Yes, you can fish dry flies when it is getting so dark that you can not see the fish take your dry fly. Since you must be aware of the strikes in order to set the hooks the easiest way to do this is to fish down and across stream with a tight line so you can feel the strike. I do this when fishing for smallmouths during the white fly hatch, hexagenia hatch and caddisfly hatch. It also works for trout on these hatches if the stream is large enough for a downstream approach not to scare the trout. It’s a great trout technique with the Neversink Skate like Ed Shenk’s ties.