The first time I saw a grannom caddis emerge onto the water surface I prepared to defend myself! It was huge. One of the biggest flies I've seen on any British water. At over 1?" they are the largest of our caddis flies, at the other end of the scale they are impossible to imitate on a hook.
When a caddis gets dislodged or begins its swim to the surface it is at its most vulnerable to predation by trout. Not only during a hatch do imitations of caddis pupa produce fish, they are "remembered" by trout as a good meal, so they will grab them whenever they get the chance. This makes imitations of caddis pupa great searching patterns, Often my first choice when there is no hatch going on.