THE LIFE-CYCLE OF THE MILLIPEDE POLYDESMUS-ANGUSTUS - ANOTHER CASE OFCOHORT-SPLITTING

The life cycle of Polydesmus angustus in the Centre of France is descr ibed on the basis of data collected both in the field and in culture. The analysis of the stadial composition in litter and soil samples has shown that most individuals from a forest population complete their l ife cycle in two years and breed in late spring. Results from pitfall trapping have confirmed that but have also suggested that some individ uals could develop and breed in one year, cohorts splitting into slow and fast groups each year. Cultures made it possible to check the foll owing points: (i) individuals from a same nest, kept in the same tempe rature conditions, can actually reach maturity either in one year (spr ing) or in more than a year (autumn); (ii) whether maturity is achieve d in spring or in autumn, adults usually breed in spring and early sum mer and they die after reproduction; (iii) all females have an extende d period of oviposition and lay their eggs in several nests, but those which overwinter as adults begin to breed earlier in spring than thos e which overwinter as subadults. The discussion deals with the proxima te factors that could be involved in the process of cohort-splitting.