A new study published in the journal Current Biology adds to the understanding of how mosquitoes acquire human targets to bite, proving that they use a so-called triple threat system which combines visual, thermal and olfactory signs.
At one point, the study offers an example of a female mosquito which finds herself in need of providing a blood meal to her young. The mosquito searches for a blood host – in towns, cities or other populated areas humans are the most likely targets. And easy at the same time, as the carbon dioxide we exhale attracts them. At the same time, they can of course spot a victim if it has line of sight towards it, but also get thermal cues which makes it easily detect body heat.
However, the main question remained in what ways do they use all this information to determine their next victim. To get an answer, the researchers released hungry female mosquitoes into wind tunnels with different controlled sensory signals. For example, one experiment saw the injection of a plume with a high carbon dioxide concentration into the wind tunnel, in an attempt to resemble olfactory cues from humans.
To differentiate between human exhaling and normal air composition signals, the same set of mosquitos was then treated to a normal air composition plume. As expected, they pursued the CO2 plume while ignoring the other one.
Going to the next phase, they inserted a dark object into the tunnel and then released the same CO2 and non-CO2 plumes to examine the insects’ response. In the CO2 case they pursued the dark object, while in the other variants they simply ignored it; this came to prove that mosquitoes put more emphasis on olfactive cues rather than visual ones.
To also test their response to thermal signals, two glass objects which were applied a chemical substance that made capable of emanating heat were released into the wind tunnels. One of them was heated to human body temperature, whilst the other was kept at room readings, which were about 15 degrees lower. They also released the CO2 and background air plumes; apparently, the mosquitoes had a clear preference for the warm body regardless of the presence of CO2 traces near it.
In the end, researchers were able to map the way in which a mosquito find its target; it all relies on distance. The insects first pick up olfactive cues which they can smell from 10 to 50 meters away. Flying closer reveals the blood host visually, within 5-15 meters, while also revealing if the target body is emanating heat. The CO2 plume is not necessary in case the mosquito is already in range to detect heat, apparently.
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