other mysteries. In the modern day (let's say 1900 - present), there have been several theories about what triggers a trout to hit a fly. Imitation, impression and presentation are the usual suspects, although other theories abound too. In another thread, it's noted that there have been 'scientific studies' done by fly fishers and that's true. The problem with every one of those studies is in any conclusion the study may have made. There's not one that has a conclusion that can be proved because no one can duplicate the experiment or create a 'control group' upon which an experiment can be created. Oh, and by the way, if any fly fisherman had been able to decipher the mystery of 'trigger' what would be the fun and why bother to create flies that didn't cause this effect? No two waters are the same and the conditions of any one water cannot be duplicated. Someone once wrote, "You can never step into the same water twice".
What triggers a trout in a slow moving limestone creek, a shallow slow moving crystal clear stream, a fast tumbling clean river or waters of other conditions, as well as the overall climate of any particular day can change at the drop of a hat. Or more realistically, as a large cloud passes over.
As a specific statement, I don't know what triggers a trout and never will. What I hope is that by carefully observing the specific water I happen to be fishing, I can make a reasonably educated guess what might fool a few fish at that time. Sometimes that works out and sometimes not. If it always worked, we'd call it 'Catching'.

Allan