Charles Ellicott Commentary Exodus 8:19

"Then the magicians said unto Pharaoh, This is the finger of God: and Pharaoh`s heart was hardened, and he hearkened not unto them; as Jehovah had spoken." — Exodus 8:19 (ASV)

The finger of God. —Rather, of a goal. The magicians meant to say, “This is beyond the power of man: it is supernatural; some god must be helping Moses and Aaron.” They did not mean to profess a belief in One God.

Pharaoh’s heart was hardened. —The mosquitoes did not impress Pharaoh as the frogs had done (Exodus 8:8–15). His heart remained hard. He had no need to harden it by an act of his will. Probably the visitation affected him very little, since he would possess mosquito curtains, and could inhabit the loftier parts of his palace, which would be above the height to which the mosquito ascends (Herod, ii. 95).