The phrase "a fly which" is correct and usable in written English.
It can be used when providing additional information or description about a fly in a sentence.
Example: "I saw a fly which was buzzing around the room, landing on various surfaces."
Alternatives: "a fly that" or "a fly who".
"I feel like a fly which wants to stand up against that tank".
After that, I started to get a little lost, and was then distracted by a fly, which I managed to swat at and kill with the book.
In 1965 a Swiss graduate student, Walter Gehring, discovered a fly which had grown legs on its head in the place where it should have had antennae.
Scientists at Harvard University have already made a robot modelled on a bee, the size of a fly, which they have christened "RoboBee".
One memorable moment in the film was improvised: in response to Paul Newman's kiss, Neal swats a fly, which she happened to notice on the set.
When Burnett completes his follow-through after a strikeout, as he did four times against the Orioles, his right arm flies upward as if he is swatting a fly, which, in effect, he was doing Thursday night.
He passed out a flyer, which read in part: "Dealing with the increasing problem of abandoned and vacant homes is at the forefront of our efforts to continue improving our community...
A flyer which gave the recipient free entry in to La Poubelle - 1960's nightclub based in London's Soho.
Parents received the information via a flyer which a researcher also discussed with the parents.
Nevertheless, the mobile team will contact each patient in the control group, providing them with general information in the form of a flyer which will outline the existence of the mobile team, and provide relevant addresses and telephone numbers.
We have assembled transcript sequences from the complete life cycle of T. biloba, a sepsid fly which exhibits primary gain of a novel trait, and identified many developmentally important genes.
