A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
exo 8:1PLAGUE OF FROGS. (Exo 8:1-15)
the Lord spake unto Moses, Go unto Pharaoh--The duration of the first plague for a whole week must have satisfied all that it was produced not by any accidental causes, but by the agency of omnipotent power. As a judgment of God, however, it produced no good effect, and Moses was commanded to wait on the king and threaten him, in the event of his continued obstinacy, with the infliction of a new and different plague. As Pharaoh's answer is not given, it may be inferred to have been unfavorable, for the rod was again raised.
exo 8:2I will smite all thy borders with frogs--Those animals, though the natural spawn of the river, and therefore objects familiar to the people, were on this occasion miraculously multiplied to an amazing extent, and it is probable that the ova of the frogs, which had been previously deposited in the mire and marshes, were miraculously brought to perfection at once.
exo 8:3bedchamber . . . bed--mats strewed on the floor as well as more sumptuous divans of the rich.
ovens--holes made in the ground and the sides of which are plastered with mortar.
kneading-troughs--Those used in Egypt were bowls of wicker or rush work. What must have been the state of the people when they could find no means of escape from the cold, damp touch and unsightly presence of the frogs, as they alighted on every article and vessel of food!
exo 8:5Stretch forth thine hand with thy rod over the streams, &c. The miracle consisted in the reptiles leaving their marshes at the very time he commanded them.
exo 8:7the magicians did so with their enchantments--required no great art to make the offensive reptiles appear on any small spot of ground. What they undertook to do already existed in abundance all around. They would better have shown their power by removing the frogs.
exo 8:8Pharaoh called, . . . Intreat the Lord, that he may take away the frogs from me--The frog, which was now used as an instrument of affliction, whether from reverence or abhorrence, was an object of national superstition with the Egyptians, the god Ptha being represented with a frog's head. But the vast numbers, together with their stench, made them an intolerable nuisance so that the king was so far humbled as to promise that, if Moses would intercede for their removal, he would consent to the departure of Israel, and in compliance with this appeal, they were withdrawn at the very hour named by the monarch himself. But many, while suffering the consequences of their sins, make promises of amendment and obedience which they afterwards forget; and so Pharaoh, when he saw there was a respite, was again hardened [Exo 8:15].
exo 8:16PLAGUE OF LICE. (Exo 8:16-19)
smite the dust of the land, &c.--Aaron's rod, by the direction of Moses, who was commanded by God, was again raised, and the land was filled with gnats, mosquitoes--that is the proper meaning of the original term. In ordinary circumstances they embitter life in Eastern countries, and therefore the terrible nature of this infliction on Egypt may be imagined when no precautions could preserve from their painful sting. The very smallness and insignificance of these fierce insects made them a dreadful scourge. The magicians never attempted any imitation, and what neither the blood of the river nor the nuisance of the frogs had done, the visitation of this tiny enemy constrained them to acknowledge "this is the finger of God"--properly "gods," for they spoke as heathens.
exo 8:20PLAGUE OF FLIES. (Exo 8:20-32)
Rise up early . . . Pharaoh; lo, he cometh forth to the water, &c.--Pharaoh still appearing obdurate, Moses was ordered to meet him while walking on the banks of the Nile and repeat his request for the liberation of Israel, threatening in case of continued refusal to cover every house from the palace to the cottage with swarms of flies--while, as a proof of the power that accomplished this judgment, the land of Goshen should be exempted from the calamity. The appeal was equally vain as before, and the predicted evil overtook the country in the form of what was not "flies," such as we are accustomed to, but divers sorts of flies (Psa 78:45), the gad fly, the cockroach, the Egyptian beetle, for all these are mentioned by different writers. They are very destructive, some of them inflicting severe bites on animals, others destroying clothes, books, plants, every thing. The worship of flies, particularly of the beetle, was a prominent part of the religion of the ancient Egyptians. The employment of these winged deities to chastise them must have been painful and humiliating to the Egyptians while it must at the same time have strengthened the faith of the Israelites in the God of their fathers as the only object of worship.
exo 8:25Pharaoh called for Moses, . . . Go ye, sacrifice to your God in the land, &c.--Between impatient anxiety to be freed from this scourge and a reluctance on the part of the Hebrew bondsmen, the king followed the course of expediency; he proposed to let them free to engage in their religious rites within any part of the kingdom. But true to his instructions, Moses would accede to no such arrangement; he stated a most valid reason to show the danger of it, and the king having yielded so far as to allow them a brief holiday across the border, annexed to this concession a request that Moses would entreat with Jehovah for the removal of the plague. He promised to do so, and it was removed the following day. But no sooner was the pressure over than the spirit of Pharaoh, like a bent bow, sprang back to its wonted obduracy, and, regardless of his promise, he refused to let the people depart.