How Do Mosquitos Decide Who To Bite?

For whatever reason, I haven't been bitten by a mosquito in the continental US since I was about 15 years old.  My wife, on the other hand, gets treated like a mobile buffet by mosquitos and related bloodsuckers any time she leaves the house after dark.  The bite-affinity thing seems at least a little bit hereditary-- one of my grandfathers had the same no-mosquito-bites deal going while he was alive, and various other members of my family seem to have varying degrees of it (mine seems to work the best, a rare bright spot in the scorched and time-bomb-strewn wreckage of my genome). One evening while I was enjoying a lakeside sunset and my wife was enjoying having most of the blood removed from her body by carnivorous insects though, I got to wondering 1) why they bite some people and not others, and 2) as a corollary, how they locate prey.

I'm actually insecure enough to be slightly jealous that they like her so much better.

Because I'm an electrical engineer and have an embarrassingly simplistic understanding of nature and biology, my first thought was that they probably locate prey via infrared vision.  Mosquitos hunt at night, so visible light isn't going to be that useful, and their prey (any endothermic animal, such as my wife) light up like light bulbs if you're seeing them in the right range of infrared.  The hypothesis made some superficial sense-- I've got a lowish standard body temperature of about 97.9, while my wife is closer to the "normal" 98.6.  It's also at least anecdotally true that mosquitos like to bite hot, sweaty people more.  Giving the IR hypothesis a little more thought though, differentiating between infrared sources with a temperature variation of less than one degree Fahrenheit would be quite a feat, definitely way beyond the capability of any sensor we've ever built that isn't way bigger than a mosquito.  That's not necessarily a deal-breaker; nature is annoyingly full of stuff that science can't replicate (yet), but it did make the whole thing questionable.

As usual, my "infrared hypothesis" was absolutely dead wrong.  Like most bugs and other small critters, it turns out that mosquitos do most of their sensing and hunting via sophisticated chemical detection, also occasionally known as "smell."  As anyone who's ever used a public restroom knows, the olfactory (smell) sense is about as good as it gets sensor-wise in terms of bang for the buck-- a single olfactory receptor can be triggered by a single molecule of some chemicals, even in humans (who have kind of low-end olfactory systems compared to most other things out there).  Mosquitos' senses of smell are tuned to detect very specific things; the important ones for our purposes are carbon dioxide (exhaled by everything with lungs, as well as secreted through the skin to varying degrees) and various organic molecules that show up as trace chemicals in human sweat.  So people who sweat more are going to get bitten more, not because they're showing up better in infrared but because they're secreting way more of the things that mosquitos use to locate prey.  In general, I don't sweat a whole lot compared to my wife, which probably accounts for our difference in popularity among the bloodsucking set.  In addition to demonstrating (yet again) my general ignorance of the world around me, my little odyssey of discovery here is an excellent example of how easy it is to come to completely the wrong conclusion about something using only a limited set of data and a few bad assumptions.

Interestingly, some more reading appears to indicate that blood type also plays a role in mosquito affinity.  That seems counterintuitive on its face, since the mosquito obviously doesn't know what kind of blood you have til it's already biting you, but apparently different blood types secrete different amounts of the chemicals mosquitos go looking for.  Again, this roughly correlates with my anecdotal observations-- I'm type O-something, which means I've got the lowest possible amount of extra stuff in my blood.  My wife is either A or B, I'm pretty sure; theoretically, she'd be in even worse shape if she was AB (I have no idea if the positive/negative factor matters).  Another, more disturbing metric is blood-alcohol level-- an elevated one apparently increases your likelihood of getting bitten by quite a bit, either because alcohol in your blood slightly alters its chemical composition (and probably as a result your sweat-signature) or possibly just because drunk people tend to sweat more.  Because science is important, I'll test this hypothesis by sitting out in the backyard with a 12-pack as soon as its warm enough for mosquitos and other living things to be outside where I live.