Mosquitoes prefer some people over others. The preferred victims sweat smells more attractive than others because of the proportions of the carbon dioxide, octenol, and other compounds that make up body odor. The most powerful semiochemical that triggers the keen sense of smell of Culex quinquefasciatus is nonanal. Another compound identified in human blood that attracts mosquitoes is sulcatone or 6-methyl-5-hepten-2-one, especially for Aedes aegypti mosquitoes with the odor receptor gene Or4. A large part of the mosquitos sense of smell, or olfactory system, is devoted to sniffing out blood sources. Of 72 types of odor receptors on its antennae, at least 27 are tuned to detect chemicals found in perspiration. In Aedes, the search for a host takes place in two phases. First, the mosquito exhibits a nonspecific searching behavior until the perception of a hosts stimulants, then it follows a targeted approach.Most mosquito species are crepuscular (dawn or dusk) feeders. During the heat of the day, most mosquitoes rest in a cool place and wait for the evenings, although they may still bite if disturbed. Some species, such as the Asian tiger mosquito, are known to fly and feed during daytime.Prior to and during blood feeding, blood-sucking mosquitoes inject saliva into the bodies of their source(s) of blood. This saliva serves as an anticoagulant; without it the female mosquitos proboscis might become clogged with blood clots. The saliva also is the main route by which mosquito physiology offers passenger pathogens access to the hosts bloodstream.
On the opposite movement, the mandibles are pushed deeper into the skin by levering against the maxillae. The maxillae do not slip back because the toothed blades grip the skin. The hypopharynx and the labrum are both hollow. Saliva with anticoagulant is pumped down the hypopharynx to prevent clotting, and blood is drawn up the labrum. To understand the mosquito mouthparts, it is helpful to draw a comparison with an insect that chews food, such as a dragonfly. A dragonfly has two mandibles, which are used for chewing, and two maxillae, which are used to hold the food in place as it is chewed. The labium forms the floor of the dragonflys mouth, the labrum forms the top, while the hypopharynx is inside the mouth and is used in swallowing. Conceptually, then, the mosquitos proboscis is an adaptation of the mouthparts that occur in other insects. The labium still lies beneath the other mouthparts, but also enfolds them, and it has been extended into a proboscis. The maxillae still "grip" the "food" while the mandibles "bite" it. The top of the mouth, the labrum, has developed into a channeled blade the length of the proboscis, with a cross-section like an inverted "U". Finally, the hypopharynx has extended into a tube that can deliver saliva at the end of the proboscis. Its upper surface is somewhat flattened so, when the lower part of the hypopharynx is pressed against it, the labrum forms a closed tube for conveying blood from the host.