How do mosquitoes find you in the dark?

Mosquitoes find you mainly by smell. A female that needs blood follows the carbon dioxide and a chemical called 1-octen-3-ol in your breath, and she also uses sight. Then she homes in on the odours of your skin. Smelling needs no daylight, which suits an insect that mostly feeds at dawn or dusk: your body is broadcasting.

Only females drink blood. All mosquitoes drink nectar, but the females of many species also drink blood, which supplies the proteins she needs to make eggs. Males only drink sugary fluids and have less specialised mouthparts. So a bite is, above all, a mother shopping for her eggs.

The smell system is built for the job. A large part of her olfactory system is devoted to sniffing out blood sources, and of 72 types of odour receptors on her antennae, at least 27 are tuned to chemicals found in perspiration. In the species Aedes, the search comes in two phases: she flies about until she detects a host's odorants, then flies towards them, using the concentration of odorants as her guide.

Why you and not your friend? Mosquitoes are reported to prefer people with type O blood, an abundance of skin bacteria and high body heat, and they also favour pregnant women. Much of the odour on your skin is made when skin bacteria break down sweat and sebum, and how attractive you are to mosquitoes has a heritable, genetic component.