Canst thou make him afraid as a grasshopper? - Or, rather, "as a locust" - כארבה kā'arbeh. This is the word which is commonly applied to the locust considered as gregarious, or as appearing in great numbers (from רבה râbâh, "to be multiplied"). On the variety of the species of locusts, see Bochart "Hieroz." P. ii. Lib. iv. c. 1ff The Hebrew word here rendered "make afraid" (רעשׁ râ‛ash) means properly "to be moved, to be shaken," and hence, to tremble, to be afraid. In the Hiphil, the form used here, it means to cause to tremble, to shake; and then "to cause to leap," as a horse; and the idea here is, Canst thou cause the horse, an animal so large and powerful, to leap with the agility of a locust? See Gesenius, "Lex." The allusion here is to the leaping or moving of the locusts as they advance in the appearance of squadrons or troops; but the comparison is not so much that of a single horse to a single locust, as of cavalry or a company of war-horses to an army of locusts; and the point of comparison turns on the elasticity or agility of the motion of cavalry advancing to the field of battle.
The sense is, that God could cause that rapid and beautiful movement in animals so large and powerful as the horse, but that it was wholly beyond the power of man to effect it. It is quite common in the East to compare a horse with a locust, and travelers have spoken of the remarkable resemblance between the heads of the two. This comparison occurs also in the Bible; see Joel 2:4, "The appearance of them is as the appearance of horses; and as horsemen so shall they run;" Revelation 9:7. The Italians, from this resemblance, call the locust "cavaletta," or little horse. Sir W. Ouseley says, "Zakaria Cavini divides the locusts into two classes, like horsemen and footmen, 'mounted and pedestrian.' "Niebuhr says that he heard from a Bedouin near Bassorah, a particular comparison of the locust with other animals; but he thought it a mere fancy of the Arabs, until he heard it repeated at Bagdad. He compared the head of a locust to that of a horse, the breast to that of a lion, the feet to those of a camel, the belly with that of a serpent, the tail with that of a scorpion, and the feelers with the hair of a virgin; see the Pictorial Bible on Joel 2:4.
The glory of his nostrils is terrible - Margin, as in Hebrew, "terrors." That is, it is fitted to inspire terror or awe. The reference is to the wide-extended and fiery looking nostrils of the horse when animated, and impatient, for action. So Lucretius, L. v.:
Et fremitum patulis sub naribus edit ad arma.
So Virgil, "Georg." iii. 87:
Collectumque premens voluit sub naribus ignem.
Claudian, in iv. "Consulatu Honorii:"
Ignescunt patulae nares.

Canst thou make him afraid as a grasshopper?.... Which is frightened at every noise, and at any approach of men; but not so the horse; or canst thou move him, or cause him to skip and jump, or rather leap like a grasshopper? that is, hast thou given, or canst thou give him the faculty of leaping over hedges and ditches, for which the horse is famous? so Neptune's war horses are said (q) to be good leapers;
the glory of his nostrils is terrible: which may be understood of his sneezing, snorting, pawing, and neighing, when his nostrils are broad, spread, and enlarged; and especially when enraged and in battle, when he foams and fumes, and his breath comes out of his nostrils like smoke (r), and is very terrible.
(q) Homeri Iliad. 13. v. 31. (r) "Iguescunt patulae nares". Claudian. in 4. Consul. Honor.

make . . . afraid--rather, "canst thou (as I do) make him spring as the locust?" So in Joel 2:4, the comparison is between locusts and war-horses. The heads of the two are so similar that the Italians call the locusts cavaletta, "little horse."
nostrils--snorting furiously.

