How do mosquitoes sense so sensitively (I mean they swarm around you in hundreds within seconds if you’re outside in the Yukon during the summer months) the presence of blood? Is there any truth to the suggestion that one shouldn’t eat foods with a high content of sugar in them? Is there any truth to the suggestion that citronella works to deter them? What do they do when they’re not feeding - just hanging around in the air??
What they are sensing is carbon dioxide and/or warm, moist air. Both of these will be downwind of a mammal. Once a lady mosquito senses your air column by the hairlike sensors on her long snoot, she turns into that column, following it right to you.
In a mosquito-rich place such as the Yukon, there aren’t enough mammals to go around, so many of these little keening vampires will die without laying eggs, because they can’t find a blood meal.
Doug has some relevant information in his SDSAB article about repellents.