Mosquito heat seeking is driven by an ancestral cooling receptor.

Mosquitoes transmit pathogens that kill >700,000 people annually. These insects use body heat to locate and feed on warm-blooded hosts, but the molecular basis of such behavior is unknown. Here, we identify ionotropic receptor IR21a, a receptor conserved throughout insects, as a key mediator of heat seeking in the malaria vector Although mediates heat avoidance in , we find it drives heat seeking and heat-stimulated blood feeding in At a cellular level, is essential for the detection of cooling, suggesting that during evolution mosquito heat seeking relied on cooling-mediated repulsion. Our data indicate that the evolution of blood feeding in involves repurposing an ancestral thermoreceptor from non-blood-feeding Diptera.