Exodus 8:20-32 -- The Fourth Plague

Exodus 8:20 The Lord said to Moses, “Get up early in the morning and present yourself to Pharaoh when you see him going out to the water. Tell him: This is what Yahweh says: Let My people go, so that they may worship Me. 21 But if you will not let My people go, then I will send swarms of flies against you, your officials, your people, and your houses. The Egyptians’ houses will swarm with flies, and so will the land where they live. 22 But on that day I will give special treatment to the land of Goshen, where My people are living; no flies will be there. This way you will know that I, Yahweh, am in the land. 23 I will make a distinction between My people and your people. This sign will take place tomorrow.”

This is the first plague that the Israelites did not have to endure.  The Lord is showing Egypt who he favors and who he is punishing by where they flies will be residing.

Exodus 8:24 And the Lord did this. Thick swarms of flies went into Pharaoh’s palace and his officials’ houses. Throughout Egypt the land was ruined because of the swarms of flies. 25 Then Pharaoh summoned Moses and Aaron and said, “Go sacrifice to your God within the country.”

Here Pharaoh is attempting to compromise with God.  You can go sacrifice, but you must do it here.  You cannot leave to sacrifice.

Exodus 8:26 But Moses said, “It would not be right to do that, because what we will sacrifice to the Lord our God is detestable to the Egyptians. If we sacrifice what the Egyptians detest in front of them, won’t they stone us? 27 We must go a distance of three days into the wilderness and sacrifice to the Lord our God as He instructs us.”
28 Pharaoh responded, “I will let you go and sacrifice to the Lord your God in the wilderness, but don’t go very far. Make an appeal for me.”

Ok, you can go out of the country, but don't go far... Again trying to compromise.  Pharaoh even asks Moses to appeal to God to remove the flies.  Pharaoh recognizes the power of God but refuses to obey him completely.

Exodus 8:29 “As soon as I leave you,” Moses said, “I will appeal to the Lord, and tomorrow the swarms of flies will depart from Pharaoh, his officials, and his people. But Pharaoh must not act deceptively again by refusing to let the people go and sacrifice to the Lord.” 30 Then Moses left Pharaoh’s presence and appealed to the Lord. 31 The Lord did as Moses had said: He removed the swarms of flies from Pharaoh, his officials, and his people; not one was left. 32 But Pharaoh hardened his heart this time also and did not let the people go.

Again, the discomfort was removed and Pharaoh again became comfortable and refused to let them go.  This is twice now he said they could go and then backed out.

I read a commentary that said the swarm of insects, though most probably a flying insect, could be any number of insects that swarm.   They state that the Hebrew used is simply the word swarm, and no specific species of insect is given.

So, if Exodus does not mention exactly what insect, can we know what kind it really was?  Psalms gives us a clue

Psalms 78:45
He sent among them swarms of flies,which fed on them,

Remember the Hebrew probably did not include "of flies," but in this verse we learn that the swarm fed on them, which means they were biting insects which probably drank blood, like biting flies or mosquitoes.  Though we cannot know for sure exactly what insects swarmed the Egyptians, we do know the result.  They devastated the land.  The people of Egypt were probably so weary of all the biting and swarming that they had little rest and were not able to perform their labors as they once could.  The insects would have been everywhere: on and in their cloths, in their food, in their water... everywhere you can think of their would have been these insects.  We have no idea how long it took Pharaoh to summon Moses after the swarms appeared, so there is no telling how long the people suffered before Pharaoh made his petition.