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Female blood-suckers zero in on human prey by smelling our breath

Mosquito coast

Deadly female blood-suckers have been shown to zero in on their living prey by sniffing out the CO2 emitted in exhaled breath, according to new experiments.

The blood-drinking femme fatales in question are mosquitoes: but in all fairness mosquitoes are probably more of an issue for the human race than sexy vampires on any given day so it's difficult to fault the priorities of the investigating boffins.

Having settled that, we find that the said boffins looked into the ways in which female mosquitoes - thirsty for human (or animal) blood to feed their voracious young - actually lock onto their prey.

“Very little was known about what a host looks like to the mosquito and how a mosquito decides where to land and begin to feed,” says biologist Jeff Riffell.

Following some complicated experiments in a wind tunnel full of mosquitoes, it seems that the primary mechanism is detection of CO2 exhalations. Mosquitoes can pick up this natural biological exhaust from as far off as 50 metres, according to the experiments, and follow it until they are close to the source. At this point it seems they then look out for visual clues - anything that stands out from its surroundings - which brings them in to say a metre or so from the target if all has gone well (for the mosquito).

The final stage of guidance is heat, with the tiny airborne bloodsuckers appearing able to pick up typical body-heat signatures within a metre and use this to close in for a tasty draught of blood.

"These experiments show that the attraction to a visual feature and the attraction to a warm object are separate. They are independent, and they don't have to happen in order, but they do often happen in this particular order because of the spatial arrangement of the stimuli: a mosquito can see a visual feature from much further away, so that happens first. Only when the mosquito gets closer does it detect an object's thermal signature," explains Floris van Breugel, Cal tech postdoc who worked on the study alongside Riffell and other boffins.

One can't argue with such painstaking research, but it is possible to quibble with some of the conclusions drawn by van Breugel and the rest of the team. They write:

Even if it were possible to hold one's breath indefinitely, another human breathing nearby, or several meters upwind, would create a CO2 plume that could lead mosquitoes close enough to you that they may lock on to your visual signature. The strongest defense is therefore to become invisible, or at least visually camouflaged. Even in this case, however, mosquitoes could still locate you by tracking the heat signature of your body . . . The independent and iterative nature of the sensory-motor reflexes renders mosquitoes' host seeking strategy annoyingly robust.

Rather than holding one's breath, it would make more sense to simply breathe through a filter full of sodalime absorbent, of the sort used in rebreather sets to scrub CO2 from a diver's breathing loop. In this way the bloodsuckers would never know anybody was around. It wouldn't be practical in all situations, but it could be less faff than citronella candles or creams, smoke repellents etc.

Anyway. The full boffinry is here, and published in the journal Current Biology. ®

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