Or else, if you will not let my people go, behold, I will send swarms of flies on you and your servants and your people, and into your houses. And the houses of the Egyptians shall be filled with swarms of flies, and also the ground on which they stand.
Exodus 8:21

This verse doesn't say "flies" in Hebrew. It says "swarms" without specifying what kind of swarm. The literal meaning of the Hebrew "arob" is related to a mixture or churning. Some interpret this to mean a mix of different kinds of animals. I wonder if it could also have been a divinely ironic reference to the mixed multitude that Egypt held in slavery: "Since you won't let my swarm of people go, I'll send another of my swarms to make you let them go."

Swarms of flies can also be a symptom of the absence of God. The plague of flies was absent from the land of the Hebrews in Egypt.