Aharon strikes the dust and every grain of it becomes a biting insect. Targum Pseudo-Jonathan on Exodus 8:13 is emphatic: all the dust of the earth was changed to become insects, in all the land of Mizraim.
This totality matters. Egypt was a nation built on the cleanliness of its priests, who shaved their heads and bodies daily to remain ritually pure. Lice are the single ritual enemy a shaved priest cannot tolerate. Now every ounce of dust, the most common substance on earth, has become the one substance the Egyptian priesthood cannot live with.
The meturgeman highlights upon the flesh of men and of cattle — on priests and pharaohs, on slaves and livestock, with no exemption. The first two plagues had struck Egypt's waters and houses from outside. This one crawls onto their skin. The body itself is no longer a private space.
And the detail that makes it comic is that the plague arrives from dust — the very ground Moses had once used to hide a sin. God has taken the most humble material in creation, the stuff underfoot, and weaponized it against the most proud empire on earth. Pharaoh's priests, who counted themselves pure, now stand in a desert of lice.
The takeaway: God fights empires with dust. When the oppressor is overwhelmed by the contents of his own sandals, it is time to let the slaves go.