In Book XXII of The Odyssey, why does Homer use the similes of the fly and the cows, then the vulture, in his description of the battle?
When Odysseus and Telemachus attack the suitors in Book XXII of The Odyssey, they flee in terror, described by Homer as follows:
And then, as cattle stung with a gad-fly,
In heat of summer run about the field,
So round about the hall the suitors fly.
This particular simile references the nature of the attack. Odysseus and Telemachus are like the aforementioned gad-fly; they appear to be harmless at first, but are ultimately a terrifying nuisance. Their attack is sudden and speedy and results in complete mayhem within the hall, as the suitors try to frantically escape. Comparing the suitors to cattle also suggest that they are mindless creatures who run in herds and are spooked by the smallest threat.
The attack is further described by Homer with another simile:
As when the vultures stoop down from the hill
Upon the fowl, these couch close to the plain,
Threatened with heavy clouds, they slay and kill,
These cannot fly away...
Again, we receive a vivid description of the manner in which Odysseus and Telemachus are attacking, swooping down upon the suitors and picking them mercilessly so that they have no chance of escape.
Ultimately, both these choices of simile suggest the animalistic behavior of the attackers and their victims and present to the reader a way of understanding the bloody events. Odysseus, who has witnessed these men take up residence on his land and attempt to prey on his wife, has no patience or mercy for the parasitic suitors--a truth which is demonstrated in the figurative language used.
(A note on sources: the translation used for this answer is by Sir William Molesworth.)
In Book XXII, Odysseus and Telemachus begin to slaughter the suitors of Penelope. The goddess Athene causes the suitors to become fearful and confused, running to the other end of the court "like a herd of cattle maddened by the gadfly in the early summer when the days are at their longest."
Odysseus and his men attack the sutors as vultures do the smaller birds. The sens here is of helplessness to do anything to protect themselves. They are at Odysseus' mercy as a slaughters them in his righteous rage. Though some beg for mercy, Odysseus has none. One by one, they all suffer the same fate.