24 MILLIPEDES 



Occasionally they have crossed railways and been squashed in 

 such numbers that locomotives have been impeded and sand has 

 had to be strev^n on the lines before their driving wheels would 

 grip. At other times cattle have refused to graze on invaded pas- 

 tures, wells have been filled with drowned corpses and workmen 

 cultivating the fields have become nauseated and dizzy from the 

 odour of millipedes crushed by their hoes. However, such mass 

 migrations are of rare occurrence and local in extent, so that their 

 net effect on distribution is probably negligible. 



The subject of migration in myriapods has been reviewed by 

 Cloudsley-Thompson (1949b; 1951a) and it has been suggested 

 that although certain aspects of the phenomenon are still not ex- 

 plained, the evidence lends support to the hypothesis that mass 

 migration is merely an extreme case of the more familiar sudden 

 attacks on crops due to extremely favourable local conditions, fol- 

 lowed by drought and possibly accompanied by abnormal physio- 

 logical conditions of reproduction. 



An explanation of the problem of how dispersal can take place 

 has been suggested as a result of recent work in which it has been 

 shown that millipedes are markedly nocturnal and show a diurnal 

 cycle of rhythmic activity. In O. gracilis and B. guttulatus this is 

 primarily a response to light and darkness, but is also correlated 

 with the stimulus of falling temperature in the evening. Aktograph 

 experiments on two large West African species of millipedes have 

 demonstrated an endogenous diurnal rhythm independent of fluc- 

 tuating light and temperature and persisting in Ophistreptus sp. up 

 to nineteen days. Locomotory activity is stimulated by increases or 

 decreases of temperature, and it is probable that in tropical forms 

 temperature fluctuations are of primary importance in the initia- 

 tion of diurnal rhythms. Perhaps in their natural gloomy habitat 

 in tropical forests, light is an insignificant environmental factor 

 (Cloudsley-Thompson, 1951b). Thus it is at night that millipedes, 

 like other myriapods and woodlice, are able to disperse themselves 

 and overcome the restrictions inherent in the physiology of their 

 integuments. 



Blower (in Kevan, 1955)* has suggested that millipedes are dis- 

 tributed at various depths in the soil according to their water- 

 relations and body forms. Nematophora and Polydesmoidea which' 



