What figurative language is: this was the last straw upon the back of a manny a moderate man

in this passage from chapter 6 of "animal farm", by george orwell, what the author is ridiculing in this passage is the frivolity of some rules.

clover, the mare, believes she remembers that the fourth commandment forbade sleeping in beds because it was something that humans did. snowball, napoleon, and the pigs are brain-workers who look after the welfare of the farm. they are the obvious leaders because they are smart, and so they are entitled to eat the best food and sleep in beds. it is squealer, who says these lines making the difference between sleeping with sheets, what humans do, and sleeping in beds, what pigs do. the unwillingness to question authority makes the oppressed to obey without thinking.