Exodus 8:20, commentary and pictures

God sent Moses with Aaron to confront Pharaoh once more with the demand to let Israel go that they might serve Him, warning that refusal would be followed by judgment. The threatened judgment was grievous swarms of flies upon Pharaoh and his servants and his people and their houses and land, fulfilled by a corrupting swarm over all Egypt. These were not ordinary harmless flies but large and venomous flies that filled the houses and swarmed upon the ground so that the land was corrupted, and their stings brought great pain to man and beast. God promised to sever the land of Goshen where Israel dwelt so that no flies would be there, thus putting a division between His people and Pharaoh's people by the next day to show that He is the Lord in the midst of the earth.

To serve Him meant going three days' journey into the wilderness to sacrifice as He commanded, not sacrificing in Egypt. Sacrifice could not be made in Egypt because Israel had to offer animals the Egyptians worshiped as sacred and held so sacred that to slay one even accidentally was regarded as an unpardonable offense punishable by death, so doing so before Egyptian eyes would provoke stoning. Under chastening Pharaoh first offered to let Israel sacrifice inside Egypt itself, which was refused, then consented to let them go into the wilderness provided they did not go far away, and asked for intercession and prayer for removal of the plague.

Moses warned Pharaoh not to deal deceitfully about letting the people go to sacrifice, and interceded that the flies be removed by the next day. God removed them completely so that none remained. Yet Pharaoh hardened his heart through persistent rebellion and still refused to let the people go. As part of the plagues account, the episode exposed the Egyptian idols as powerless and demonstrated Jehovah's supremacy.