Insects like mosquitoes locate us through their sense of smell, not sight. The odors in our sweat and breath are extremely attractive to them. Specifically, carbon dioxide, lactic acid and an alcohol-based compound called 1-octen-3-ol are the main allure. The olfactory system of an insect contains odorant receptors, used to recognize the smell of prey. DEET interferes with these receptors, but scientists aren't sure exactly how. Does DEET smell bad to mosquitoes? Or does it simply make them unable to smell at all? Research results vary.
A study in 2008 showed that DEET targets certain odorant receptors in the insect, confusing its olfactory system and therefore "blinding" the bug to the odor that usually attracts it to us [source: Science Daily]. So, if this is true, DEET isn't actually a repellent. It's more like an invisibility cloak. However, a more recent study suggests that mosquitoes can actually smell DEET. A University of California Davis study showed that mosquitoes avoid DEET because they think it stinks [source: Fountain].