Fig. 2. Pattern of pH changes in the upper vas deferens (UVD) lumen of Spodoptera littoralis. (A) Under a 16 h:8 h L:D cycle, rhythmic changes are detected in the luminal pH, with significant acidification of the UVD occurring in the middle of the dark phase. (B) The robust pH rhythm continues on the second day in constant darkness. (C) The rhythm is abolished in constant light. Each data point represents the pH measured in 10 individuals. Values are means ± S.E.M. The horizontal bars represent the day (open), night (filled) and former day (hatched) portions of the photo-regime.