The proceeding verse could give a clue.
Revelation 9:13-19 “The sixth angel blew his trumpet. And I heard one voice from the horns of the golden altar that is before God say to the sixth angel who had the trumpet: “Untie the four angels who are bound at the great river Eu·phraʹtes.” And the four angels who have been prepared for the hour and day and month and year were untied to kill a third of the people. The number of the armies of cavalry was two myriads of myriads; I heard the number of them. And this is how I saw the horses in the vision and those seated on them: They had fire-red and hyacinth-blue and sulfur-yellow breastplates, and the heads of the horses were like the heads of lions, and fire and smoke and sulfur came out of their mouths. A third of the people were killed by these three plagues, by the fire and the smoke and the sulfur that came out of their mouths. For the authority of the horses is in their mouths and in their tails, for their tails are like serpents and have heads, and with these they inflict harm.”
These are angelic beings being brought upon mankind.
Regarding the features of the locusts, this is I could come up.
1). Crowns signify authority or dominion, a prince or ruler, of which they once had. Just as the angel of Persia is called “prince” of Persia where he fought with Michael, and the prince of Greece.
2) Human faces, or faces of men— to signify these angels’ “carnality” or flesh-like personality.(Galatians 5:19-21)
3) It’s not just a breastplate, but “like iron breastplates”, which is dark in color or even black. Could this just mean sturdiness, or unrighteousness?
4)Having a long hair is a disgrace or dishonor to man, of which these angels subjected themselves.(1 Corinthians 11:14)