Trillions of insects embark, largely unnoticed, on epic journeys every year across mountain ranges, deserts and seas, and it is only now, as their numbers suffer huge declines, that scientists are tracking their movements On a cloudless sunny day in October 1950, ornithologists Elizabeth and David Lack stood on a mountain pass in the Pyrenees and observed a once-in-a-lifetime spectacle – clouds of migrating insects. Up to 500 butterflies were fluttering past them every hour through the 2,200m-