lights and light traps.
- In the western banded glowworm of California, males carry genes between areas, while the wingless females stay close to home.
Sources [4, 6]
15–65 mm full-grown larva
- The long, soft, segmented body looks like a larva's, even in adult females.
- A full-grown larva is 15–65 mm long, depending on the species. Females are much larger than males.
- An adult female can be told from a larva by a small opening for laying eggs near the tail end.
Sources [4, 7]
- Six short, larva-like legs, each ending in a single claw, carry females and larvae through soil and leaf litter.
- Larvae glow in wet soil, and in humid tropical forests they also climb on bark and leaves.
Sources [4]
- Glowworm beetle larvae are hunters that feed on millipedes.
- Many millipedes defend themselves with bad-tasting chemicals called benzoquinones.
- A Phengodes larva paralyses its millipede so fast that the chemicals stay locked in their glands. At the end, only the millipede's hard outer skeleton and its sealed chemical sacs are left.
Sources [4, 5]

Real glowworm beetles

Photos from Wikimedia Commons. Some of them can repaint the 3D model.

Female (railroad worm)Larva-shaped for life: a long, soft, segmented body with green-yellow lights along the sides and, in Phrixothrix, red lights on the head.Wingless adult · glows · lays eggsLights on, lights off: the two looks of a railroad worm. Photo: Aaron Pomerantz, CC BY 2.0
Female (railroad worm)Larva-shaped for life: a long, soft, segmented body with green-yellow lights along the sides and, in Phrixothrix, red lights on the head.Wingless adult · glows · lays eggsPhrixothrix hirtus, the red-light railroad worm, in Brazil. Photo: Thomas Fuhrmann, CC BY-SA 4.0
LarvaAlmost identical to the female, but with no egg opening. It grows to 15–65 mm before turning into a pupa.Hunts millipedes · glowsA Phengodes in Missouri: a female or a larva. Photo: Andy Reago & Chrissy McClarren, CC BY 2.0
Female (railroad worm)Larva-shaped for life: a long, soft, segmented body with green-yellow lights along the sides and, in Phrixothrix, red lights on the head.Wingless adult · glows · lays eggsFemale or larva? Experts check for the tiny egg opening. Photo: Benjamin Smith, CC BY 2.0
Western banded glowworm (Zarhipis integripennis)Photo: nancybeetoo, CC BY 2.0
Winged maleA small brown-to-black beetle, about 6–35 mm long, with short wing covers and big feathery antennae.Flies at night · follows scent · short-livedA winged male: note the feathery antennae. Photo: Insects Unlocked, CC0
European glow-wormA different beetle: the wingless female of a firefly, Lampyris noctiluca. She glows steadily at night to attract flying males.Firefly family (Lampyridae) · larvae eat snailsThe European glow-worm is a firefly, not a railroad worm. Photo: Tero Keski-Valkama, CC0
European glow-wormA different beetle: the wingless female of a firefly, Lampyris noctiluca. She glows steadily at night to attract flying males.Firefly family (Lampyridae) · larvae eat snailsEuropean glow-worm in the UK. Photo: AJC1, CC BY-SA 2.0

Glowworm beetle by the numbers

Measured by scientists in the field and in the lab.

Species: ≈250; 244 counted in 2023, almost all in the Americas⁠[2, 7]
Side lights: 11 pairs; in Phrixothrix, plus 2 red head lights⁠[1]
Head light colour: 628 nm; deep red: the reddest beetle light measured⁠[1]
Side light colour: 535–565 nm; green to yellow in Phrixothrix⁠[3]
Larva length: 15–65 mm; full-grown larva, depending on species⁠[4]
Male length: 6–35 mm; small winged beetles⁠[4]
Eggs: 53; laid by one Phengodes female (1906 record)⁠[4]
Time as a pupa: 12–35 days; western banded glowworm: 12–13 days for females, 20–35 for males⁠[4]; Female (longest)13 daysMale (longest)35 days
Genome size: ≈3.4 Gb; Phrixothrix hirtus: the largest known in its beetle superfamily⁠[2]
Red light efficiency: 15%; of reactions give off light, vs over 40% for green beetle enzymes⁠[14]; Red enzyme15 %Green enzymes40 %

Glowworm beetle myths and facts

The word glowworm causes a lot of mix-ups. Test what you know.

0 of 8 answered

“Glowworms are a kind of worm.”
“Every animal called a glowworm is the same kind of creature.”
“Railroad worms flash on and off like fireflies.”
“The railroad worm's red head light is the only red light made by a land animal.”
“Male glowworm beetles find females by following their glow.”
“Glowworm beetle larvae eat snails.”
“Male glowworm beetles never glow.”
“A female railroad worm is just a baby that never grew up.”

