Locusts are common in the eastern countries. How could Pharaoh know that it was a plague sent by Hashem?
Oznayim la'Torah: By informing Pharaoh exactly when they would arrive - 'tomorrow.' And by specifying the exact location 1 where they would wreak havoc - 'within your borders.' 2
Compare to Maharal (refer to 9:18:1.01:2, regarding the Makah of Barad).
What does the name "Arbeh" mean, and what was their nature?
Maharal (Netzach Yisrael Ch. 5, p. 24): Locusts are called Arbeh (root Reish-Beis-Hei) due to their great power to multiply. Conceptually, each locust stands alone as a separate entity; as in the verse, "The locusts have no king" (Mishlei 30:27). The function of a king would be to unify the people - whereas locusts are so numerous that they have no such unity. 1
Maharal (Chidushei Agados Vol. 1, p. 24, to Shabbos 32b): This is why locusts are cruel.
