Presence of circadian rhythms in the locomotor activity of a cave-dwellingmillipede Glyphiulus cavernicolus sulu (Cambalidae, Spirostreptida)

The locomotor activity of the millipede Glyphiulus cavernicolus (Spirostrep tida), which occupies the deeper recesses of a cave, was monitored in light -dark (LD) cycles (12h light and 12h darkness), constant darkness (DD), and constant light (LL) conditions. These millipedes live inside the cave and are apparently never exposed to any periodic factors of the environment suc h as light-dark, temperature, and humidity cycles. The activity of a consid erable fraction of these millipedes was found to show circadian rhythm, whi ch entrained to a 12:12 LD cycle with maximum activity during the dark phas e of the LD cycle. Under constant darkness (DD), 56.5% of the millipedes (n = 23) showed circadian rhythms, with average free-running period of 25.7h +/- 3.3h (mean +/- SD, range 22.3h to 35.0h). The remaining 43.5% of the mi llipedes, however, did not show any clear-cut rhythm. Under DD conditions f ollowing an exposure to LD cycles, 66.7% (n = 9) showed faint circadian rhy thm, with average free-running period of 24.0h +/- 0.8h (mean +/- SD, range 22.9h to 25.2h). Under constant light (LL) conditions, only 2 millipedes o f 11 showed free-running rhythms, with average period length of 33.3h +/- 1 .3h. The results suggest that these cave-dwelling millipedes still possess the capacity to measure time and respond to light and dark situations.