How we cite our quotes:
"for now were all transformed
Alike, to serpents all as accessories
To his bold riot" (10.519-21)
Satan and his legions are turned into serpents after the Fall as a result of their role in it. Serpents can't walk upright like angels or humans; in Paradise Lost, the ability to stand "erect," or to stand at all, is a mark of either distinction or proximity to God. This is a fitting punishment, for it further removes the fallen angels from the realm of humans, angels, etc.