Invertebrates
Railroad Worm
Phrixothrix hirtus · also called railroad worm, trem-de-ferro
The railroad worm is not a worm but a glowing beetle larva from the Americas, and on a dark night it looks exactly like its name. A row of yellow-green lights runs down each side of its body like the lit windows of a passing train — and at the front, on its head, sits a pair of lamps that glow red. No other land animal on Earth is known to make red light.
Overview
The glowing form is a soft, flattened, segmented larva or larva-like adult female, about 4–6 cm long, found in moist forest, grassland, and gardens from Mexico to Argentina. It is a predator, hunting millipedes and other soft-bodied invertebrates, which it subdues with a paralysing bite.
Two colours of light
Eleven pairs of yellow-green lights run along the body, and a separate pair of organs on the head glows red to orange. A disturbed railroad worm lights up like a train with headlamps.
The only red light on land
Bioluminescent red light is otherwise known only from a few deep-sea fish. The railroad worm’s head organs use a version of the light-making enzyme tuned to emit at a longer wavelength than its green organs. The green side lights seem to work as a warning to predators; the dim red head lights are thought to be a covert torch, letting the worm see its prey while staying invisible to animals that cannot detect red.
The male
Adult males are ordinary winged beetles that do not feed and barely glow. The female keeps the glowing, wingless body for life — a pattern called larviform.
Status
The railroad worm has not been assessed. The group is widespread and locally common across the Americas, though sensitive to habitat loss and light pollution.
Railroad Worm: common questions
Is the railroad worm actually a worm?
No. It is an insect, specifically the larva, or a wingless adult female that keeps a larva-like body, of a beetle in the family Phengodidae. The name comes from its appearance when it lights up at night — a line of glowing spots along each side of the segmented body looks like the lit windows of a passenger train, and in the species that also have head lights, the red glow at the front completes the illusion of a locomotive with its lamps on. It is a beetle relative of fireflies and click beetles rather than any kind of true worm.
Why can the railroad worm make red light when almost nothing else on land can?
Bioluminescence works by an enzyme, a luciferase, acting on a small molecule to release light, and the exact colour depends on the shape of the enzyme. Most bioluminescent land animals, including fireflies and the glowing side organs of the railroad worm itself, produce green to yellow light. The railroad worm's head organs use a slightly different version of the enzyme that shifts the emission into the red. On land this is unique. In the deep sea a handful of fish independently evolved red light for the same reason the railroad worm seems to use it, to see without being seen.
What are the two sets of lights for?
They appear to do different jobs. The green lights running along the body are switched on when the animal is disturbed and are generally interpreted as a warning signal, advertising to a predator that the railroad worm is distasteful or defended. The red head lights are much dimmer and are switched on while the worm is hunting. Because red light travels poorly and most nocturnal predators and the worm's millipede prey are insensitive to it, the head lamps may act as a covert torch, illuminating prey at close range without giving the hunter away.
Are railroad worms rare or endangered?
They have not been formally assessed, and the group as a whole is not considered threatened. Railroad-worm beetles occur widely across the Americas and can be locally common in moist forest, grassland, and even suburban gardens, where people occasionally find a glowing larva after rain. That said, like other bioluminescent insects they depend on dark nights and healthy soil and leaf-litter communities, so heavy habitat clearance, pesticide use, and artificial lighting can reduce local populations, and some specialists regard light pollution as an underappreciated threat to glowing insects generally.