And there came out of the smoke locusts upon the earth: and unto them was given power, as the scorpions of the earth have power.
And it was commanded them that they should not hurt the grass of the earth, neither any green thing, neither any tree; but only those men which have not the seal of God in their foreheads [Rev. 9:3–4].
And out of the smoke came forth locusts upon the earth, and power was given to them as the scorpions of the earth have power. And it was said to them in order that they should not hurt the grass of the earth nor any green things, nor any tree, but only (except) the men who do not have the seal of God on their foreheads.
To me this beggars description. John uses symbolic language which describes creatures so frightful that this is the only way he could speak of them.
These are locusts, but they are of a very unusual character. As Govett remarks (The Apocalypse Expounded by Scripture, pp. 185–186), they are “no common locusts,” and he gives the following reasons:
1.     for they eat no vegetable productions;
2.     the locusts of the earth have no king (Prov. 30:27); these have;
3.     in the plague of Egypt the inspired recorder had said, “Before them there were no such locusts as they, neither after them shall be such” (Exod. 10:14);
4.     yet they are literal creatures resembling the literal animals named: the lion, the horse, the scorpion, the man.
This is a plague of locusts which is as literal as the plague of locusts in Egypt. Joel prophesied of a coming plague of locusts (see Joel 1). Again, a working knowledge of the Old Testament is essential to the understanding of Revelation. The difference between the locusts here and the locusts in Joel is the character of the locusts and the object of their destruction. They sting as scorpions, and their objects are evil men.
And to them it was given that they should not kill them, but that they should be tormented five months: and their torment was as the torment of a scorpion, when he striketh a man [Rev. 9:5].
And it was given to them in order that they should not kill them, but in order that they should be tormented five months; and their torment was as the torment of a scorpion, when it striketh a man.
The scorpion is shaped like a lobster and lives in damp places. His sting is in his tail; though it is not fatal, it is very painful indeed. This is the picture we are given here. These were mentioned by Joshua when he spoke of the hornet, “And I sent the hornet before you, which drave them out from before you …” (Josh. 24:12). Therefore you can see that believers, living during the Great Tribulation who will be acquainted with the Old Testament, will understand what John is talking about regarding these scorpions.
And in those days shall men seek death, and shall not find it; and shall desire to die, and death shall flee from them [Rev. 9:6].
And in those days shall the men seek death, and shall not find it; and they shall earnestly desire to die, and death fleeth from them.
Satan is given the key to this long shaft (which evidently is what is called sheol in the Old Testament and hell [hades] in the New Testament). The shaft leads to the abyss where the spirits of the dead of the ages past have gone. This is where the Lord Jesus went to announce the redemption that He had wrought on the cross. Satan does not want his crowd to die, and it is only his crowd that are attacked by these locusts. Men during this period try to commit suicide and are unable to do it—this reveals something of the awfulness of that day. Satan wants them here because there is a battle between light and darkness that is being waged. There are others who think that maybe it is God who will not let these men die because sinful man must face the consequences of his rebellion—there is no escape. It is not a laughing matter to reject Jesus Christ; it is not a simple thing to ignore Him. People say there are so many important things in this life—and I am willing to grant that many things take second, third, and fourth place—but the most important thing is your decision concerning Jesus Christ.
McGee, J. Vernon: Thru the Bible Commentary. electronic ed. Nashville : Thomas Nelson, 1997, c1981, S. 5:967-968

 

9:3 locusts. A grasshopper-like insect that descends in swarms so thick they can obscure the sun and strip bare all vegetation. In the 1950s a locust swarm devoured every growing thing for several hundred thousand square miles in the Middle East. These are not normal locusts, however, but specially prepared ones that are merely the outward form of demons, who, like locusts, will bring swarming desolation (see notes on Joel 2:1–5). “Like” appears 9 times in John’s description; he finds it difficult to describe what he sees in a way the reader can understand. scorpions. An arachnid that inhabits warm, dry regions and has an erect tail tipped with a venomous stinger. A scorpion’s victim often rolls on the ground in agony, foams at the mouth, and grinds his teeth in pain. The demons in locust form are able to inflict the physical—and perhaps, spiritual—pain like the scorpion (v. 5).
9:4 men who do not have the seal of God. Everyone on earth except the two groups mentioned in chap. 7—the 144,000 Jewish evangelists and their converts (see note on 7:3).
9:5 five months. The normal life cycle of locusts is 5 months, usually from May to Sep.
9:6 seek death and will not find it. The tormented will find no relief. Even their unimaginable attempts to end their misery in suicide will be unsuccessful.
MacArthur, John Jr: The MacArthur Study Bible. electronic ed. Nashville : Word Pub., 1997, c1997, S. Re 9:3