Glowworm beetle records

Small beetles, big superlatives. Each one is backed by a study.

Only red light on land
622–628 nm
Phrixothrix hirtus
The only land animal known to make true red bioluminescence.⁠[2, 1]
Widest range of light colours
Green to red
Railroad worms (Phengodidae)
Their enzymes make the widest range of colours of any beetle group, from about 542 nm green to 628 nm red.⁠[8, 24, 1]
Biggest genome in its superfamily
≈3.4 Gb
Phrixothrix hirtus
The largest genome described among the click beetles, fireflies and their relatives (Elateroidea).⁠[2]
Oldest glowing relative
≈99 million years
Cretophengodes azari
A male beetle in amber from Myanmar, with a light organ on its abdomen, linking glowworm beetles to harder-bodied relatives.⁠[11]
First orange railroad-worm enzyme
600 nm
Euryopa clarindae
In 2024 scientists described the first orange-glowing railroad worm luciferase, from males of a rare species of the Brazilian Cerrado.⁠[8]

The railroad worm's red light

Why scientists study one small Brazilian beetle's head lamp.

628 nm
reddest beetle light
Measured from the head lights of Phrixothrix hirtus. The side lights of a related species glow green at 542 nm.⁠[1]
71%
same enzyme recipe
The red and green luciferases share 71% of their building blocks. Small changes shift the colour.⁠[1]
15 in 100
reactions make red light
The red enzyme is dimmer than green ones, which manage more than 40 in 100.⁠[14]
9.8×
brighter in the lab
By changing a few building blocks, scientists made red enzymes up to 9.8 times brighter in cell extracts.⁠[23]
Red
passes through tissue
Blood and skin absorb red light less than green, so researchers use red-glowing enzymes to follow infections and disease inside living animals.⁠[15]

Railroad worm red light comes from the same luciferin fuel as a firefly's yellow-green; only the enzyme differs.

Life of a glowworm beetle

From a glowing egg to a winged male or a larva-shaped mother. Timings come from the few species that have been studied.

Egg
Laid in a group
A mother lays her eggs together on the ground, 53 in one record, and curls around them.⁠[4]
Up to 1 month
Eggs light up
The eggs glow, but it can take up to a month after laying before they become luminous.⁠[4]
Larva
Millipede hunter
The larva hunts millipedes in soil and leaf litter and grows to 15–65 mm.⁠[4, 5]
Pupa
The big change
In the western banded glowworm, a female spends 12–13 days as a pupa and a male 20–35 days.⁠[4]
Adult female
Larva shape for life
She stays wingless and larva-like, glows, and gives off a scent that draws in males.⁠[4]
Adult male
Short night flights
The winged male follows her scent through the dark. Males are short-lived and probably do not feed.⁠[4, 7]
Mother
Glowing guard
After mating she curls around her new eggs, often glowing continuously, perhaps until they hatch.⁠[4]

Railroad worm vs European glow-worm vs New Zealand glowworm

Three animals, one confusing name. Only the first is a glowworm beetle.

Railroad worm (Phengodidae) vs European glow-worm (Lampyris noctiluca) vs New Zealand glowworm (Arachnocampa luminosa)
	Railroad worm	European glow-worm	New Zealand glowworm
What it is⁠[4, 18, 21, 22]	A beetle (family Phengodidae)	A firefly beetle (family Lampyridae)	A fly: the larva of a fungus gnat (Keroplatidae)
Who glows⁠[4, 20, 17, 22]	Larvae, wingless females, and the males of some groups	Above all the wingless adult female	The larva
Light colour⁠[3, 1, 18, 25, 9]	Green-yellow sides (535–565 nm), red head (600–628 nm) in Phrixothrix	Green, a little bluer than an American firefly's	Blue-green (about 488 nm in an Australian relative)
Why it glows⁠[4, 17, 22]	Thought to warn predators it tastes bad	To attract flying males	To lure flying insects to its sticky threads
Larva's food⁠[4, 5, 19, 26, 22]	Millipedes	Snails	Small flying insects caught on sticky silk lines
Where⁠[7, 20, 17, 25]	The Americas, from southern Canada to Chile (a few in Turkey and the Middle East)	Europe	New Zealand
Light chemistry⁠[3, 18, 25]	Firefly-type luciferin	Firefly-type luciferin	A completely different luciferin

Glowworm beetles and European glow-worms are both beetles, yet genome studies suggest beetles evolved light more than once.

Are glowworm beetles endangered?

Nobody knows yet. Glowworm beetles are rarely seen, many species have not even been named, and none has been assessed for the IUCN Red List.

