Mosquitoes hunt by homing in on infrared emissions from warm-blooded animals, which is a whole lot easier when air temperature is cooler than the animal's body temperature. This is why these bugs only come out at dusk when the temperature isn't too hot but it's lower than their prey's body temperature.
Only female mosquito's bite human's. They use our blood to help fertilize their eggs. Also the reason a mosquito bite swells and gets itchy is because before sucking our blood they inject a poison to make it easier to suck our blood
It's easier to find warm things using their infrared sensors when everything else is cooler. Let me put it like this: You drink blood. If you could only see things that were warm as red and what was cool as blue, but no shapes, would you wanna run into anything that's been sitting in the sun for a while and is now red in your vision? Or would you want to be able to clearly see what is really warm and what is really cold?
There are several good points, and I confess I don't know why they come out at night. I believe the cooler temperature is the most correct. However, they will and do come out in the hottest temperatures (but stay close to water usually). The infrared may also be true, as far as their sight, but they are not drawn to anything by infrared vision. Mosquitoes, both male and female are drawn by carbon dioxide. Since this is given off by warm blooded animals when they breathe, it makes the most sense (and it a proven scientific fact as well). The female is the only one to draw blood, but the male can also bite, they don't often, because they don't need to. The poison injected is more of a blood thinner, which is why you get a bump, the blood builds up under the skin where you were bitten.