INDIVIDUAL VARIATION IN THE SEASONAL REPRODUCTION OF THE TERRESTRIAL ISOPOD ARMADILLIDIUM-VULGARE LATR, (CRUSTACEA, ONISCIDEA)

Under particular conditions of photoperiod and temperature, Armadillid ium vulgare females, originating from a single population, might exhib it individual differences in the onset of reproduction and duration of the breeding period. In a population issued from a strain from middle latitudes, some females underwent only one parturial moult (northern tendency) and others three parturial moults (southern tendency). Femal es with an atypical northern phenology are the most numerous and tend to be found near the Danish population. In the latter, there is an asy mmetrical response to laboratory selection (favourable to females with a longer breeding period). The asymmetrical variation in atypical ind ividuals acts as a safety device against the unpredictability of the e nvironment. The adaptation of this species, originally from the Medite rranean periphery, to a northern environment has led to a reduction in its capacity to breed over long periods of time. Populations from mid dle latitudes can undergo one or several parturial moults which enable s the species to successfully colonize even far-away countries. These intrapopulation differences have an essential role and explain why Arm adillidium vulgare is one of the most widely distributed species among Oniscidea. (C) Elsevier, Paris.