A/C needed for Venice apartment in mid-September?

Yes, you need A/C in Venice in September. It is still quite warm and as you mentioned there are mosquitoes. Screens are rare in Italy, which is insane. <Maybe with Zika spreading they will adopt window screens, but most Italians I know close up tight at night, even in summer when they do not have A/C. I don't get it. I need fresh air or A/C in summer!

It's a matter of preference. I stayed in Siena, Montalcino in August one year and never used the AC It was hot but the window shades kept the room cool when we got back at night. I did use the AC one night on Montepulicano on that trip though. Those are further south then Venice.

Weather questions are always a total guess but looking at historical averages the last couple of years. Venice in mid-Sept will likely be highs in the mid to low 70's and lows in the upper 50's to low 60's
I would not use Air Conditioning in such temps. even if humid but some would I guess.
You can leave the windows open during the day, as dusk comes the weather will be cooler so would not worry so much about shutting them if you need to.

Lots of apartments in historical centers, not only in Venice, don't have air conditioning. September can still be hot, but probably not as bad as earlier in the summer.
I know that San Franciscans are not used to temperatures over 60 F and you start missing the fog whenever temperatures hit 70 F, but most of us in the Bay Area can survive the 70's and even low 80's without air conditioning so I wouldn't have a problem with that apartment. Just ask if they have a fan (or buy one once there). Mosquitoes have a hard time landing on your body with high 'tail winds', they are like little Cessna planes.

Just get a place with AC and that way you don't have to worry about it anymore. You need AC so you can close the windows and keep those insane mosquitos out! No, it's not just a matter of personal perference...that would be for temperature only. When mosquitos are added to the equation there's no question about it - get AC.

Thank you, everyone! I enjoy warm nights b/c we get so few of them here in the Bay Area (as Roberto mentioned), but I would much prefer to leave the windows open if it's above 60F. Sounds like that's a bad idea on the mosquito side of the equation -- although the fan idea is intriguing, and I've also read (here, I think) about some sort of plug-in option.

The only other time I've traveled in Italy before it was also Sept., and none of the hotels/convents/B&Bs I stayed in had A/C -- or at least I never noticed it on, or turned it on myself. Don't remember any mosquito bites. I didn't visit Venice that trip, but did visit Florence and Rome.

My parents never installed air conditioning, and Florence is awful both for humid heat and mosquitoes. Italians (especially older generations) have this crazy belief that air conditioning and cool breezes are the root of all evils and cause pretty much all diseases known to mankind, from common cold, to pneumonia, to arthritis and, I suppose, Ebola. An easy way for me to get my father pissed was to turn a fan on, or even just create a draft by opening two windows on the opposite side of the house. He would simply accuse me of trying to kill him. He died of kidney disease a few years ago, I presume due to my insistence of turning the fan on during my yearly visits.

When you are in Italy, just buy some Vape products, like Vapemat, which you can plug in the socket, to keep the mosquitoes away while the windows are open. Just make sure you pronounce it the Italian way when you buy it at any hardware store (ferramenta). It's VA- PAY MUTT. Vape is a leading manufacturer of insect repellents.
http://www.vape.com/en/products/

Mosquitoes have a hard time landing on your body with high 'tail winds', they are like little Cessna planes.

Love it!!

We are often reluctant to keep the windows open at night because of noise. So if you are in an area with street noise, that's another reason to get A/C.

Not much street noise in Venice, certainly no traffic noise. We have used the VAPE devices andthet do work. Available also at supermarkets, cost about five euro, I recollect.