Behold, I give unto you power to tread on serpents and scorpions, and over all the power of the enemy. The older authorities read here, "I have given." The only recorded instance of a literal fulfilment of this promise was in the case of Paul at Melita, after the shipwreck (Acts 24:3-5).
A similar promise was made during the "forty days". It seems however, best, in the case of this peculiar promise, to interpret the Lord's words as referring to spiritual powers of evil, taking the serpent and scorpion as symbols of these.
It should be remembered that the subject of conversation between the Master and his servants was the conflict with and victory ever these awful powers restlessly hostile to the human race (see Psalms 91:13).