Explanation and summary
The third plague of the 10 plagues that the God of the Israelites sent as judgment upon Egypt, through Aaron, was gnats (or lice), as recorded in the book of Exodus. In this plague, all the dust of Egypt becomes a swarm of gnats/lice. The Egyptian god this plague insulted was Seth (an earth god) and perhaps others as well.
It is impossible to determine what particular troublesome insect pest of Egypt is meant by the third plague, whether body-lice or mosquitoes or sandflies or ticks or fleas. Those who have experience of these pests in Egypt are quite ready to accept any of them as adequate for the plague. Lice seem rather to be ruled out, unless different kinds of lice were sent, as there is no one kind that torments both man and beast.
All the other insect pests appear in incredible numbers out of the "dust" when the pools have dried up after the receding of the waters. The assertion that the account of this plague is not complete, because it is not recorded that Pharaoh asked its removal or that Moses secured it, is amazing. Perhaps Pharaoh did not, in fact, ask its removal. There seems also at this time some difficulty in Moses having access to Pharaoh after this plague (Exodus 8:20).
Biblical Text (KJV)
16 And the Lord said unto Moses, Say unto Aaron, Stretch out thy rod, and smite the dust of the land, that it may become lice throughout all the land of Egypt.
17 And they did so; for Aaron stretched out his hand with his rod, and smote the dust of the earth, and it became lice in man, and in beast; all the dust of the land became lice throughout all the land of Egypt.
18 And the magicians did so with their enchantments to bring forth lice, but they could not: so there were lice upon man, and upon beast.
19 Then the magicians said unto Pharaoh, This is the finger of God: and Pharaoh's heart was hardened, and he hearkened not unto them; as the Lord had said.
Perhaps the plague was not removed at all.
The Egyptians are disposed to think it was not! Certainly that season of the year spent in Egypt, not in a dahabiyeh on the Nile, but in a native village, will furnish very satisfying evidence that stinging and biting insects are a very real plague in Egypt yet.
The magicians failed with their enchantments and acknowledged that divine power was at work, and seem to have acknowledged that Yahweh was supreme (Exodus 8:19), but Pharaoh would not heed them.
International Standard Bible Encyclopedia, which is in the public domain (with minor edits).