Subperineurial glial cells express PER. Close up of the optic lobe surface in brains of moody-GAL4>GFP flies. (A) Large GFP-positive nuclei of subperineurial glia (arrowheads) are PER-positive in the brains of young (5 day) flies at ZT22, but PER protein is almost absent in glia of old brains at ZT22, although PER is detected in the outer layer of perineurial glia not labeled by GFP (asterisks). Other PER-positive cells in young brains at ZT22 not marked by GFP are likely different types of glia. Subperineurial glia in young and old brains are PER-negative at ZT10. Scale bars equal 10 μm. (B) Brain outline indicating location of cells shown in (A) at the surface of the optic lobe. (C) Graph showing the average relative fluorescence of PER in subperineurial glia at ZT22. PER level is significantly lower in old brains (**p = 0.0031, t = 3.719, df = 11.56). Number of brains analyzed are shown within each bar; error bars indicate SEM.