Commentary Inf XVII 82

The 'stairs' to which Virgil refers are monstrous creatures, as he looks ahead to their means of conveyance over the great gaps in walkable terrain that they will face at Cocytus ([Inf XXXI 142-143], Antaeus will be their 'stairs' then) and at the center of the Earth ([Inf XXXIV 73-75], where Satan's legs will be the ladder Virgil climbs to begin to draw Dante up from hell).