Why are there mosquitoes?

Why weren't there any when it was hot, but now there are so many? I clearly remember when I was younger, there were lots of mosquitoes when it was hot and none when it got cooler. But as time went by, there were mosquitoes in the summer because it was summer, and then there were some in the fall when it got cooler because they could survive. Now, though, there are none when it's hot and tons of them right now. It's the complete opposite... lol