Mass aggregations and migrations of millipedes despite numerous attempts to find causes for their occurrences are still an enigma. They have been reported from both southern and northern hemisphere countries, from highlands and lowlands of both tropical and temperate regions and they can involve species belonging to the orders Julida and Spirobolida, Polydesmida and Glomerida. According to the main suggestions put forward in the past, mass occurrences in Diplopoda occur: (1) because of a lack of food and a population increase beyond sustainable levels; (2) for the purpose of reproduction and in order to locate suitable oviposition sites; (3) to find overwintering or aestivation sites; (4) because of habitat disruption and changes in the local environment; (5) as a consequence of weather conditions the year (or winter and spring) before. A recent outbreak (November 2014) of a mass migration of the polydesmid
Chamberlinius hualienensis
Wang 1956 on the Japanese Izu Island of Hachijojima 300 km to the south of Tokyo gave this author an opportunity to review the existing literature on millipede mass migrations and to carry out additional observations on the phenomenon in the field as well as the laboratory. Hitherto unreported heavy infestations with phoretic deutonymphs of the mite
Histiostoma
sp. as well as dense populations of internal rhabditid nematodes (
Oscheius necromena and an unidentified species of the genus
Fictor
), suggest that infestations of this kind could be necromenic and either have been a contributing factor for the mass migration or been a consequence of so many individuals occurring together at close proximity. It is concluded that mass migrations and aggregations in millipedes do not have one common cause, but represent phenomena that often are seasonally recurring events and appear identical in their outcome, but which have evolved as responses to different causes in different millipede taxa and therefore need to be examined on a case-to-case basis.
