Exodus 8:18

“And the magicians did so with their enchantments, to bring forth lice, etc.] They made use of their magical art, and juggling tricks they were masters of, to produce the like sort of creatures, or at least to make such appear, or seem to appear, to the eyes of men: but they could not; God would not suffer them to do it, to impose upon Pharaoh, and deceive him and the Egyptians any longer; and a stop is put to them, when such small and despicable creatures were produced, the more to put them to shame and confusion, and to show that what they did before was not real, and that what they did in appearance was only by divine permission; so there were lice upon man and upon beast; these lay in…”

“And the magicians did so - That is, endeavoured to do so. Exodus 8:19”

“the magicians: Exo 7:11 they could: Exo 9:11; Gen 41:8; Isa 19:12, Isa 47:12, Isa 47:13; Dan 2:10, Dan 2:11, Dan 4:7, Dan 5:8; Luk 10:18; Ti2 3:8, Ti2 3:9 Exodus 8:19”

“"The magicians did so with their enchantments (i.e., smote the dust with rods), to bring forth gnats, but could not." The cause of this inability is hardly to be sought for, as Knobel supposes, in the fact that "the thing to be done in this instance, was to call creatures into existence, and not merely to call forth and change creatures and things in existence already, as in the case of the staff, the water, and the frogs." For after this, they could neither call out the dog-flies, nor protect their own bodies from the boils; to say nothing of the fact, that as gnats proceed from the eggs laid in the dust or earth by the previous generation, their production is not to be regarded as a direct…”