Original: Their larvae, called "leatherjackets", "leatherbacks", "leatherback bugs" or "leatherjacket slugs", because of the way they move, consume roots (such as those of turf grass) and other vegetation, in some cases causing damage to plants.
Simplified: Crane fly larvae are called leatherjackets or 'leatherjacket slugs' because of the way they move and eat roots (such as those of grass in lawns) and other vegetation.