I have pike flies that weight more than most of my lures.Good question. Some flies are actually also called lures depending on how they are fished.
But if I take your meaning, I'd guess a fly would become a lure when it becomes too heavy to cast with a fly rod.
True true! And I have flies that I can cast with my 10 weight that wouldn't budge with a 5 weight. I think maybe we could simply say that a fly IS a lure, intended for use with a fly rod. But then there are flies that as you said have spinners etcetera that are best used with spinning gear.I have pike flies that weight more than most of my lures.
It would depend on your definition of a lure and a flies, LOL. A lot of streamers can be called lures, even though they don't have the action of a lure you would cast with your spinning rod. Then there are flies that are tied with spinners, grub tails and some that are tied to look and move like lures. This is a topic than can go on and on.
Hi;If you spend time down south you will be asked "whats ya useing for bait?" It applies to everything you use from a live minnow to a fly. I use salt water flies and there isn't a natural part to the fly. I quess if I can cast it with a fly rod it is a fly! Or a bait or a lure. The only worry is where fishing is restricted to a fly.