Female mosquitoes require the proteins in blood to produce their eggs, driving their search for a host. They rely on a complex, multi-stage sensory system to successfully locate a blood meal. The sequential use of different cues allows the insect to navigate a complex environment and accurately target a host, from long-range detection to final landing.
Carbon Dioxide as the Long-Distance Beacon
The initial signal a mosquito uses to detect a potential host is the plume of carbon dioxide (CO2) released from breath. Specialized receptors on the mosquito’s maxillary palps, small appendages near the mouth, are exquisitely sensitive to CO2 concentration. These receptors allow the insect to detect exhaled breath from significant distances.
Human breath contains approximately 4 to 5% CO2, significantly higher than the ambient atmospheric level. The mosquito detects this concentration gradient, which activates its flight and host-seeking behavior. Following the increasing concentration of CO2, the mosquito is guided closer to the source. This gaseous signal acts as an initial filter, confirming the presence of a large, breathing organism.
The Specifics of Human Scent Signatures
Once the mosquito enters the CO2 plume, it switches focus to the volatile organic compounds (VOCs) emanating from the host’s skin. These chemical attractants differentiate a human from other breathing animals. Lactic acid is a prominent attractant, produced when sweat is metabolized, and its presence synergizes strongly with the CO2 signal.
The unique “human scent” is produced by resident skin microbiota. Bacteria such as Staphylococcus and Corynebacterium metabolize compounds in human sweat and sebum, releasing a complex bouquet of VOCs. These include specific short-chain carboxylic acids and ammonia.
The specific composition and concentration of these bacterial byproducts determine an individual’s attractiveness. The mix of these compounds, rather than a single chemical, creates the unique scent signature the mosquito tracks for short-range targeting.
Non-Chemical Attractants (Heat, Vision, and Movement)
In the final stage of targeting, the mosquito relies on non-chemical, physical cues to pinpoint the exact location for feeding. Movement is a strong visual attractant that draws the mosquito’s attention from a modest distance.
The detection of CO2 activates the mosquito’s visual system to scan for certain colors. Mosquitoes are strongly attracted to colors in the long-wavelength range, specifically red, orange, black, and cyan. Since all human skin, regardless of pigmentation, emits a signal in the red-orange range, these colors act as a visual beacon.
A thermal sensing system allows the mosquito to detect the host’s body heat and home in on warm areas. Using these heat sensors, the insect distinguishes the temperature of the skin from the ambient environment. This thermal cue helps the mosquito locate a warm surface for the final landing and puncture.