He spake, and there came a swarm of flies By the word spake the Psalmist intimates that the flies and lice came not forth by chance. The command, we know, was uttered by the mouth of Moses; for although God could have given the command himself, he interposed Moses as his herald. God, however, gave no less efficacy to his word, when he commanded it to be uttered by a man, than if he himself had thundered from heaven. When the minister executes his commission faithfully, by speaking only what God puts into his mouth, the inward power of the Holy Spirit is joined with his outward voice. Here again it is to be observed, that the Egyptians were afflicted with the plague of the flies and lice, that God, with the greater ignominy, might subdue their rebellion and obstinacy. When it is said, that he gave them hail for rain, it denotes a hail of such appalling violence, that it could not be attributed to natural causes. It is probable that Egypt is not so subject to this annoyance as other countries, and it is very seldom visited even with rain, being watered with the Nile. This made it appear to the Egyptians the more wonderful that their country was stricken with hail. To render this calamity the more dreadful, God also mingled with it fire. The hail, then, was accompanied with a tempestuous whirlwind, that the Egyptians who had hardened themselves against the other miracles, inspired with terror, might know that they had to deal with God.

He (p) spake, and there came divers sorts of flies, [and] lice in all their coasts.
(p) So that this vermin did not come by chance, but as God had appointed, and his prophet Moses spoke.

He spake, and there came divers sorts of flies,.... Or a "mixture" (z) of various insects, and, as some interpret it, of wild beasts; and so Kimchi says evil beasts are meant, which were raised up to go into all their borders; the Arabic version renders it "dogflies", after the Septuagint; See Gill on Psalm 78:45, this was the fourth plague, Exodus 8:24.
And lice in all their coasts; this is the third plague, and what the magicians could not imitate, but were obliged to own the finger of God was in it, Exodus 8:16. God can make use of mean and despicable instruments to do his work; the weakness of God is stronger than men.
(z) "mixtura insectarum", Pagninus, Montanus; "colluvies insectarum", Piscator, Gejerus, Michaelis; "mista bestiarum", Vatablus; "colluvies animalium", Junius & Tremellius.

Coasts - In all their land, even to the utmost borders of it.