Mosquitoes are a ubiquitous presence in human life, yet only the adult female seeks out and takes a blood meal from a host. This behavior separates her from the male mosquito, who is not equipped for or interested in biting humans or animals. Understanding this distinction involves examining the specific nutritional needs of the female and the sophisticated sensory systems she employs to complete her reproductive cycle.
Why Only Female Mosquitoes Bite
The fundamental difference in feeding behavior between male and female mosquitoes is driven entirely by reproduction. Both sexes rely on plant nectar, fruit juices, and other sugary substances as their primary food source for daily energy needs, such as flying and metabolic functions. However, only the female requires the specialized nutrients found in the blood of vertebrates to produce viable eggs.
The complex process of egg development necessitates a high concentration of proteins and lipids, which are abundant in blood and largely absent in plant sugars. This biological requirement initiates the gonotrophic cycle: the sequential process of host-seeking, blood-feeding, blood digestion, egg maturation, and egg-laying. Without the proteins and iron acquired from a blood meal, the female cannot synthesize the components needed to mature her eggs.
A single blood meal can be sufficient for a female mosquito to lay one batch of eggs, a process that typically takes two to four days. Once the eggs are laid, the female re-enters the host-seeking phase to obtain another blood meal and start the cycle anew. The male mosquito, whose reproductive role ends with mating, has no such nutritional need. Consequently, he lacks the specialized mouthparts—the piercing proboscis—required to penetrate skin and access a capillary.
How Female Mosquitoes Locate Humans
The female mosquito’s search for a blood host is a sophisticated, multi-stage process involving the integration of various sensory cues over different distances. The initial, long-range alert is the detection of carbon dioxide (\(\text{CO}_2\)), which all vertebrates exhale. Female mosquitoes can detect \(\text{CO}_2\) plumes from distances up to 50 meters, using specialized organs on their antennae called maxillary palps.
As the mosquito follows the \(\text{CO}_2\) gradient, the elevated concentration triggers a change in her behavior, enhancing her sensitivity to other cues at an intermediate range. Once she closes the distance to about five to fifteen meters, visual cues become important as she tracks the host’s silhouette. Mosquitoes are attracted to dark or high-contrast objects that stand out against the horizon.
At the short range, within one meter, thermal and volatile chemical signatures confirm the target. The mosquito detects the heat radiating from the body, specifically the thermal infrared signature of skin, which averages about \(34^{\circ}\text{C}\). This thermal detection is paired with an olfactory assessment of the unique cocktail of human-produced volatile chemicals found in sweat and on the skin. Key attractants include L-lactic acid, octenol (1-octen-3-ol), and ammonia, which are breakdown products of human skin bacteria and metabolism.
Female Mosquitoes as Disease Vectors
The female mosquito’s requirement for blood makes her an effective vector, capable of transmitting pathogens from one host to another. Transmission occurs not when the mosquito ingests blood, but when she injects saliva into the host before feeding. Mosquito saliva is a complex fluid containing vasodilators and anticoagulants, which prevent the host’s blood from clotting and ensure a smooth blood meal.
If a female mosquito has previously fed on an infected host, viruses or parasites will have multiplied and migrated from her gut to her salivary glands. When she probes a new host, she injects these pathogens along with her saliva into the skin. Components of the saliva actively assist the infection process by suppressing the host’s local immune response, allowing the pathogen to establish itself more easily.
This mechanism is responsible for the spread of some of the world’s most severe illnesses. Female Anopheles mosquitoes transmit the parasitic protozoa that cause Malaria, a disease responsible for millions of deaths globally. Species from the Aedes genus, such as Aedes aegypti, are vectors for various arboviruses, including Dengue, Zika, and Chikungunya viruses. Furthermore, Culex mosquitoes transmit West Nile Virus and various forms of Encephalitis.