The "locusts" of revelation 9 could have been a reference to the Roman Army under Titus that besieged Jerusalem, since the duration of the "locust" plague (5 months) matches the duration of of Titus' siege (from april to september, 5 months). Apparently, there was a period of time during the siege where the Romans allowed a large number of Christians to escape, just as the "locusts" were forbidden from harming those sealed by God.