Opposite changes in the resistance to cold and desiccation, which occur during the development of the millipede Polydesmus angustus (Diplopoda: Polydesmidae)

Abstract

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.

Keywords

• diplopoda
• polydesmus angustus
• arthropod egg
• supercooling
• cold hardiness
• desiccation resistance
• life cycle