PER is weakly express in GFP-positive cortex glia. Images of the region containing PER-positive dorsal lateral neurons (arrows) and GFP-positive cortex glia (arrowheads) in brains of NP2222-GAL4>GFP flies. (A) In comparison to neurons, very weak PER staining is observed in GFP-positive cortex glia at ZT22 in young flies and is further reduced in old flies. No PER is detected in young or old brains at ZT10. Scale bars equal 10 μm. Right: enlarged images of the outlined regions show weak but discernible PER signal in cortex glia in young and old brains at ZT22. Scale bars equal 2 μm. (B) Brain outline indicating location of cells shown in (A) in the lateral region of the brain. (C) Graph showing the average relative PER fluorescence in cortex glia at ZT22. PER level is significantly lower but still detectable in old brains (**p = 0.0099, t = 3.784, df = 5.743). Number of brains analyzed are shown within each bar; error bars indicate SEM.