Cortex glia surround neuronal cell bodies that reside underneath the surface glia. To visualize cortex glia cells, we used two drivers, NP2222-GAL4 or NP577-GAL4 (Awasaki et al., 2008) combined with UAS-GFP. GFP-positive cells were abundant in both young and old flies having a mesh-like appearance as described previously (Awasaki et al., 2008). Given their large population and prolific distribution in the cortex, we focused on a small subset of GFP-positive cortex glia in the vicinity of the dorsal lateral pacemaker neurons (Figures 3A,B). PER signal was detected in both cortex glia lines at ZT22 but not at ZT10 similar to the circadian expression of PER in the lateral neurons (Figure 3A). The relative level of PER signal in cortex glia in brains of NP2222-GAL4>GFP flies was much lower than in the lateral neurons located nearby, but was present at ZT22 and not at ZT10 (Figure 3A). Analysis of PER signal in cortex glia in this region showed that the average PER levels were significantly reduced in old fly brains compared to young (Figure 3C, p-value ≤ 0.0099, t-value = 3.784, df = 5.743).