Else, if thou wilt not let my people go, behold, I will send swarms of flies upon thee, and upon thy 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.

Wesley's Notes

8:21 Flies - Or insects of various kinds; not only flies, but gnats, wasps, hornets; and those probably more pernicious than the common ones were.

King James Translators' Notes

swarms...: or, a mixture of noisome beasts, etc

Matthew Henry's Concise Commentary

8:20-32 Pharaoh was early at his false devotions to the river; and shall we be for more sleep and more slumber, when any service to the Lord is to be done? The Egyptians and the Hebrews were to be marked in the plague of flies. The Lord knows them that are his, and will make it appear, perhaps in this world, certainly in the other, that he has set them apart for himself. Pharaoh unwillingly entered into a treaty with Moses and Aaron. He is content they should sacrifice to their God, provided they would do it in the land of Egypt. But it would be an abomination to God, should they offer the Egyptian sacrifices; and it would be an abomination to the Egyptians, should they offer to God the objects of the worship of the Egyptians, namely, their calves or oxen. Those who would offer acceptable sacrifice to God, must separate themselves from the wicked and profane. They must also retire from the world. Israel cannot keep the feast of the Lord, either among the brick-kilns or among the flesh-pots of Egypt. And they must sacrifice as God shall command, not otherwise. Though they were in slavery to Pharaoh, yet they must obey God's commands. Pharaoh consents for them to go into the wilderness, provided they do not go so far but that he might fetch them back again. Thus, some sinners, in a pang of conviction, part with their sins, yet are loth they should go very far away; for when the fright is over, they will turn to them again. Moses promised the removal of this plague. But let not Pharaoh deal deceitfully any more. Be not deceived; God is not mocked: if we think to cheat God by a sham repentance and a false surrender of ourselves to him, we shall put a fatal cheat upon our own souls. Pharaoh returned to his hardness. Reigning lusts break through the strongest bonds, and make men presume and go from their word. Many seem in earnest, but there is some reserve, some beloved, secret sin. They are unwilling to look upon themselves as in danger of everlasting misery. They will refrain from other sins; they do much, give much, and even punish themselves much. They will leave it off sometimes, and, as it were, let their sin depart a little way; but will not make up their minds to part with all and follow Christ, bearing the cross. Rather than that, they venture all. They are sorrowful, but depart from Christ, determined to keep the world at present, and they hope for some future season, when salvation may be had without such costly sacrifices; but, at length, the poor sinner is driven away in his wickedness, and left without hope to lament his folly.