Abstract
The water spider Argyroneta aquatica (Argyronetidae) is the only spider that lives under water. We conducted observation on the time budget of daily activity and some life history characteristics of this spider under rearing condition. All adult males died until August, but adult females were alive until next year and produced three or four egg sacs throughout their life. Females protected her egg cases after oviposition. Juveniles were observed to get out of their egg sacs about 3 days after hatching, and dispersed by swimming, not by ballooning. Juveniles matured next Spring. The water spider seems to complete their life within a year. The activity of the spider was recorded with a video. Juveniles and adult females acted mostly at night, but they spent most of the time in the air dome. Consequently, the proportion of time spent for activity to the total observation time was less than a few percent. In contrast, the activity of adult male was observed frequently also in the daytime.
