Mosquito bites can range from being a minor annoyance to a significant health threat, particularly in regions where certain species of mosquitoes spread deadly diseases. Aedes aegypti, for example, is responsible for transmitting viruses like dengue, Zika, and yellow fever, leading to over 100 million cases annually. Another species, Anopheles gambiae, spreads the parasite that causes malaria, which the World Health Organization estimates results in over 400,000 deaths each year. This makes mosquitoes the deadliest animals on Earth.
While male mosquitoes are harmless, female mosquitoes require blood for egg development, which drives their relentless search for hosts. For over a century, scientists have studied how these insects locate their targets. It is now understood that mosquitoes do not rely on a single cue but instead integrate information from multiple senses to find a host.
Recent research led by a team at the University of California, Santa Barbara, has added a new dimension to our understanding of mosquito behavior by discovering that mosquitoes can detect infrared (IR) radiation. This new sense significantly enhances their ability to locate hosts. The study, published in Nature, reveals that infrared radiation, when combined with carbon dioxide (CO2) and human odor, doubles the mosquitoes’ host-seeking activity. The researchers also identified the location of the infrared detector in the mosquito’s body and described its function on both a morphological and biochemical level.
Aedes aegypti is known for its ability to use various cues such as CO2 from exhaled breath, skin odors, and body heat to locate humans. However, each of these cues has limitations. For instance, mosquitoes have poor vision, and strong winds or rapid movements can disrupt their ability to track chemical signals. This led researchers to investigate whether mosquitoes could use a more reliable directional cue, such as infrared radiation, to find their hosts.
The researchers conducted experiments in which female mosquitoes were exposed to human odors and CO2, with one group also exposed to infrared radiation at skin temperature. The results showed that the addition of IR significantly increased the mosquitoes’ host-seeking behavior. This confirmed that infrared radiation is a critical sense that mosquitoes use to detect humans, effective at distances up to 70 centimeters.
The study also explored how mosquitoes detect IR. Unlike visible light, which activates rhodopsin proteins in animal eyes, IR radiation has too low energy to be detected in the same way. The researchers found that mosquitoes detect IR indirectly through heat-sensitive neurons located at the tips of their antennae. These neurons contain a protein called TRPA1, which responds to heat generated by IR radiation. The researchers discovered that when the tips of the mosquitoes’ antennae were removed, the insects lost their ability to detect IR.
Further investigations revealed that the tips of the mosquito’s antennae have specialized structures called peg-in-pit, which are well-suited for sensing radiation. These structures allow IR to enter and warm up the peg, which in turn activates the TRPA1 protein, enabling the mosquito to detect the radiation.
Interestingly, the researchers found that the activity of TRPA1 alone does not fully explain the range at which mosquitoes can detect IR. They discovered that other proteins, specifically two rhodopsins (Op1 and Op2), are also involved. These rhodopsins, though typically associated with light detection, are also sensitive to small increases in temperature and work alongside TRPA1 to extend the mosquito’s ability to detect IR over a longer range.
This discovery has significant implications for controlling mosquito populations and preventing mosquito-borne diseases. For example, incorporating thermal IR into mosquito traps could make them more effective. The findings also explain why loose-fitting clothing is effective at preventing bites; it not only prevents the mosquito from reaching the skin but also allows IR to dissipate, making it harder for mosquitoes to detect their targets.
