This far and no further (Exodus 8)

The Nile and all the waters of Egypt were turned to blood. Then the frogs. Then the gnats. Then the flies. But still Pharoah hardened his heart.

It is interesting that the magicians could not conjure up the gnats. They could turn staffs into snakes; they could turn water into blood; they could produce frogs out of thin air. But not gnats. What’s the difference? Are gnats harder to come up with than frogs or snakes?
It is as if the LORD said to the magicians, the sorcerers and the demonic forces behind them, “This far and no further.” Isn’t this exactly what the Almighty said to Satan, when Satan requested permission to destroy Job, the servant of God?