Mosquito-borne diseases affect hundreds of millions of people annually and disproportionately impact the developing world. One mosquito species, Aedes (Ae.) aegypti, is a primary vector of viruses that cause dengue, yellow fever, and Zika. The attraction of Ae. aegypti females to humans requires integrating multiple cues, including CO2 from breath, organic odors from skin, and visual cues, all sensed at mid and long ranges, and other cues sensed at very close range. Here we identify a new cue that Ae. aegypti use as part of their sensory arsenal to find humans. We demonstrate that they sense the infrared radiation (IR) emanating from their targets and use this information in combination with other cues for highly effective mid-range navigation. Detection of thermal IR requires the heat-activated channel TRPA1, which is expressed in neurons at the tip of the antenna. Two opsins are co-expressed with TRPA1 in these neurons and promote the detection of lower IR intensities. We propose that radiant energy causes local heating at the end of the antenna, thereby activating temperature-sensitive receptors in thermosensory neurons. The realization that thermal IR is an outstanding mid-range directional cue expands our understanding as to how mosquitoes are exquisitely effective in locating hosts.

Recordings of mosquitoes in an arena that was custom fabricated by the UCSB Physics Machine Shop. Arena contained five panels of 1/2" acrylic (Cat. no. 8560K268, McMaster-CarrⓇ, Los Angeles, CA, USA) . Two 10 cm x 10 cm cutouts were made in one panel with enough tolerance to hold the two Peltier plates (10cm x 10cm) securely ​​(TEC plate, model TCP-50, Advanced Thermoelectric, Nashua, NH, USA). The panels were assembled using stainless steel socket head screws (McMaster-Carr®, Los Angeles, CA, USA). An LED light bar was mounted to illuminate the wall that contained the Peltier devices (ASIN: B07CVCF8JF, YEEZEN, Shenzhen, China). 

All but one of the interior faces of the arena panels were covered in white PVC adhesive (ConTactⓇ, Kittrich Corp., Pomona, CA, USA) paper to limit unwanted visual stimuli in the mosquito visual field and produce high contrast images (dark mosquito bodies vs. light background) for subsequent object tracking. The Peltier plates were also covered with white ConTactⓇ paper (emissivity is 0.92). Additionally, the Peltier devices housed in the arena wall were recessed from the interior face of the arena by 0.5 cm to provide an air gap when it covered (Extended Data Fig. 1b). Behind the panel that was left clear, we mounted a webcam (Logitech c920, Logitech, Newark, CA, USA) trained on the arena wall that housed the Peltier plates. Experiments were video recorded using the Logitech Webcam Software (v.2.51) at 10 FPS for 300 seconds. Mosquitoes were placed inside arena in 30 cm x 30 cm x 30 cm mosquito cages (BugDorm-1, #DP1000, MegaView Science Co, Ltd., Taiwan). To improve imaging quality, we replaced one mesh panel of the cage with a 1/16” thick, clear acrylic panel (Cat. no. 8560K171, McMaster-CarrⓇ, Los Angeles, CA, USA). This modification allowed us to achieve a sharper, higher contrast image which improved our tracking ability.