Another group of glial cells marked with GFP via NP6520-GAL4 driver was observed in the medulla segment of the optic lobe (Figures 5A,B). Based on their position and large oblong nuclei, these cells appear to represent the giant glial cells of the medulla (Tix et al., 1997). Cell nuclei were PER negative at ZT10, but PER was detected at ZT22 albeit with somewhat variable intensity from cell to cell (Figure 5A). Nevertheless, the average intensity of PER signal was significantly reduced in these cells in the brains of old files compared to young (Figure 5C, p-value ≤ 0.0100, t-value = 3.056, df = 12) indicating that PER levels in these glial cells are reduced as the function of age similar to other glia types discussed above.