"Let My people go, that they may serve Me. Or else, if you will not let My people go, behold, I will send swarms of flies on you and your servants, on your people and into your houses. The houses of the Egyptians shall be full of swarms of flies, and also the ground on which they stand. And in that day I will set apart the land of Goshen, in which the people dwell, that no swarms of flies shall be there, in order that you may know that I am the LORD in the midst of the land. I will make a difference between My people and your people. Tomorrow this sign shall be" (Exo. 8:20-23).
This is the first plague where a distinction is noted between the Israelites and Egyptians. It would seem that the Israelites had endured the prior plagues with the Egyptians but that will change here. Why did they need to experience any of the plagues? They needed to learn firsthand that the plagues were real and that their God, Jehovah, was all powerful. This separation also shows the extent of God's power; He can control precisely where the plague starts and stops geographically. This also would prove to the Egyptians that the power behind these plagues was not just any deity, it was Jehovah, God of Israel!
God, unlike Pharaoh, keeps His promises. "And the LORD did so. Thick swarms of flies came into the house of Pharaoh, into his servants' houses, and into all the land of Egypt. The land was corrupted because of the swarms of flies" (8:24). We should note that the text doesn't actually specify "flies" here, but such is quite possible. Some suggest that the swarming insects God sent were perhaps flying beetles and capable of fastening themselves to one's body, particularly the edges of the eyelids (which was known to result in blindness). Even today some beetles are deified in Egypt. Amon-Ra, the creator and king of the Egyptian gods, had the head of a beetle. Why didn't he stop the swarms? Surely the Egyptians would have called out to him for help.
This plague is intense and Pharaoh cannot stand it, so he summoned Moses and Aaron and told them: "Go, sacrifice to your God in the land" (8:25). This is Pharaoh's first offer at a compromise. But, the Israelites sacrificing in the land of Egypt would never work, as Moses knew. "It is not right to do so, for we would be sacrificing the abomination of the Egyptians to the LORD our God. If we sacrifice the abomination of the Egyptians before their eyes, then will they not stone us?" (8:26). The way in which the Israelites would sacrifice would not be in accordance with Egyptian sacrificial rites. It would be very offensive to them. Also, sacrificing in the land was not what the LORD had instructed them to do! Thus, Moses went on to state - "We will go three days' journey into the wilderness and sacrifice to the LORD our God as He will command us" (8:27; cf. Acts 5:29). Pharaoh realizes the danger to himself and all of Egypt if the Israelites were to travel three days into the wilderness. They might keep going with no intention of returning!
Pharaoh is desperate, however, and makes a second promise - "I will let you go, that you may sacrifice to the LORD your God in the wilderness; only you shall not go very far away. Intercede for me" (Exo. 8:28). Moses is willing to entreat the LORD that the swarms would be removed from the land the next day, though he warned Pharaoh not to act deceitfully as he had done previously. Pharaoh behaved exactly as before, however, and violated his word once the plague was removed. He would not let the Israelites go.