"Then the Lord said to Moses, 'Get up early in the morning and confront Pharaoh as he goes to the river and say to him, "This is what the Lord says: Let my people go, so that they may worship me. If you do not let my people go, I will send swarms of flies on you and your officials, on your people and into your houses. The houses of the Egyptians will be full of flies; even the ground will be covered with them."'" NIV translation
Once again, Moses is sent to meet Pharaoh and to give him a warning. The flies that were spoken of were in fact beetles or scarabs. They were thought to be sacred to their sun god.
"'"But on that day I will deal differently with the land of Goshen, where my people live; no swarms of flies will be there, so that you will know that I, the Lord, am in this land. I will make a distinction between my people and your people. This sign will occur tomorrow."'" NIV translation
Until this point, the plagues had affected all of the people in the land of Egypt. Now, God is going to protect His people so that the Egyptians see that he is in control. This is also a picture of the sealing of the witnesses as described in Revelation.
"And the Lord did this. Dense swarms of flies poured into Pharaoh's palace and into the houses of his officials; throughout Egypt the land was ruined by the flies." NIV translation
God sent the beetles and their so-called sacred thing became a curse. This is a reminder to us that those who put their hope in the things of the world will be cursed by those things.
"Then Pharaoh summoned Moses and Aaron and said, 'Go, sacrifice to your God here in the land.'" NIV translation
Once again, Pharaoh tries to get Moses and Aaron to compromise in what God had said to them. This is the way that Satan works in the world today. That compromise can cause separation as we know that it is wrong. Satan can then use that separation to make us feel that we are unable to turn back to our heavenly Father.