Exodus 8:21, commentary and pictures

The warning was given through Moses and Aaron that if Pharaoh would not release Israel to serve Him, grievous swarms of flies would come upon Pharaoh himself, upon his servants, upon his people, into their houses and upon the ground where they were. The houses of the Egyptians would be filled with the swarms, the swarms would cover the land, and the land would be corrupted by them. The flies were not the ordinary harmless flies that annoy in some seasons of the year, but large and venomous flies. Their sting caused severe pain to both people and animals.

The plague was appointed to begin tomorrow as a sign to prove that the LORD is in the midst of the earth. With it God put a division between His people and Pharaoh's people by severing the land of Goshen where Israel dwelt, so that no swarms appeared throughout their coasts and the Israelites were spared while Egypt was struck. It exposed the impotence of Beelzebub, the Egyptians' fly-god believed to control swarms, showing he could not protect them from the very swarms he was thought to govern, since the flies came at Moses' command and relief had to be sought from Jehovah.

Pharaoh's offer under chastening to let Israel sacrifice to God inside Egypt was refused because Israel would sacrifice animals the Egyptians worshiped as sacred, which was an abomination to them. The Egyptians counted the killing of such a beast, even accidentally, an unpardonable offense punishable by death and answered only by the offender's life. Sacrificing before their eyes would therefore provoke stoning, so Israel had to go three days' journey into the wilderness to sacrifice as God commanded. At Pharaoh's request for intercession Moses entreated the LORD for the flies to depart, with the departure set for the next day, and God removed them completely so that not one remained upon Pharaoh, his servants and his people. Yet Pharaoh dealt deceitfully, hardened his heart, and still refused to let the people go.