Craneflies look like mutant midges. They are oversized mosquitoes pretty much, but don't feed on blood (I think...). The larval stages of them are a trout's delight, because they look like a fat, tumbling midge larvae on steriods in the river. I would say that in a river system, the larva itself is important to trout, especially around here on some of the rivers when the water is high and murky during snow runoff in May/June. The big larvae are easy targets as the faster water kicks them up from the bottom and they're pretty big and visible. As for the adult stages, I don't know if they're an essential part of a trout's diet, but I would imagine that trout feed on them at one point or another, they are opportunistic enough. Their hatches aren't prolific on any river I've been on, but maybe they are elsewhere...