A railroad worm is a larva, or larviform, female adult of a beetle of the genus Phrixothrix in the family Phengodidae, characterized by the possession of two different colors of bioluminescence. It has the appearance of a caterpillar. The eleven pairs of luminescent organs on their second thoracic segment through their ninth abdominal segment can glow yellowish-green, while the pair on their head can glow red;[1] this is due to different luciferases in their bodies, as the reaction substrate, called luciferin, is the same.