The tone of "The Fly" is conversational and colloquial, as the narration often mimics how the characters themselves speak. This means that—in the beginning of the story, at least—the tone is somewhat humorous, since it aligns with the casual, glib comments that Londoners might say in jest to each other. Consider, for example, the following passage, in which Mr. Woodifield's wife and daughters try to imagine what he does when he goes into London once a week:
Though what he did there the wife and girls couldn't imagine. Made a nuisance of himself to his friends, they supposed.... Well, perhaps so.
This passage pokes fun at Woodifield by implying that even his own family sees him as a bumbling old man who putters around and makes "a nuisance of himself." The ellipsis after "they supposed" subtly suggests that his wife and daughters aren't necessarily wrong to assume he's an annoyance to his old coworkers, as does the narrative interjection, "Well, perhaps so"—an interjection that adds humor to this moment, as if the omniscient (and otherwise absent) narrator is winking at readers and inviting them to laugh at Woodifield.
As the story progresses, though, the tone shifts slightly. Although it remains conversational, the narration adopts a somewhat militaristic, stereotypically masculine tone when the boss watches the drowning fly:
He's a plucky little devil, thought the boss, and he felt a real admiration for the fly's courage. That was the way to tackle things; that was the right spirit. Never say die; it was only a question of ... But the fly had again finished its laborious task, [...].
The phrase "plucky little devil" is still very colloquial, but it also sheds light on the boss's performance of masculinity, as he thinks about the fly as if it's a brave and scrappy little soldier and he's its drill sergeant. When the narration goes on to say things like, "Never say die," the story's tone hints at the boss's impulse to stand strong in the face of hardship while maintaining the quintessentially British "stiff upper lip" (that is, an even-tempered refusal to succumb to strong emotions). In this way, the tone of "The Fly" sheds light on the boss's determination to avoid dwelling on the harrowing loss of his son.