Something interesting is that their defense mechanism is "predator satiation" which means that they coordinate their emergence in these massive groups because the predators eat their fill and simply can't eat all of them. They're slow, can't fly well and it's the end of their life cycle anyway.
Unfortunately, like all nature news it's coupled with information that humanity is screwing things over by paving over their emergence sites and climate change is screwing up their synchronization. Hooray!