They don't really look like much of anything. And yet, the "#18 pale yellow bodied Wet with a grey mallard wing & sparse partridge collar" mentioned above is an absolute killer during spinner falls of sulfurs and PMDs (although I personally use a light dun hackle on that particular fly.) And it usually has to be dead drifted, so I'm pretty sure it's not being taken as an emerger. It's one of those occasions where a wing wet outfishes a wingless one.