Revelation 9:9

breastplates
θώρακας [thōrakas] . This word may denote an actual breastplate or “the part of the body covered by the breastplate, the chest.”¹ It is the word from which we get thorax. Interestingly, we use this word to describe insects. “Thorax: The second or middle region of the body of an arthropod, between the head and the abdomen, in insects bearing the true legs and wings.”² See commentary on Revelation 9:7.

like the sound of chariots with many horses running into battle
The sound of their flight is compared to chariots in battle, so great is their number. Once again, Joel and John agree: “With a noise like chariots Over mountaintops they leap, Like the noise of a flaming fire that devours the stubble, Like a strong people set in battle array.” [emphasis added] (Joel Joel 2:5).

Notes

¹ Frederick William Danker and Walter Bauer, A Greek-English Lexicon of the New Testament and Other Early Christian Literature (Chicago, IL: University of Chicago Press, 2000), 367.

² American Heritage Online Dictionary, Ver. 3.0A, 3rd ed. (Houghton Mifflin, 1993), s.v. “thorax.”