First terrestrial bioluminescent animals to emit red light

The first terrestrial bioluminescent animals known to emit red light are the railroad worms, i.e., the worm-like larvae and adult worm-like females of phengodid glow-worm beetles belonging to the New World genus Phrixothrix, typified by Brazil's P. hirtus. Like all other bioluminescent beetle species (which number over 3,000), railroad worms emit greenish light (peak wavelength = 546–570 nm), in their case by way of 11 pairs of light-emitting organs running along most of their thoracic and abdominal segments. These give them a superficial resemblance to lit-up train carriages, hence their common name. However, uniquely among land-based creatures, they also emit intense ruby-red light (620–638 nm), via a single pair of light organs on their head.

Researchers believe that the railroad worms' lateral light organs, emitting green light, are used for defensive purposes, i.e., warding off potential predators. Conversely, it has been suggested that their unique red light-emitting cephalic light organs may be utilized for providing the railroad worms with exclusive illumination when hunting prey, because not all animals can see red light, so would not realize that they are being illuminated and thus exposed to the vision of the railroad worms.

Interestingly, in the deep sea, certain bioluminescent fishes, notably the black dragonfishes, emit red light that cannot be detected by their prey, thereby camouflaging these predatory fishes when seeking prey.