John Calvin Commentary Psalms 105:31

"He spake, and there came swarms of flies, And lice in all their borders." — Psalms 105:31 (ASV)

He spake, and there came a swarm of flies. By the word spake, the Psalmist suggests that the flies and lice did not come forth by chance. The command, we know, was uttered by the mouth of Moses; for although God could have given the command Himself, He appointed Moses as His herald.

God, however, gave no less effectiveness 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 should be observed that the Egyptians were afflicted with the plague of the flies and lice, so that God, with greater disgrace, might subdue their rebellion and obstinacy.

When it is said that He gave them hail for rain, this indicates a hail of such appalling violence that it could not be attributed to natural causes.

It is probable that Egypt is not as subject to this annoyance as other countries, and it is very seldom visited even with rain, as it is watered by the Nile. This made it appear all the more wonderful to the Egyptians that their country was struck with hail. To make this calamity more dreadful, God also mixed fire with it. The hail, then, was accompanied by a tempestuous whirlwind, so that the Egyptians who had hardened themselves against the other miracles, filled with terror, might know that they had to deal with God.