Tails (ουρας). Old word, in N.T. only in Revelation 9:10; Revelation 9:19; Revelation 12:4.

Like unto scorpions (ομοιας σκορπιοις). Aleph A wrongly have ομοιοις (agreeing with σκορπιοις instead of with ουρας). It is a condensed idiom for "like unto the tails of the scorpions" as we have it in Revelation 13:11 (cf. Matthew 5:20; 1 John 2:2).

Stings (κεντρα). Old word from κεντρεω (to prick, to sting), in N.T. only here, Acts 26:14 (about Paul); 1 Corinthians 15:55 (about death). It is used "of the spur of a cock, the quill of the porcupine, and the stings of insects" (Vincent). It was the goad used for oxen (Proverbs 26:3; Acts 26:14).

In their tails (εν ταις ουραις αυτων). This locates "their power to hurt" (η εξουσια αυτων αδικησα, infinitive here, ινα αδικησουσιν in Revelation 9:4) in their tails. It might have been in other organs.