We know that mosquitos pick up clues that humans are around by detecting our heat, carbon dioxide shedding, and our scents. Human scent contains a lot of ingredients and varies from person to person, but mosquitos are very good at detecting when a human is around. For some species, their lives depend on it. And they've evolved a complex system to make sure they keep that ability.
Scientists genetically altered mosquitos to erase an entire family of human odor-sensing proteins from their genes. But those mosquitos were still able to sniff out a human presence! On further study, they found out that the mosquito olfactory system is different from other insects, and indeed other animals. Normally, an olfactory neuron is armed with receptors to detect one kind of odor. It is only with billions of olfactory neurons that an animal can detect and identify a myriad of smells. But mosquito neurons have more than one type of receptor per olfactory cell. They can lose receptors and still detect clues that a human is near.
While this may seem like a setback in the quest to disable mosquitos and prevent bites (and the associated diseases), it opens up a new frontier in research to develop bait for mosquito traps. Read what we've learned about how mosquitos smell humans at the Guardian. -via Damn Interesting