Animals experience the world through elegantly coordinated senses. Some, like birds, rely heavily on vision to navigate the skies. Others, like many rodents, have poor eyesight but a great sense of smell. What they all have in common is the ability to synchronize multiple senses to perform critical tasks such as finding food. Female mosquitoes find us by our odour, the carbon dioxide we breathe out, the warmth on our skin and our shape. It is a seemingly redundant arrangement that makes humans a conspicuous prey to this tiny foe, which wants to feed on our blood and can make us sick. But how would a mosquito cope if its sense of smell was impaired? In a recent study, Takeshi Morita (The Rockefeller University, USA) and colleagues from the USA, Germany and Canada asked how the yellow fever mosquito (Aedes aegypti) may overcome such loss. They wondered whether these insects could display some form of sensory compensation, similar to how blind humans enhance their hearing and touch.
As mosquitoes are a potent foe, scientists have long been interested in how they function, so they have mutated many of their genes, providing Morita and colleagues with a catalogue of mutated mosquitoes ready for them to test the modified mosquitoes’ sense of smell and ability to detect warmth. The experimenters released 40 to 50 female mosquitoes lacking part of their smell system into a chamber and filmed them for up to 10 min as they flew by or landed on hot surface, a human arm, a plume of carbon dioxide or a combination of them. Using software that tracked the positions of the insects as they flew around, the experimenters recorded how many approached the stimuli mimicking aspects of the human body.
The scientists discovered that the mosquitoes with an impaired sense of smell were more attracted to heat than the normal (unmutated) mosquitoes. Far from having trouble finding a human, the insects that were unable to smell properly outperformed the regular ones with all their senses intact at their people-finding skills. The mosquitoes’ boosted attraction for body temperature was so robust that it even persisted when repellent was added around the hot surface or sprayed on the human arm.
Intrigued by this finding, the scientists speculated that mosquitoes compensated for sense loss by fine-tuning another sense. Mosquitoes smell with their antennae, the two feathery organs on their head. As the insects that lost their sense of smell ramp up their ability to sense heat, the team wondered where these sensors may occur in the mosquitoes’ bodies. Morita and colleagues removed either the tip of the antenna or different parts of the legs and then tested the mosquitoes’ ability to find a warm surface. Cutting part of the antenna, hind or mid legs made no difference to the mosquito's heat-seeking performance. However, when the tip of the front legs was removed, the mosquitoes had trouble finding warmth. The researchers also observed that, for mosquitoes with an incomplete smell system, exposing the tip of the front legs to heat increased nerve activity more than it did for normal (unmodified) mosquitoes. This finding challenges the previous notion that temperature detection is mostly the business of the antennae. Moreover, it suggests that the heat sensors responsible for the compensation between odour and heat sensing reside on the front legs.
Hopefully, understanding how mosquitoes adjust their senses will improve disease prevention, as scientists may be able to use this knowledge to design more effective repellents. But in the meantime, we can admire how hard evolution has worked to make this terrorizing insect an enemy worth fighting.
