“A guy walks into a bar…” is a typical form of what has been called the “bar joke.” A version with a fly is:
 
“A blowfly goes into a bar and asks: ‘Is that stool taken?’”
 
The pun relies on a different kind of “stool.” “Two flys go into a bar and ask ‘Is that stool taken?’” was cited in a newsgroup on 2000. “A fly walks into a bar and asks, ‘Is this stool taken?’” was cited on Twitter on August 23, 2011.
 
“A blowfly goes into a bar and asks: ‘Is that stool taken?’” was included with other science jokes in The Guardian (UK) on December 28, 2013.
 
   
Wikipedia: Calliphoridae
The Calliphoridae (commonly known as blow flies, blow-flies, carrion flies, bluebottles, greenbottles, or cluster flies) are a family of insects in the order Diptera, with 1,100 known species. The family is known to be polyphyletic, but much remains disputed regarding proper treatment of the constituent taxa, some of which are occasionally accorded family status (e.g., Bengaliidae, Helicoboscidae, Polleniidae, and Rhiniidae).
       
Google Groups: novell.community.chat
Two flys
Martin Flexman
11/19/00
Two flys go into a bar and ask “Is that stool taken?”
 
MumblingNerd’s Mumbling Blog
A man walks into a bar…
12 FEBRUARY 2010 (The joke doesn’t appear on this page in 2012.—ed.)
(...)
A blowfly walks into a bar and says to the bartender; “Is that stool taken?”
 
Twitter
Michael Parisi
‏@MPunlocked
A fly walks into a bar and asks, “Is this stool taken?”
12:01 AM - 23 Aug 2011
 
Twitter
adrianbruce
‏@adrianbruce
Dung beetle walks into a bar. “Is that stool taken?” hehehehe
12:59 AM - 20 Nov 2011
   
The Guardian (UK)
Scientists tell us their favourite jokes: ‘An electron and a positron walked into a bar…’
Saturday 28 December 2013 19.05 EST
(...)
. A blowfly goes into a bar and asks: “Is that stool taken?”
No idea where I got this from!
Amoret Whitaker, entomologist, Natural History Museum
     
Facebook
TeamOcho
July 31, 2014 ·
Another bar joke:
A blowfly goes into a bar and asks: “Is that stool taken?”
 
Earth Unfiltered
September 16, 2015 by Thomas McAuley-Biasi
15 Awful Nerdy Biology Jokes
(...)
6. A blowfly goes into a bar and asks- “Is that stool taken?“
 
Twitter
Fungus Among Us
‏@McGillMush
A blowfly goes into a bar and asks: “Is that stool taken?”
- Amoret Whitaker, entomologist, Natural History Museum
#sciencejokes #ENVB222
12:17 PM - 10 Nov 2015