Jamieson, Fausset and Brown on Joel 1:4

4. This verse states the subject on which he afterwards expands. Four species or stages of locusts, rather than four different insects, are meant (compare Le 11:22). Literally, (1) the gnawing locust; (2) the swarming locust; (3) the licking locust; (4) the consuming locust; forming a climax to the most destructive kind. The last is often three inches long, and the two antennæ, each an inch long. The two hinder of its six feet are larger than the rest, adapting it for leaping. The first "kind" is that of the locust, having just emerged from the egg in spring, and without wings. The second is when at the end of spring, still in their first skin, the locusts put forth little ones without legs or wings. The third, when after their third casting of the old skin, they get small wings, which enable them to leap the better, but not to fly. Being unable to go away till their wings are matured, they devour all before them, grass, shrubs, and bark of trees: translated "rough caterpillars" (Jr 51:27). The fourth kind, the matured winged locusts (see on Na 3:16). In Joe 2:25 they are enumerated in the reverse order, where the restoration of the devastations caused by them is promised. The Hebrews make the first species refer to Assyria and Babylon; the second species, to Medo-Persia; the third, to Greco-Macedonia and Antiochus Epiphanes; the fourth, to the Romans. Though the primary reference be to literal locusts, the Holy Spirit doubtless had in view the successive empires which assailed Judea, each worse than its predecessor, Rome being the climax.