Cohort-splitting in the millipede Polydesmus angustus (Diplopoda: Polydesmidae): No evidence for maternal effects on life-cycle duration

Under seasonal conditions, Polydesmus angustus individuals born in the first part of the breeding season have a 1-year life cycle and those born later have a 2-year life cycle (cohort-splitting). In this study, 249 juveniles from four early broods (born in midJuly) and four late broods (born in September) were reared under similar laboratory conditions, to test for possible maternal influences on life-cycle duration. Development times of earlyand late-born individuals were compared under four combinations of day length and temperature (16 h – 18°C, 16 h – 16°C, 12 h – 18°C and 12 h – 16°C). The results showed that development time varied significantly in response to day length, temperature and sex, but that of individuals in the early and late broods did not differ significantly (mean development times ± SE: 180 ± 6 and 183 ± 8 days, respectively). There were no significant interactions between birth period and other factors, indicating that the effects of day length, temperature and sex on development time were similar in earlyand late-born individuals. This indicates that the extended life cycle of millipedes born late in the season is not maternally determined and that cohort-splitting is controlled entirely by the environmental conditions experienced by the offspring during their development. This conclusion is supported by the absence of significant variation in offspring live weight at birth measured at different times in the breeding season. The results are discussed in relation to the bet-hedging theory, which is often put forward to account for cohort-splitting in arthropods. In P. angustus, the results are consistent with either bet-hedging or adaptive plasticity, but further studies are required to decide which interpretation is correct.