Locusts are insects that were cast upon Egypt in retaliation against an impious Pharaoh.  Moses cast a number of plagues onto Egypt to punish the impious Pharaoh.  The Pharaoh is punished because he enslaved the Hebrews, and refused piety to God.  Moses took his powerful rod and cast plagues onto the lands and water of Egypt.  He polluted the waters oft he Nile so that no one could drink, he cast plagues of flies, gnats, frogs, and locust.  The locust were compared to the fallen angels gathering because they are innumerable.  A plague of locusts can reach over one thousand square miles.  The devastation that Arabic lands still suffer today is vast and horrible.  Milton uses this image to convey the darkness cast into hell when the angels fell.  Milton also uses the reference to Moses to reiterate the power Satan had over his minions.