Mosquitoes are feeling the heat in their hunt

Dominic Acquista, Departments Editor dominic.acquista@photonics.com

Is there anything more annoying than a mosquito bite? They’re itchy, scratchy (a whole bevvy of synonyms), and might just leave your skin a bit bloody if you dig your claws into them long enough. If that didn’t make you ponder these airborne menaces enough, then here is a fun fact: Only female mosquitoes bite people, because they need the blood for egg development. It’s enough to make those who love being in nature hot under the collar.

Courtesy of iStock.com/ARTPUPPY.

Well, it turns out, that’s exactly what mosquitoes count on.

The little buggers also have the capacity to spread diseases such as dengue, yellow fever, Zika, and malaria. Two species of mosquito alone, the Aedes aegypti and Anopheles gambiae, can account for more than 100 million total cases of these diseases per year. There are multiple mechanisms by which mosquitoes can detect human skin, which makes it difficult for scientists to ebb the flow of preventable insect-borne diseases. But their work is at least landing where some of these answers lie.

A team led by researchers at the University of California, Santa Barbara have discovered that mosquitoes can sense IR radiation from human skin while they are in pursuit of their next meal. This has been added to a repertoire of previously discovered avenues, including odor and exhaled CO₂, through which mosquitoes land on human skin.

According to the researchers, the insects can detect the heat rising from skin within ~10 cm as well as when they land on their target. However, they can detect IR radiation from the same source at up to ~70 cm, making this sense imperative to their success. In fact, during a test in which they compared host-seeking activities between two groups of female mosquitoes, the group that tested with IR, CO₂, and odor variables demonstrated double the host-seeking activity than the group that tested for only odor and CO₂.

How do they do it? To settle the buzz, a separate lab found a temperature-sensitive protein called TRPA1 at the tips of a mosquito’s antennae that is known for detecting IR radiation. As they describe it, the tip of each antenna has peg-in-pit structures that are well adapted to sensing radiation. The pit shields the peg from conductive and convective heat, enabling the highly directional IR radiation to enter and heat the structure. Essentially, they have built-in sanguine sonar in their heads. The researchers found that if the mosquitoes had these tips removed, their ability to detect IR radiation would be nonexistent, but because there are an estimated 1 trillion mosquitoes in the world, it would be just slightly improbable to remove all of them.

This would explain, however, why mosquitoes are less likely to bite someone if he or she is wearing baggy clothes — not because they love skintight fashion or hate early 2000s skate culture, but because the radiation is able to dissipate between the potential victim’s skin and the fabric.

These findings could provide a way to improve methods for suppressing mosquito populations, such as incorporating thermal IR sources around skin temperature into mosquito traps. In the meantime, those heading out for an evening jaunt would be advised to bring their hottest (warmest) friend with them, and dress in their best ’90s grunge.