Of the 12,000 known millipede species, only eight are known to glow in the dark. All eight belong to the genus Motyxia and live in three counties in California. They don’t glow for each other, though—these millipedes are blind.
To test whether the nocturnal arthropods are glowing for another reason, scientists at the University of Arizona and elsewhere collected 164 living M. sequoiae from Giant Sequoia National Monument and painted half to conceal their glow. They also created 300 clay millipedes and painted half of them with a luminescent pigment. They then left their millipede collection out overnight, distributing them randomly along a line and tethering the live ones to the ground.
When they returned the next morning, “it was just carnage,” said lead researcher Paul Marek. “We were really surprised at the predation rate on these millipedes. Overall, about one-third of them—both real and fake—had been attacked.”
Luminescent millipedes were attacked less than half as often as their dark counterparts. Rodents, likely southern grasshopper mice, inflicted most of the bite marks.
The glowing, greenish-blue light is probably a warning to them: When blind millipedes are disturbed, they generate a hydrogen cyanide toxin. Most species display a warning color—yellow, orange or red. Motyxia millipedes, however, instead glow.
The study appears in Current Biology.