Charles Ellicott Commentary Exodus 8:31

"And Jehovah did according to the word of Moses; and he removed the swarms of flies from Pharaoh, from his servants, and from his people; there remained not one." — Exodus 8:31 (ASV)

There remained not one. —The sudden and entire removal of a plague like this at the word of Moses was almost as great a miracle as its sudden coming at his word, and is therefore, when it happened, carefully recorded. (See Exodus 10:19.) It seems not to have happened with the frogs (Exodus 8:11–13) or with the mosquitoes.