This is probably a very stupid question, but I ask it in all seriousness and seek an answer because I haven't come across one in the brief introduction I've had by reading in books about fly fishing for trout.

When I was a small boy drowning worms to take bluegills, my dad advised me to thread the worm on the hook so that the hook was not shown to the fish. I dutifully did so. I'm not sure whether my dad actually believed this bit of lore anymore than he did his advice to spit on the worm before casting it. I followed that bit of advice also.

What bothers me now is understanding how "educated" trout become suspicious of indicators, and indicators of certain colors, brightly colored fly lines, flies a size too large, etc., but seem to ignore a fully evident hook on flies they take.

I understand that from their position of looking up at a dry, the hook might be less visible, but what about a nymph or wet tumbling slowly along near the bottom?

Thanks in advance for your answers.