The magicians tried to produce gnats using their magic arts, but they could not. And the gnats remained on man and beast.
18. And the magicians did so. They “did” is here put for “they tried to do;” for they did not succeed, as presently appears. They are therefore said to have done, what they in vain attempted, or what they essayed, but without success.
Here is a short account of the plague of lice. It does not appear that any warning was given of it before. Pharaoh's abuse of the respite granted to him might have been a sufficient warning to him to expect another plague: for if the removal of an affliction harden us, and so we lose the benefit of it, we may conclude it goes away...
Commenting on Exodus 8:16-19
Then the magicians said unto Pharaoh, this is the finger of God,.... This is to be ascribed to a power superior to human, to a divine power; so long as they could do something similar, or impose upon the senses of men, and make them believe they did the like, they would not acknowledge divine omnipotence; but when they no longer could deceive the sight...