The female mosquito possesses a specially adapted proboscis that is used like a syringe to extract blood from a host species. The blood nourishes her eggs. Mosquito saliva contains an anticoagulant, which thins the blood, thereby making it easier to extract. Mosquitoes locate hosts by detecting movement and body heat, and by sensing chemical cues such as carbon monoxide (produced by respiration) and lactic acid (contained in animal sweat). Male mosquitoes do not 'bite'; they obtain nourishment from the nectar of flowering plants. There are more than 2,700 species of mosquitoes, of which 200 occur in North America.