Else, if thou refuse. Moses denounces the extreme dearth and famine of the land of Egypt, because the locusts will suddenly arise, altogether to consume the remaining produce of the year; for half of it had already been destroyed by the hail. But, although ancient histories bear witness, and it has happened also in our time, that not only cornfields, but that pastures have been devoured by locusts, still we may gather from the circumstances, that this was an extraordinary instance of the divine vengeance; because Moses both appoints the next day, and also relates that an incredible multitude suddenly burst forth, and adds, that such had never been seen; and, lastly, threatens that no house should be exempt from their invasion. Moreover, it is worth while again to remark the nature of the scourge, that God collects and arms a host of vile insects, whereby He may insultingly overcome this indomitable tyrant with all his forces. The ingratitude of Egypt, too, was worthy of this return, since it was too great an indignity that the posterity of Joseph should be tyrannically persecuted in that. country, which a little more than 250 years before he had preserved from famine by his energy. What follows in verse 6, that "he turned himself, and went out from Pharaoh," is recorded as a token of his indignation; as though Moses, worn out with the perverseness of the tyrant, had hastily withdrawn himself from him, without bidding him farewell. Therefore, although he was otherwise of a mild disposition, this peremptory harshness was to be adopted as a reproof of the arrogance with which the tyrant spit in the face of heaven itself. But, let the Pharaohs of our age also learn, that when they impede by their cruel menaces the pure worship of God, it is in His strict justice that fanatics, like locusts, assail their kingdoms with their impious errors, and infect their people with contagion.

The locusts - The locust is less common in Egypt than in many eastern countries, yet it is well known, and dreaded as the most terrible of scourges. They come generally from the western deserts, but sometimes from the east and the southeast. No less than nine names are given to the locust in the Bible, of which the word used here is the most common (ארבה 'arbeh); it signifies "multitudinous," and whenever it occurs reference is made to its terrible devastations.

To-morrow will I bring the locusts - The word ארבה arbeh, a locust, is probably from the root רבה rabah, he multiplied, became great, mighty, etc.; because of the immense swarms of these animals by which different countries, especially the east, are infested. The locust, in entomology, belongs to a genus of insects known among naturalists by the term Grylli; and includes three species, crickets, grasshoppers, and those commonly called locusts; and as they multiply faster than any other animal in creation, they are properly entitled to the name ארבה arbeh, which might be translated the numerous or multiplied insect. See this circumstance referred to, Judges 6:5; Judges 7:12; Psalm 105:34; Jeremiah 46:23; Jeremiah 51:14; Joel 1:6; Nahum 3:15; Judith 2:19, 20; where the most numerous armies are compared to the arbeh or locust. The locust has a large open mouth; and in its two jaws it has four incisive teeth, which traverse each other like scissors, being calculated, from their mechanism, to grip or cut. Mr. Volney, in his Travels in Syria, gives a striking account of this most awful scourge of God: -
"Syria partakes together with Egypt and Persia, and almost all the whole middle part of Asia, in the terrible scourge, I mean those clouds of locusts of which travelers have spoken; the quantity of which is incredible to any person who has not himself seen them, the earth being covered by them for several leagues round. The noise they make in browsing the plants and trees may be heard at a distance, like an army plundering in secret. Fire seems to follow their tracks. Wherever their legions march the verdure disappears from the country, like a curtain drawn aside; the trees and plants, despoiled of their leaves, make the hideous appearance of winter instantly succeed to the bright scenes of spring. When these clouds of locusts take their flight, in order to surmount some obstacle, or the more rapidly to cross some desert, one may literally say that the sun is darkened by them."
Baron de Tott gives a similar account: "Clouds of locusts frequently alight on the plains of the Noguais, (the Tartars), and giving preference to their fields of millet, ravage them in an instant. Their approach darkens the horizon, and so enormous is their multitude, it hides the light of the sun. They alight on the fields, and there form a bed of six or seven inches thick. To the noise of their flight succeeds that of their devouring actively, which resembles the rattling of hail-stones; but its consequences are infinitely more destructive. Fire itself eats not so fast; nor is there any appearance of vegetation to be found when they again take their flight, and go elsewhere to produce new disasters."
Dr. Shaw, who witnessed most formidable swarms of these in Barbary in the years 1724 and 1725, gives the following account of them: "They were much larger than our grasshoppers, and had brown-spotted wings, with legs and bodies of a bright yellow. Their first appearance was towards the latter end of March. In the middle of April their numerous swarms, like a succession of clouds, darkened the sun. In the month of May they retired to the adjacent plains to deposit their eggs: these were no sooner hatched in June than the young brood first produced, while in their caterpillar or worm-like state, formed themselves into a compact body of more than a furlong square, and, marching directly forward, climbed over trees, walls, and houses, devouring every plant in their way. Within a day or two another brood was hatched, and advancing in the same manner, gnawed off the young branches and bark of the trees left by the former, making a complete desolation. The inhabitants, to stop their progress, made a variety of pits and trenches all over their fields and gardens, which they filled with water, or else heaped up therein heath, stubble, etc., which they set on fire; but to no purpose: for the trenches were quickly filled up and the fires extinguished, by infinite swarms succeeding one another; while the front seemed regardless of danger, and the rear pressed on so close that retreat was altogether impossible. In a month's time they threw off their worm-like state; and in a new form, with wings and legs, and additional powers, returned to their former voracity." - Shaw's Travels, 187, 188, 4th edition.
The descriptions given by these travelers show that God's army, described by the Prophet Joel, Joel 2:1-11, was innumerable swarms of locusts, to which the accounts given by Dr. Shaw and others exactly agree.

