Yellow-spotted millipedes (Harpaphe haydeniana) are found in most coastal forests from Southeast Alaska to Monterey California. They are often associated with redwood forests. Found in leaf litter, in soil and under large moist logs, these millipedes break down organic matter and are important in returning nutrients to the soil.
These flat-backed (polydesmidan) millipedes are dark colored with yellow-tipped keels (the outer edges of each segment). Mature yellow-spotted millipedes grow between 1.6 and 2 inches and have about 20 segments. Males have 30 pairs of legs and females have 31 pairs of legs. The difference in leg number is because in males one pair of legs is modified into gonopods or appendages that facilitate the transfer of sperm to the female during mating.
When threatened, yellow-spotted millipedes exude hydrogen cyanide as a defense. Shrews, other small mammals and insects find the hydrogen cyanide toxic. The hydrogen cyanide smells like almonds and gives H haydeniana two of its common names, cyanide millipede and almond-scented millipede. The distinctive coloration of yellow-spotted millipedes acts as a warning signal to predators of their toxicity (aposematic coloration) However, one ground beetle, Promecognathus laevissimus, does not find yellow-spotted millipedes toxic.
Yellow-spotted millipedes also exhibit bioluminescence and glow a blueish color in the dark. The common name, night-train millipede, refers to this bioluminescence.
I do not know the derivation of another colloquial name, clown millipede.
There is another millipede (Harpaphe pottera) that morphologically is so close to H haydeniana that they can only be distinguished by examination of the gonopods. This could be H pottera, but I will assume it is the more widespread yellow-spotted millipede.
These yellow-spotted millipedes were along the Myrtle Creek Trail in the Smith River National Recreation Area (DelNorte County CA). Now I am anxious to go back and try to see these millipedes glowing at night.