Weird: I was just thinking about the same thing last night as I was tying up some golden stone nymphs with the usual light belly, dark top. Stands to reason that an upside-down insect would look more vulnerable and thus trigger a stronger response. This might be even more true of an upside-down baitfish (anyone ever seen a dead or dying fish float right-side-up?). As with all aspects of fly-tying, someone has probably already written a book about this...