Studies in diurnal rhythms. I. Rhythmic behaviour in millipedes.

In recent years the phenomenon of 24 hr. periodicity in animals has attracted considerable attention. Diurnal rhythms have been reviewed by Welsh (1938), Park (1940) and Calhoun (1944), and present many interesting physiological and ecological problems. It is evident that they are common and manifest in diverse ways. Park, Lockett & Myers (1931), in a short study of nocturnal ecology among forest animals, including the millipede Polydesmus serratus, reached some important conclusions: the nocturnal species have a definite rhythm of activity and are inactive by day. The maximum activity is usually in the middle of the night, and tends to increase with increasing relative humidity and decreasing air temperature and rate of evaporation. Conversely, activity decreases with decreasing relative humidity, increasing temperature and evaporation rate. Nocturnal activity was later investigated by means of an aktograph with the large tropical Spirobolus marginatus (Park. 1935), and it was found that closely marked cycles of activity persisted during starvation for periods up to 18 days, showing that the activity was not associated with feeding periods or the regular occurrence of hunger rhythms. Although the millipedes were kept in darkness at constant temperature and humidity, the general character of the rhythm did not change, the animals being preponderantly active at night. Therefore Park concluded that Spirobolus has an 'inherent nocturnal rhythm'. In his 1940 paper the term 'endogenous activity' was used, and such rhythms were divided into 'inherent' (i.e. genetical) and 'habitual' ones. Calhoun (1944) considered all endogenous rhythms to be of the latter category because the genetical determination of 24 hr. rhythm had in no case been demonstrated.