Poole's Outline of Revelation 9

At the sounding of the fifth angel a star falleth from heaven, to whom is given the key of the bottomless pit, 1, he opens the pit, and there come forth locusts like scorpions, who have power to hurt men for a time, 2-11. The first woe past, 12. At the sounding of the sixth angel four angels which were bound are loosed, and bring great plagues on the earth for a limited time, 13-21.