Albert Barnes Commentary Exodus 8:21

"Else, if thou wilt not let my people go, behold, I will send swarms of flies upon thee, and upon they servants, and upon thy people, and into thy houses: and the houses of the Egyptians shall be full of swarms of flies, and also the ground whereon they are." — Exodus 8:21 (ASV)

Swarms of flies - Generally, they are supposed to be the dog-fly, which at certain seasons is described as a plague far worse than mosquitoes. Others, however, adopt the opinion that the insects were a species of beetle, which was revered by the Egyptians as a symbol of life, of reproductive or creative power. The sun-god, as creator, bore the name Chepera, and is represented in the form, or with the head, of a beetle.