Researchers have captured unprecedented images of the mechanisms that allow mosquitoes, the world’s deadliest animal, to target our blood. Credit: Erik Jepsen/ UC San Diego.

Biologists have used cutting-edge imaging technology to probe mosquitoes’ anatomical adaptations designed to target humans.

It’s bound to happen at a summer picnic, a peaceful walk in the woods or simply sitting in your backyard… a mosquito targets your blood for its next meal. You’ve been bitten.

But how do mosquitoes find you? Among several methods used to locate new hosts for blood sucking, mosquitoes feature a keen ability to detect carbon dioxide.

As we breathe out, we emit CO₂ into the air around us, which mosquitoes can sense. But how? Scientists have been aware of the mosquito’s ability to detect our carbon dioxide expirations but the intricate underlying physiological structures enabling these capabilities have largely remained unclear.

University of California San Diego researchers in the School of Biological Sciences and School of Medicine have now uncovered the first elaborately detailed visualisations of these mechanisms.

At UC San Diego’s National Center for Microscopy and Imaging Research, the researchers used serial block-face electron microscopy, a technique that repeatedly slices tissue and images with an electron beam to generate detailed serial pictures, to construct 3D nanoscale models of the mosquito’s carbon dioxide-detecting neurons.

Professor Chih-Ying Su, whose lab is in UC San Diego’s Neurobiology Department, said: “In the past people have speculated about these mosquito mechanisms but they were hard to appreciate.

“Now we have a realistic 3D morphological model that provides quantitative measurements of the sensory surface area. This is the first time we’re seeing this level of detail.”

Intricate dendrite sheets of specialised carbon dioxide-sensing neurons (foreground) aid the detection skills of a yellow fever mosquito, lurking in the background. Credit: Hassan Tahini @ ScienceBrush.

Sensilla

Mosquitoes detect carbon dioxide through sensory hairs known as sensilla. These hairs contain olfactory receptor neurons, or ORNs, including neurons specialised for CO₂ sensing.

The researchers focused on these features in Aedes aegypti mosquitoes, which are known to spread yellow fever, dengue, chikungunya and Zika viruses.

The new results provide key insights into the mosquito’s sensing mechanisms that have been evolutionarily adapted to seek blood sources, a feature that contributes to their status as the world’s deadliest animal.

The new 3D visualisations reveal several remarkable specialised structures.

Within the hairs, the new images revealed anatomical adaptations along sensory dendrites, branches that project out of neurons.

Within capitate peg or ‘cpA’ neurons, the researchers found enlarged CO₂-sensing surface areas and a unique axonal architecture enriched with mitochondria — the energy components within cells — suggesting a high-energy-demand area.

Such features likely allow heightened sensitivity to carbon dioxide.

“These characteristics suggest that ORNs have evolved specific metabolic and structural adaptations to support their essential role in host-seeking,” the researchers wrote in their paper.

Trigger

The researchers also compared their new visualisations with similar structures found in fruit flies.

They found that the analogous area in fruit flies is much less prominent, highlighting key differences between the insects.

Su said: “Fruit flies also have a sensing area for carbon dioxide but it’s much smaller.

“Sensing chemical cues is important for all animals but in fruit flies it serves as an alarm signal — they avoid carbon dioxide.

“For mosquitoes, carbon dioxide is an arousal cue that helps them find us. It’s a trigger for their whole host-seeking behaviour.”

The researchers hope the new findings will provide valuable new information to further science’s understanding of the mosquito’s unique anatomical structures and their functions in seeking new blood hosts.

The study is published in Proceedings of the National Academy of Sciences.