First-stadium juveniles of Polydesmus angustus born each month from May to September were reared throughout their life cycle under controlled seasonal conditions. At maturity, the reproductive patterns of 62 females were studied individually. It was confirmed that females born from May to August have a 1-year life cycle and those born from late August onwards a 2-year life cycle (cohort-splitting). A third type of life cycle - interseasonal iteroparity - was observed in a few females born late in the season. On average, annual females started to reproduce when 11.4 months old and produced 3.6 broods per female over 1.8 months; the later they were born from May to August, the later they reproduced the following year. Biennial females started to reproduce when 19.9 months old and produced 3.8 broods per female over 2.2 months; all reproduced early in the breeding season. These results indicate that only annual females can produce an appreciable proportion of biennial offspring from late August onwards, which rules out direct genetic determination of life-cycle duration. The reproductive characteristics of P. angustus suggest a non-genetic mechanism that can drive cohort-splitting. Because individual females reproduce for about 2 months on average, this automatically results in cyclic variation in life-cycle duration (annual/biennial/annual) in the long-term progeny of any female.
Supercooling point (SCP), survival at low temperatures, rate of water loss in dry air at 20°C and survival under desiccating conditions of eggs of Polydesmus angustus (Diplopoda) were determined. The results were compared with those obtained previously for the eight post-embryonic stadia, to obtain an overview of the changes in resistance to cold and desiccation throughout the species' development. The SCP temperatures of egg batches ranged from -14.8 to -30.6°C and were significantly lower than those of the active stadia. Eggs were not affected by prolonged exposure to low temperature above 0°C and survived much better than active stadia when cooled to -6 and -10°C. This indicates that the cold hardiness of P. angustus is highest in the egg stage and decreases during development. On the other hand, the rate of water loss was significantly higher from eggs than from active stadia. When eggs were taken out of their protective nest, they lost water at the high rate of 7% min-1 in dry air. They also survived for a shorter time than active stadia at 76% RH and 20°C. The resistance to desiccation of P. angustus is lowest at the egg stage and increases during development. The results suggest that the life cycle of P. angustus may have responded to selection pressures other than cold and drought, and do not support the hypothesis that cold hardiness and resistance to desiccation are overlapping adaptations in terrestrial arthropods.