IUCN Red List

Not Evaluated (NE)

Trend unknown

No glowworm beetle species has an IUCN assessment, so how they are doing is unknown.⁠[27, 4]

Not on the IUCN Red List (checked October 2026)

Threats

Forest loss>80%
Many Brazilian railroad worms live in the Atlantic Forest and Cerrado. More than 80% of the Atlantic Forest's remaining patches are smaller than 50 ha.⁠[20, 28]
Light pollution15 nights
Artificial light at night harms many insects. In the European glow-worm, females near lights glowed for up to 15 nights without finding a mate.⁠[29, 30]
Too little known≈100
Around 100 species may be waiting in museum collections to be named. It is hard to protect animals nobody has described.⁠[4]

What’s helping

Darker nights
Switching off or dimming outdoor lights helps night insects. In glow-worm experiments, red and yellow lamps disturbed mating less than white or blue ones.⁠[29, 31]
Naming and studying
Scientists are still describing species and using DNA to match larvae with their adults, which had been done for only one Brazilian species before 2025.⁠[20]
Look, don't take
Females cannot fly and move only short distances, so local groups depend on the females staying where they are.⁠[6]

Glowing relatives

European glow-worm (Lampyris noctiluca)Near Threatened
Listed as Near Threatened. Artificial light makes it harder for females to attract mates.⁠[27, 17, 30]
North American firefliesVaries by species
Of 132 species assessed, at least 18 (14%) are threatened, mainly by habitat loss, light pollution and climate change.⁠[32]

Glowworm beetle quiz

12 questions. Every answer is somewhere on this page.

Question 1 of 12Score 0

What kind of animal is a glowworm beetle?

Glowworm beetle field notes

Diet, food web, safety and badges from the Snappit field guide.

Diet & Feeding

Glowworm larvae are tiny hunters. As far as scientists know, they hunt millipedes.

Primary Foods

Millipedes

Age differences: Adult males generally do not eat, while larvae are active predators.

Ecological connections

American MillipedeHunts
Glowworm larvae prey on these millipedes on the forest floor.
Masked ShrewPossible predator (unconfirmed)
Shrews might eat glowworm larvae that live in leaf litter, but this has not been confirmed.
American RobinPossible predator (unconfirmed)
Ground-feeding birds like robins might eat glowworm larvae, but this has not been confirmed.

Safety

Danger 1/5 · Very low

Avoid touching or disturbing glowworms in their natural habitat to ensure they can continue their important role in the ecosystem.

Traits

Collections

Insects

Classification

Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Coleoptera
Family: Phengodidae

Sources for these glowworm beetle facts

32 studies and references · checked Oct 16, 2026

Frequently Asked Questions

How to identify Glowworm beetle?

Look for hard wing covers meeting in a line down the back, then compare the antennae, body shape, legs and markings. Metallic colors shift a lot with lighting. Beetle Identifier: upload a photo and compare the visible details.

How big is Glowworm beetle?

6-65 mm

How long does Glowworm beetle live?

200-700 days

What does Glowworm beetle eat?

Glowworm larvae are tiny hunters. As far as scientists know, they hunt millipedes.

Where is Glowworm beetle usually found?

Forests and dry areas

How many legs does Glowworm beetle have?

What is the difference between a glowworm and a firefly?

Glowworm is a name, not one animal. Glowworm beetles of the Americas (Phengodidae) are their own beetle family. The European glow-worm is actually a firefly whose females have no wings. The New Zealand glowworm is a fly larva.

Why is it called a railroad worm?

Its rows of glowing spots along the body look like the lit windows of a train at night.

Why do glowworm beetles glow?

Scientists think the glow warns night-time predators that they taste bad. Males find females by scent, not light, so the glow is probably not a love signal.

What do glowworm beetles eat?

The larvae and larva-shaped females hunt millipedes, paralysing them quickly with digestive juice from their hollow jaws. Adult males are short-lived and probably do not feed.

Where do glowworm beetles live?

Almost all live in the Americas, from southern Canada to Chile, in places from dry parts of California to tropical forests. A small group lives in Turkey and the Middle East.

Do male glowworm beetles glow?

Usually not, but males in the group that includes Phrixothrix glow all their adult life, and Stenophrixothrix males give off yellow light.

Are glowworm beetles dangerous?

There is no sign that they are a danger to people: their jaws are made for millipedes. Still, don't pick them up. When handled, a railroad worm larva may release a reddish liquid that may taste bad or irritate. Watch without touching, and wash your hands if you do touch one.

How can I see glowworm beetles in the wild?

Look on warm, damp nights on soil and leaf litter. Use a dim red light, which disturbed glow-worm mating less than white or blue light in experiments. Watch without touching and leave them where they are.

Take the discovery with you