Exodus 8:22
And I will sever in that day the land of Goshen, in which my people dwell, that no swarms of flies shall be there; to the end thou mayest know that I am the LORD in the midst of the earth. And I will sever in that day the land of Goshen, in which my people dwell, that no swarms of flies shall be there; to the end thou mayest know that I am the LORD in the midst of the earth.
In the plague of swarms of flies God severed the land of Goshen where His people dwelt so that no swarms would be there, while Pharaoh, his servants, his people, their houses, the ground, and all the land of Egypt were filled with a grievous swarm that corrupted the land. Throughout the coasts of Israel in Goshen He suffered no flies to appear, making a visible separation between His people and Pharaoh's people. The flies sent upon Egypt were not harmless common flies that merely annoy in some seasons of the year, but large and venomous flies whose sting was very painful upon both man and beast.
The division was announced beforehand as a sign to happen the next day, and it occurred exactly as foretold when the Lord did as He said. Its purpose was that Pharaoh might know that the Lord is in the midst of the earth, present and ruling with power to protect His own people while judging Egypt. The plague also challenged Beelzebub, the Egyptian fly-god believed to protect the land from swarms, showing that idol to be powerless and leading the people to look elsewhere for relief.