[stephen f. polyak]

Boy, we're having a run with this one! In my opinion, David nailed it - it's for ventilation. Think of the greenhouse effect and imagine sitting in that small cockpit, suited with flying kit, on a hot/sunny day and add engine heat! To gain some air circulation and temperature relief, just slide the upper panel open; adjust it as desired with altitude and airspeed. The upper panel could also allow fumes/smoke to escape in an fire emergency. (A possible disadvantage is that the fire may then also “breath” and intensify.) Later models of the 109 with better closed-canopy ventilation, pressurization, or improved blown canopies eliminated the upper sliding panel. Any possibility a like panel was fitted to early Bf 110s (hinged canopy)? As noted, a/c with a progressive sliding canopy (e.g., Fw 190) would not need such a feature.