tails like scorpions, and there were stings in their tails
Stings is κέντρα [kentra] , used also of a pointed stick (Acts Acts 9:5; Acts 26:14) and figuratively to describe the power of death to hurt (1Cor. 1Cor. 15:55). The sting denotes not only the ability to pierce, but possibly also the pain of a poisonous tip, or stinger.1 See commentary on Revelation 9:5. The sting of some natural scorpions is extremely painful:
Of a boy stung in the foot by a scorpion, Laborde relates that, although of a race which bears everything with remarkable patience, he rolled on the ground, grinding his teeth, and foaming at the mouth. It was a long time before his complainings moderated, and even then he could make no use of his foot, which was greatly inflamed. And such is the nature of the torment which these locusts from the pit inflict.2
2 J. A. Seiss, The Apocalypse: Lectures on the Book of Revelation (Grand Rapids, MI: Zondervan Publishing House, 1966), 206.