The CO2 and humidity senses of insects in a changing world.

The ability to detect carbon dioxide and water vapor is essential for insect survival. Insects possess specialized receptors and anatomical structures that confer remarkable sensitivity to these environmental factors. As both CO2 and humidity are prominent greenhouse gases, understanding how insects sense and respond to these variables is crucial given global fluctuations in climate. This Review consolidates current insights into the molecular and neural mechanisms underlying CO2 and humidity detection in insects, with a focus on their roles in respiration, water balance and osmoregulation. It highlights case studies of context-dependent functions of these sensory systems in insect-plant interactions and host-seeking behaviors of blood-feeding species. At broader spatial scales, the roles of CO2 and humidity detection are explored for orientation and long-distance navigation behaviors. With growing concerns for declining insect populations, changes to plant-pollinator networks and range expansion of disease vectors, advancing research across biological levels is essential. Finally, the breakthroughs in single-cell and long-read sequencing technologies coupled with sophisticated behavioral tools should be leveraged to fill phylogenetic gaps, explore drivers of specialization, identify vulnerable populations and uncover mechanisms of resilience.