The Twit's Death
Throughout the novel, the Twits force their monkeys to stand upside down. The monkeys have enough of this and seek their revenge, and the couple ends up glued upside-down themselves in an ironic twist of fate.
Irony of revenge
The Twits have an unhappy marriage and are constantly playing tricks on one another. However, through seeking revenge on one another they have ended up in a cycle of revenge, causing unnecessary unhappiness for themselves.
Irony of mistreatment
The Twits are cruel towards their pet monkeys and the birds surrounding their house. Ironically, these animals take revenge on the Twits, with the help of the Roly-Poly bird.