Ex 8:16 So the LORD said to Moses, "Say to Aaron, 'Stretch out your rod, and strike the dust of the land, so that it may become lice throughout all the land of Egypt.'"

17 And they did so. For Aaron stretched out his hand with his rod and struck the dust of the earth, and it became lice on man and beast. All the dust of the land became lice throughout all the land of Egypt.

18 Now the magicians so worked with their enchantments to bring forth lice, but they could not. So there were lice on man and beast.

19 Then the magicians said to Pharaoh, "This is the finger of God." But Pharaoh's heart grew hard, and he did not heed them, just as the LORD had said.

This annoying, irritating and nagging plague was God's way of discrediting the land and desert gods of Egypt like Geb and Set. The dust of the earth became lice on man and beast. Imagine them creeping into the eyes and nose of the Egyptians! This time, God, restrained Satan and his demons from mimicking it. 

Moses has already performed signs that involve the water and the air. This time, it involves the land. The point is --- the God of Israel is the God of water, air and land.