Else, if thou refuse to let my people go,.... He threatens him with the following plague, the plague of the locusts, which Pliny (x) calls "denrum irae pestis":
behold, tomorrow will I bring the locusts into thy coast; according to Bishop Usher (y) this was about the seventh day of the month Abib, that this plague was threatened, and on the morrow, which was the eighth day, it was brought; but Aben Ezra relates it as an opinion of Japhet an Hebrew writer, that there were many days between the plague of the hail, and the plague of the locusts, that there might be time for the grass and plants to spring out of the field; but this seems not necessary, for these locusts only ate of what were left of the hail, as in the following verse.
(x) Nat. Hist. l. 11. c. 29. (y) Annales Vet. Test. p. 21.

to-morrow will I bring the locusts--Moses was commissioned to renew the request, so often made and denied, with an assurance that an unfavorable answer would be followed on the morrow by an invasion of locusts. This species of insect resembles a large, spotted, red and black, double-winged grasshopper, about three inches or less in length, with the two hind legs working like hinged springs of immense strength and elasticity. Perhaps no more terrible scourge was ever brought on a land than those voracious insects, which fly in such countless numbers as to darken the land which they infest; and on whatever place they alight, they convert it into a waste and barren desert, stripping the ground of its verdure, the trees of their leaves and bark, and producing in a few hours a degree of desolation which it requires the lapse of years to repair.

To punish this obstinate refusal, Jehovah would bring locusts in such dreadful swarms as Egypt had never known before, which would eat up all the plants left by the hail, and even fill the houses. "They will cover the eye of the earth." This expression, which is peculiar to the Pentateuch, and only occurs again in Exodus 10:15 and Numbers 22:5, Numbers 22:11, is based upon the ancient and truly poetic idea, that the earth, with its covering of plants, looks up to man. To substitute the rendering "surface" for the "eye," is to destroy the real meaning of the figure; "face" is better. It was in the swarms that actually hid the ground that the fearful character of the plague consisted, as the swarms of locusts consume everything green. "The residue of the escape" is still further explained as "that which remaineth unto you from the hail," viz., the spelt and wheat, and all the vegetables that were left (Exodus 10:12 and Exodus 10:15). For "all the trees that sprout" (Exodus 10:5), we find in Exodus 10:15, "all the tree-fruits and everything green upon the trees."