The main theme of "The Fly" is death.
Perhaps the darkest of Katherine Mansfield's works, "The Fly" is an existential study of the effects of senseless death upon others and their loss of will.
On the boss's desk sits a photograph of his son, a youth taken from his father by World War I. Because the boy was buried in Belgium, the family never experienced the realism of his death. And because the boss has never fully accepted the finality of his son's death, he is quite disturbed when his former employee, Mr. Woodifield, visits and tells his old boss about his daughters' trip to Belgium. While the girls were in the cemetery in which their own brother was buried, they discovered the grave of the boss's son.
The boss makes no comment on this revelation. Instead, he makes a trite remark on Woodifield's tale of the daughters' purchase of a pot of jam, and then he follows Woodifield out the door.
For a long moment the boss stayed, staring at nothing, while the grey-haired office messenger, watching him, dodged in and out of his cubby-hole like a dog that expects to be taken for a run. Then: 'I'll see nobody for half an hour, Macey,' said the boss. 'Understand! Nobody at all.'
Visibly shaken by the reality of his son's death, the boss sits motionless. He lived for his son to take over his business. Now he senses the existential meaninglessness of life that simply ends in death. As he looks at the photograph, it seems different from all the other times that he has glanced at it. Then, a fly falls into the boss's ink pot, and the boss lifts it out with his pen. The fly is able to clean the ink from itself and test its wings. But before it can take off, the boss puts it back into the ink pot. So the fly must begin again, and again it succeeds. But, as the agent of fate, the boss "decided this should be the last time."
The fly's death may well symbolize the death of will. Mr. Woodifield has certainly been weakened; he suffers after his stroke, and he suffers from a death of the strength and will to remember. The employee Macey has a death of will, also, as he mechanically obeys the orders of the boss. Moreover, the boss has a death of the will to feel after the loss of his son because he treats his employee without concern for the man's sensibilities.