The motion of the horse, which is intended by תרעישׁנּוּ (רעשׁ, Arab. r‛s, r‛š, tremere, trepidare), is determined according to the comparison with the grasshopper: what is intended is a curved motion forwards in leaps, now to the right, now to the left, which is called the caracol, a word used in horsemanship, borrowed from the Arab. hargala-l-farasu (comp. חרגּל), by means of the Moorish Spanish; moreover, Arab. r‛s is used of the run of the ostrich and the flight of the dove in such "successive lateral and oblique motions" (Carey). nachar, Job 39:20, is not the neighing of the horse, but its snorting through the nostrils (comp. Arab. nachı̂r, snoring, a rattling in the throat), Greek φρύαγμα, Lat. fremitus (comp. Aeschylus, Septem c. Th. 374, according to the text of Hermann: ἵππος χαλινῶν δ ̓ ὡς κατασθμαίνων βρέμει); הוד, however, might signify pomp (his pompous snorting), but perhaps has its radical signification, according to which it corresponds to the Arab. hawı̂d, and signifies a loud strong sound, as the peal of thunder (hawı̂d er-ra‛d),' the howling of the stormy wind (hawı̂d er-rijâh), and the like.
(Note: A verse of a poem of Ibn-Dchi in honour of Dkn ibn-Gendel runs: Before the crowding (lekdata) of Taijr the horses fled repulsed, And thou mightest hear the sound of the bell-carriers (hawı̂da mubershemât) of the warriors (el-menâir, prop. one who thrusts with the lance). Here hawı̂d signifies the sound of the bells which those who wish to announce themselves as warriors hang about their horses, to draw the attention of the enemy to them. Mubershemât are the mares that carry the burêshimân, i.e., the bells. The meaning therefore is: thou couldst hear this sound, which ought only to be heard in the fray, in flight, when the warriors consecrated to death fled as cowards. Taijr (Têjâr) is Slih the son of Cana'an (died about 1815), mentioned in p. 456, note 1, a great warrior of the wandering tribe of the 'Aneze. - Wetzst.)
The substantival clause is intended to affirm that its dull-toned snort causes or spreads terror. In Job 39:21 the plur. alternates with the sing., since, as it appears, the representation of the many pawing hoofs is blended with that of the pawing horse, according to the well-known line,
Quadrupedante putrem sonitu quatit ungula campum
(Virgil, Aen. viii. 596);
or, since this is said of the galloping horse, according to the likewise Virgilian line,
Cavatque
Tellurem, et solido graviter sonat ungula cornu
(Georg. iii. 87 f).
חפר is, as the Arab. hâfir, hoof, shows, the proper word for the horse's impatient pawing of the ground (whence it then, as in Job 39:29, signifies rimari, scrutari). עמק is the plain as the place of contest; for the description, as now becomes still more evident, refers to the war-horse. The verb שׂישׂ (שׂוּשׂ) has its radical signification exsultare (comp. Arab. s]ts, skirta'n, of the foetus) here; and since בּכח, not בּכּח, is added to it, it is not to be translated: it rejoices in its strength, but: it prances or is joyous with strength, lxx γαυριᾷ ἐν Ἰσχύΐ. The difference between the two renderings is, however, scarcely perceptible. נשׁק, armament, Job 39:21, is meton. the armed host of the enemy; אשׁפּה, "the quiver," is, however, not used metonymically for the arrows of the enemy whizzing about the horse (Schult.), but Job 39:23 is the concluding description of the horse that rushes on fearlessly, proudly, and impetuously in pursuit, under the rattle and glare of the equipment of its rider (Schlottm. and others). רנה (cogn. of רנן), of the rattling of the quiver, as Arab. ranna, ranima, of the whirring of the bow when the arrow is despatched; to point it תּרנּה (Proverbs 1:20; Proverbs 8:3), instead of תּרנה, would be to deprive the language of a word supported by the dialects (vid., Ges. Thes.). On Job 39:24 we may compare the Arab. iltahama-l-farasu-l-arda, the horse swallows up the ground, whence lahimm, lahı̂m, a swallower = swift-runner; so here: with boisterous fierceness and angry impatience (בּרעשׁ ורגז) it swallows up the ground, i.e., passes so swiftly over it that long pieces vanish so rapidly before it, as though it greedily sucked them up (גּמּא intensive of גּמא, whence גּמא, the water-sucking papyrus); a somewhat differently applied figure is nahab-el-arda, i.e., according to Silius' expression, rapuit campum. The meaning of Job 39:24 is, as in Virgil, Georg. iii. 83f.:
Tum si qua sonum procul arma dedere,
Stare loco nescit;
and in Aeschylus, Septem, 375: ὅστις βοὴν σάλπιγγος ὁρμαίνει (Hermann, ὀργαίνεἰ μένων (impatiently awaiting the call of the trumpet). האמין signifies here to show stability (vid., Genesis, S. 367f.) in the first physical sense (Bochart, Rosenm., and others): it does not stand still, i.e., will not be held, when (כּי, quum) the sound of the war-trumpet, i.e., when it sounds. שׁופר is the signal-trumpet when the army was called together, e.g., Judges 3:27; to gather the army that is in pursuit of the enemy, 2-Samuel 2:28; when the people rebelled, 2-Samuel 20:1; when the army was dismissed at the end of the war, 2-Samuel 20:22; when forming for defence and for assault, e.g., Amos 3:6; and in general the signal of war, Jeremiah 4:19. As often as this is heard (בּדי, in sufficiency, i.e., happening at any time = quotiescunque), it makes known its lust of war by a joyous neigh, even from afar, before the collision has taken place; it scents (praesagit according to Pliny's expression) the approaching conflict, (scents even in anticipation) the thundering command of the chiefs that may soon be heard, and the cry of battle giving loose to the assault. "Although," says Layard (New Discoveries, p. 330), "docile as a lamb, and requiring no other guide than the halter, when the Arab mare hears the war-cry of the tribe, and sees the quivering spear of her rider, her eyes glitter with fire, her blood-red nostrils open wide, her neck is nobly arched, and her tail and mane are raised and spread out to the wind. The Bedouin proverb says, that a high-bred mare when at full speed should hide her rider between her neck and her tail."