Dogs (similes & metaphors)
As Japanese literature is frequently allied on the side of the cat, Akutagawa, being the contrarian of his time, prefers the metaphorical dog. However, dogs don't exactly have it good in his stories.
In "Rashōmon," the servant feels that if he did not become a thief, his dead body would be thrown under the gate like a stray dog anyway.
In "Yam Gruel," Goi truly lives "like a dog, constantly teased, [who] won't jump at a piece of meat [i.e. an offer to eat his fill of yam gruel]" (loc. 393). Later on Goi even reminisces about his life as a "homeless mongrel" (loc. 568).
In "Kesa and Morito," Kesa says that Morito had treated her like a tortured "leprous dog" (loc. 813).
"The Christians who had been standing before Santa Lucia, each and all hung their heads like the heads of wheat blown by the wind, and knelt around Lorenzo" ("The Martyr," loc. 702) (simile)
Heads of wheat blow in the wind without understanding why or how they are being moved. They hang their heads when things outside of their conception of reality are imposed on them, just as the congregation must bow their heads to the almighty power introduced by Lorenzo's martyrdom.
"[Simeon's] harmonious friendship with Lorenzo might have been compared to a fierce eagle taking loving care of a dove or a blooming vine twining around a cedar on Mt. Lebanon." ("The Martyr," loc. 581) (simile)
Here, Akutagawa uses a trite simile -- something that is rare for him. It evokes Christian imagery with the dove and Mt. Lebanon (no, not the municipality in Pennsylvania), but fails to bring any freshness to it. It seems that this story departs from Akutagawa's normal writing style in many regards.
"A year passed like a snowflake that falls into the river, a moment white and then gone forever" ("The Martyr," loc. 637) (simile)
Just as Lorenzo's excommunication was made in the blink of an eye, a year passes without a moment's notice. It seems that one year blends into the next without consideration for Lorenzo's wretched condition.
As a side note, the many metaphorical references from "The Martyr" may also be considered a stylistic choice, given that he was attempting to emulate an archaic and poetic mode.
"Truly human life is as evanescent as the morning dew or a flash of lightning" ("In a Grove, loc. 102) (simile)
The Traveling Buddhist Priest does not have much to say and even mistakes some details -- wittingly or unwittingly, the reader will never know. What we can ascertain is that he apparently reads the poetry of Samuel Butler, who is the original author of this sentence.
"As quietly as a lizard, the servant crept up to the top of the steep stairs. Crouching on all fours, and stretching his neck as far as possible, he timidly peeped into the tower." ("Rashōmon," loc. 255) (simile)
The servant's likeness to a reptile-brained creature is uncanny because of the atrocities he is about to commit against the old woman. The whole of this story centers on man's likeness to creatures when circumstances are dire. A lizard has no moral code.
