Bede the Venerable — as excerpted in the Catena Aurea on Luke 10:17-20

“That is, I give you the power of casting out every kind of unclean spirit, from the bodies possessed. And as far as regards themselves, He adds, And nothing shall hurt you. Although it might also be taken literally. For Paul when attacked by a viper suffered no injury. (Acts 28:5.) John having drunk poison is not harmed by ita. But I think there is this difference between serpents who bite with the teeth, and scorpions who sting with the tail, that the serpents signify men or spirits raging openly, scorpions signify them plotting in secret. Or serpents are those which east the poison of evil persuasion upon virtues just beginning, scorpions which go about to corrupt at last virtues which have been brought to perfection.”