1) It was noted that the larvae of the Dutch strain of Asellus aquaticus have different rates of development to a given body size and sexual differentiation according to the season in which they are born. This is independent of temperature (which was always above 18°C), crowding, and the quality and quantity of food. The determinant appears to be instead the number of hours of day light. Those born in the summer semester require less than half the time of those born in the winter, and these latter are characterized by strong variability, ranging from a minimum of 31 days to a maximum of 169. 2) The existence is suggested of genotypes sensitive and insensitive to the reduction of the hours of light. When the daily hours are greater than 12, all larvae develop in a short time (average: 42 days), with little deviation from the average. When the hours of light decrease less than 12 daily, the insensitive genotypes continue to develop, while the sensitive genotypes double or triple the time required to reach a given body size and sexual differentiation. 3) One can attribute to the different rate of development a double ecological and evolutionary importance in that it appears to be correlated a) with invernal reproductive stasis and b) with the different body sizes reached by different individuals of the same family. In fact the stasis makes indispensable for Asellus, born in August, to reach sexual, maturity before the period of sexual inactivity occurs. The difference in body size diminishes the probability of the brother-sister mating in that period of the year in which that breeding would have the highest probability of occurence due to the fact that the population is reduced to a few individuals by invernal stasis and the short life span of Asellus aquaticus.