Parafontaria

Parafontaria is a genus of "flat-backed" millipedes (order Polydesmida) consisting of 13 species native to Japan, where they are referred to as train millipedes. This is because some species exhibit periodical swarming behavior during which large numbers congregate and can impact train passage when this congregation occurs on tracks. Documentation of this event goes back to 1920. Individuals vary from around 3.5 to 6 cm (1.4 to 2.4 in) as adults, and feed on leaf litter as well as soil, making them comparable to earthworms in facilitating decomposition and soil nutrient cycling.