Exodus 8:1

The Lord spake unto Moses, Go unto Pharaoh— To render Pharaoh wholly inexcusable, sufficient warning was always given him of every approaching punishment. This plague of frogs, as well as the former, was excellently adapted to subvert the superstitions of Egypt, and to demonstrate the over-ruling power of Jehovah; for as the bank of the river Nile was the grand scene of the magical operations of the Egyptians, in which blood and frogs made a principal part of the apparatus; so, by commanding that river to produce such an infinite multitude of these creatures to annoy them, Jehovah, with wonderful propriety, suited their chastisement to the nature of their crimes: for frogs were not only the instruments of their abominations, but likewise the emblems of those impure demons, whom they invoked by their incantations. Revelation 16:13. See Owen on the Old Testament Miracles.