"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 put a division between My people and thy people: tomorrow shall this sign be" (Exodus 8:22-23).
God did a mighty thing in Egypt. In order to show the Egyptians that He was "the LORD in the midst of the earth," He took a "swarm of flies" and bound and limited them to the land of Egypt. Although Goshen was located near where Pharaoh lived and although "flies" move swiftly from place to place, these creatures never entered Goshen where the Israelties were.
Why "flies"? Why did God use a "swarm of flies" as one of the plagues to afflict Egypt? First of all, the "swarm of flies" consisted of divers sorts of flies - the god fly, cockroach, and the Egyptian beetle. These "flies" were very destructive - destroying clothes, books, plants, etc. The worship of "flies" - particularly the beetle - was a prominent part of the religion of Egypt. You could just imagine how humiliating it was for the Egyptians to have one of the gods they worshipped - the "fly" or beetle - to come and wreak total havoc in the land!
God said - "I will sever..." He also said - "And I will put a division between..." This is God's way. He makes a distinction between His people and the people of the world. The Bible tells us that He knows those that are His (2 Timothy 2:19), and He sets apart the godly for Himself (Psalm 4:3).
When God exempted His people from this plague, He was said to have redeemed or delivered them. He marvelously separated them for in the land of Goshen where they resided, it abounded with cattle whose dung was apt to breed flies. Yet, the Scripture says - "no swarm of flies shall be there." For what purpose? "To the end thou mayest know that I am the LORD in the midst of the earth." Pharaoh and all of Egypt would surely know Who is LORD!
One way or another - God will be glorifed in the earth - even if He has to use a "swarm of flies" to get His point across to the people!