Neuropil glia consists of two morphologically distinct subtypes, the ensheathing glia and astrocytes. Ensheathing glia were visualized via mz0709-GAL4 (Ito et al., 1995) or NP6520-GAL4 (Awasaki et al., 2008) drivers combined with UAS-GFP. PER was detected in GFP-positive cells in both lines; however, mz0709-GAL4 has been reported to drive expression also in the subperineurial glia (Dutta et al., 2016); therefore, NP6520-GAL4>GFP flies were used for PER signal measurement. GFP-positive cells were observed at the border between cortex and several neuropil compartments in the central brain (Figures 4A,B). At ZT10, these ensheathing glial cells were negative for PER signal in both young and old brains (not shown); however, many of these cells were PER positive at ZT22 in both ages (Figure 4A). The intensity of PER signal was variable from brain to brain and cell to cell and there was no significant difference in average PER signal between young and old brains (Figure 4C, p-value = 0.8452, t-value = 0.2017, df = 7.961).