SummaryBioluminescence — the ability of organisms to emit light — has evolved about 40–50 times independently across the tree of life [1]. Many different functions for bioluminescence have been proposed, for example, mate recognition, prey attraction, camouflage, and warning coloration. Millipedes in the genus Motyxia produce a greenish-blue light at a wavelength of 495 nm that can be seen in darkness [2]. These detritivores defend themselves with cyanide, which they generate internally and discharge through lateral ozopores [3]. Motyxia are an ideal model system to investigate the ecological role of bioluminescence because they are blind, thus limiting their visual signalling to other organisms, for example predators. While the biochemical mechanisms underlying Motyxia bioluminescence have been studied in detail [2,4], its adaptive significance remained unknown [5,6]. We here show that bioluminescence has a single evolutionary origin in millipedes and it serves as an aposematic warning signal to deter nocturnal mammalian predators. Among the numerous examples of bioluminescence, this is the first field experiment in any organism to demonstrate that bioluminescence functions as a warning signal.Video AbstractThe editors of Current Biology welcome correspondence on any article in the journal, but reserve the right to reduce the length of any letter to be published. All Correspondence containing data or scientific argument will be refereed. Queries about articles for consideration in this format should be sent by e-mail to cbiol@current-biology.com