I give a thumbs up to three of gomango's four recommendations: Cima Coppi, Kucharik and Vintgage Velos. The only reason I don't give one to Smart Wool is that I don't have any of theirs. But I sure like that full-length zipper. If gomango says Smart Wool is on a par with the other three, that is good enough for me.
Thin merino wool is the bee's knees for hot weather. It handles perspiration very well, and you will especially like not stinking to high heaven after the ride. From my experience, Vintage Velos jerseys would be the best for a hot day because they are the thinnest. (They also have a super-thin line, but I have not tried them.) My Cima Coppi jersey is a little thicker, and my two Kuchariks are a little thicker yet, although they have the longest zippers. I would hesitate to wear one of my Kuchariks in the heat, not because they are wool, but because they are comparitively thick. I'd hesitate to wear a thick non-wool jersey in the heat, too.
And here is no fabric in the world that will make riding in 115 degree weather anything other than a life-threatening suffer-fest.
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"I'm in shape -- round is a shape." Andy Rooney