In chapter 12, Miss Brent feels drowsy and thinks she hears a buzzing in her ears. She looks around and notices that there is a bumblebee buzzing in the room. She then feels the sensation of a prick in her neck and assumes it is the bee stinging her.

Meanwhile, Dr. Armstrong, William Blore, Vera Claythorne, and Justice Wargrave are all discussing the possibility that Miss Brent could be the murderer. They go to observe her demeanor and find her dead. Dr. Armstrong takes a closer look at her body and notices that there is a marking on her neck that indicates that she was injected with a hypodermic needle. He himself carries one of these needles because he is a doctor, and as he says, doctors take these needles with them wherever they travel. But when Armstrong goes to get the needle he came with, it is missing. This suggests that the murderer used his needle to kill Miss Brent.

So it was not the bee that stung her after all. But the fact that there was a bee in the room recalls the nursery rhyme about the ten little soldiers who all died in eerily similar ways to the order of characters in the book:

Six little soldiers playing with a hive; A bumblebee stung one and then there were five.